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The influence of wind speed, terrain and ventilation system on the air change rate of a single-family house
Energy 31 (2006) 719–731 www.elsevier.com/locate/energy
The influence of wind speed, terrain and ventilation system on the air change rate of a single-family house Bjo¨rn Mattsson* Department of Building Technology, Chalmers University of Technology, 412 96 Gothenburg, Sweden
Abstract When the heat balance of a building is assessed, the heat needed for the ventilation air is usually calculated according to the intended ventilation rate. However, in order to calculate the air change rate accurately several aspects have to be considered. One important parameter is the ventilation system. Whether the building has a mechanical exhaust-only, supply-only, balanced exhaust–supply or natural ventilation system will influence the air infiltration rate through cracks in the building envelope. High infiltration rates lead to an increase in the heating demand and can result in an inadequate capacity of the designed heating installation. In this paper, computer simulations of the air change rate for a detached single-family house are presented. The house is simulated in different topographical surroundings, equipped with a mechanical exhaust-only, or a balanced exhaust–supply, ventilation system. In addition, the airtightness of the building is varied, from very tight, 1 air change per hour (ACH), to quite leaky, 6 ACH, when pressurized to 50 Pa. Results from the simulations show that the same house has quite different air change rates in different surroundings with different airtightness. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction Considering the possible influence of man on the climate, it is vital to use as little energy as possible in all sectors of society. The use of energy in Sweden is mainly attributed to services and heating of buildings, 154.7 TW h, transportation, 94.2 TW h, and the manufacturing industry, 151.8 TW h, annually [1]. After the energy crises in the mid 1970s, regulations on energy use in buildings were made more stringent [2,3]. The stricter rules concerned both insulation and airtightness. Today the building code of Sweden, BBR 94 [4], states that for the production of new residential buildings the overall thermal insulation of the building envelope, i.e. the U-value, should be w0.25 W/(m2 K), or less, including * Tel.: C46 31 772 1995; fax: C46 31 772 1993. E-mail address: [email protected]. 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.04.008
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windows and doors. The code also states that when a residential building is pressurized to 50 Pa, the air leakage shall be no greater than 0.8 l/s and square meter of the building envelope. This corresponds to an airtightness of three to four air changes per hour (ACH) for an average single-family house pressurized to 50 Pa. The regulations state also that half of the energy used to heat the ventilation air has to be recovered. The minimum ventilation rate is in the same code stated to be no less than 0.35 l/s and square meter of the floor area. With a U-value of 0.25 W/(m2 K) and a ventilation rate of 0.35 l/s and square meter floor area, the energy used for heating the ventilation air is about 40–50% relative to the transmission heat loss through the building envelope, without heat recovery of the ventilation air. However, for the construction of new buildings the goal of the Swedish government is to limit the energy use to 90 kW h per year and square meter floor area by the year 2010 and, if possible, reduce it down to 60 kW h/m2 [5] in a more distant future. The energy required to heat the ventilation air of the house in the simulations presented in this paper is about 42 kW h/m2 annually if the house is located in the south of Sweden. For a location in the north of Sweden it would be about 60 kW h/m2 or more. Thus, to be able to reach the goals set up by the government a large portion of the heat in the ventilation air has to be recovered. This signifies that the energy used to heat the ventilation air has to be strictly controlled. In a leaky house, this is very difficult. 1.1. Statement of the problem In Sweden, the majority of the production of new single-family houses has a structure of wooden frame-work. In order to get the construction airtight plastic foil is often used. Even so, many buildings frequently become quite leaky. This is due to difficulties in applying the plastic foil at joints between different construction details, poor workmanship or bad fitting between the vertical studs and the sill and top plate of the framework. Fig. 1 shows a possible leakage path due to poor workmanship or bad fitting between vertical studs and the sill plate.
Wind protection foil
Plastic foil 0.2 mm Leakage path due to poor workmanship or bad fitting between vertical studs and the sill plate Air infiltrating the wall
Vertical wooden studs s 600, 45x170 and mineral wool
Gypsum board 13 mm
Sill plate 45x170 (168)
Air infiltrating the wall
Fig. 1. Leakage path through a wooden frame construction.
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Whether a house will experience a large air infiltration rate or not, is not only dependent on the overall airtightness of the building. It depends also on what kind of ventilation system the building is equipped with and in which surrounding it is located. The surrounding has a large impact on the wind speed and thus on the wind-induced pressure on the building envelope. These conditions, together with stack effect, determine the driving forces behind air leakage.
1.2. Objective The objective of this paper is to investigate the influence of ventilation system and location and shielding of the building, on air change rate and heating demand for a detached single-family house. The house is modeled as having an airtightness of 1, 3 or 6 ACH at a 50 Pa pressurization test. This range of airtightness is commonly found in modern Swedish houses. The objective is also to investigate which impact heat recovery effects, when air infiltrates or exfiltrates through a building envelope, have on the change in heating demand for a single-family house.
1.3. Method In order to meet the objective of this paper, computer simulations of a house model, subjected to different conditions, have been performed. The impact on transmission heat loss of air infiltrating through the wooden frame-work walls of the building was computed in Fluent, a commercial Computational Fluid Dynamics (CFD) code. For documentation on Fluent, see their web page, http://www.fluent.com/. In Simulink, a toolbox in the mathematical software Matlab, a model of the house was assembled. For documentation regarding Matlab and Simulink, see web page, http://www. mathworks.com/. The results from the CFD simulations were used in Simulink to take into account heat exchange effects when air infiltrates through the walls of the house.
1.4. Limitations The house model used in the computer simulations has only two leakage paths in the building envelope. One path is between the sill plate and the wind protection, board or foil, and through the mineral wool, see Figs. 1 and 2. This leakage path is located 0.5 m above the ground. The other leakage path is at the top plate and located 2.9 m above the ground. Both leakages run all around the building and are thus present at all four walls of the building envelope. In addition to the leakage paths, there are, in the case of mechanical exhaust-only ventilation, four inlet vents. These are located one at each wall, 2.6 m above the ground. The walls are modeled as two-dimensional cross sections; see Fig. 2, when calculating the heat exchange effects. The intake and exhaust vents of the mechanical ventilation systems are not influenced by wind or stack in the computer models.
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B. Mattsson / Energy 31 (2006) 719–731 Top plate 45 x 190 mm
Gypsum board 13 mm
2500
Plastic foil
Mineral wool 190 mm
Wind protection foil
Sill plate 45 x 190 mm
Fig. 2. Vertical cross section through an external wall of the building.
2. Theoretical background Roots [7] has investigated the effect of air infiltration on the transmission heat loss through a wooden frame wall. In his thesis, he studies, among other issues, the impact of air infiltration due to imperfections of the insulation material originating from electrical installations and the workmanship, on the transmission heat loss through the wall. Roots have made both measurements and computer modeling. His conclusions are that air infiltrating the wall leads to a decrease in the transmission heat loss due to dynamic, i.e. convective, insulation effects. Virtanen [8] has modeled and measured the impact on heat transfer through a wooden frame wall due to air infiltration through hypothetical cracks and the porous insulation material of the wall. His results show that the conductive heat transfer through the wall decreases with 4–10% when air infiltrates through it. Elmroth [9] measured the energy use for a detached single-family house with dynamic insulation. The part of the insulation that functioned dynamically was placed on the attic floor. A mechanical exhaust fan created an under-pressure in the indoor air relative to the air in the attic space. The pressure difference forced air to seep through the insulation and some of the transmission heat loss was brought back in to the house by the air. The decrease in the overall heating demand for the building was about 6%. One reason for this low energy gain was that only half of the ventilation air entered the house through the dynamic insulation. The rest entered through unintentional cracks in the building envelope. The CFD-program, Fluent, has been validated regarding coupled heat and air transfer through a porous insulation slab. This was done against an analytical solution, Claesson (1993) [10]. The results yield good agreement.
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3. The computational model The house model is a one-storey square building with a floor area of 10 by 10 m. The height of the walls is 2.5 m, and the air volume of the building is 240 m3. The walls of the building are made of wooden frames with mineral wool between the studs, see Fig. 2. The thermal conductivity of the mineral wool is 0.036 W/m K and the air permeability is 8!10K9 m2. The thermal conductivity of the wood is 0.14 W/m K. It is through the exterior walls that air may infiltrate the building. The leakages are modeled according to Fig. 1. Air can enter, or exit, the porous insulation material through joints between the wind protection foil and the sill and top plate, respectively, flow through the insulation and exit, or enter, via the joint between the gypsum board and top and sill plate. The house is equipped with a mechanical ventilation system, either exhaust-only or exhaust–supply. The mechanical ventilation system gives a ventilation rate of 0.5 air change per hour when there is no wind or stack effect. In the model with exhaust–supply ventilation, the mechanically supplied air volume is only 90% of the mechanical exhaust air volume. This is a common practice in order to avoid an indoor over pressure, which can lead to moisture problems in the building envelope. The model with mechanical exhaust-only ventilation has inlet vents, one on each facade. These are located 2.6 m above the ground. In theory, they are supposed to supply all the ventilation air. In reality, quite a large portion enters through leakages in the building envelope if the building is not very airtight. The flow characteristics of the leakages are modeled according to Eq. (1), and for the intentional inlet vents according to Eq. (2). The flow exponent in Eq. (1) is chosen from [6]. The exponent in Eq. (2) is obtained from the manufacturer of the vents. Ra Z C1 DP0:7
ðm3 =sÞ
(1)
Ra Z C2 DP0:5
ðm3 =sÞ
(2)
In Eqs. (1) and (2) above, Ra is the airflow, C1 and C2 are constants, and DP is the pressure difference, in Pa, across the wall. The exponents in Eqs. (1) and (2) are characterizing the flow. An exponent of 0.5 denotes fully turbulent flow and an exponent of 1.0 represents laminar flow. The transformed wind speed is modeled according to Eq. (3), which has its origin in British Standard BS5925: 1991 [6]. For the wind-induced pressure, Eq. (4) has been used. For the stack effect, i.e. pressure differences due to differences in indoor- and outdoor temperatures, Eq. (5) was used. vr Z um kr a Pw Z Cp
ðm=sÞ
rair v2r 2
ðPaÞ
Pstack Z ðrair ðTi Þ K rair ðTo ÞÞzg ðPaÞ
(3) (4) (5)
In Eq. (3) vr is the transformed wind speed at the location of the building, um is the measured wind speed at a meteorological station, r is the height (m) above the ground for which the wind speed is calculated, and k and a are constants according to the surrounding of the building. The constants, k and a, are presented in Table 1, [6].
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Table 1 Constants for transforming the measured wind speed Surrounding
k
a
Open flat country Country with scattered obstacles Urban City
0.68 0.52 0.35 0.21
0.17 0.20 0.25 0.33
In Eq. (4) Pw is the wind-induced pressure on the building envelope, Cp is a constant, see Table 3, rair is the density of the air (kg/m3) and vr is the transformed wind speed according to Eq. (3) calculated at the ridge of the roof of the considered building. For the modeled building the ridge is assumed to be 5 m above ground. In Eq. (5), Pstack is the pressure difference across the walls of the building due to density differences between the indoor and outdoor air caused by temperature differences. The variable z (m) is the distance from the neutral pressure plane, i.e. the plane, where the indoor air pressure equals the outdoor pressure and g is the constant of gravity, 9.81 m/s2. In the simulations the incident angle of the wind was chosen to 458 relative to the normal of the windward wall, see Fig. 3. The measured wind speed is modeled from 0 to 25 m/s. The measured annual average wind speed of Sa¨ve, Gothenburg, is 4.4 m/s. The outdoor temperature was chosen to be K16 8C, i.e. the temperature used when designing the heating installation system of a house in Gothenburg, Sweden. The indoor temperature is 22 8C. The simulations of the air change rate and heating demand were performed for four different types of terrain. In addition, two kinds of ventilation systems, exhaust-only and exhaust–supply, were modeled.
Facade 4
Facade 3
Facade 2
Facade 1
45°
Fig. 3. Orientation of the facades of the building and incident angle of the wind.
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Table 2 Disposition of the simulations Terrain
Mechanical ventilation system Exhaust
Exhaust–supply
Air changes of the house (h 1 3 Open flat country Country with scat-tered wind breaks Urban City
)
K1
6
1
3
6
x
x
x
x
x
x
x
x
x
x
x
x
x x
x x
x x
x x
x x
x x
This together with three different airtightnesses, 1, 3 and 6 ACH of the building resulted in simulations according to Table 2. Apart from the different situations presented in Table 2, each set up was simulated for three different shielding conditions of the house. These were: 1. The building is shielded by obstacles of equal height 2. The building is shielded by obstacles of half its height 3. The building is not shielded The shielding of the building is quite critical for the wind-induced pressure on the building envelope. In Table 3 the wind pressure coefficients, used in Eq. (4), of the four fac¸ades of the building is presented [6]. The incident angle of the wind is 458. For the orientations of the fac¸ades, see Fig. 3. The shielding of the building is normally attributed to other surrounding buildings. However, in rural areas it can be trees, bushes or other types of obstacles. Another issue that has to be discussed is the wind speed. The wind speed, which is measured at a meteorological station, is usually measured at a height of 10 m. In addition, the stations are situated in open terrain, e.g. at an airport or near the coastline. This gives wind speeds that in all cases in the simulations here presented are equal to or lower than the measured wind speed when converted to a local wind speed at the location of the building. Table 3 Wind pressure coefficients for different shielding conditions Facade
1 2 3 4
Cp Shielding 1
2
3
0.05 K0.3 0.05 K0.3
0.1 K0.35 0.1 K0.35
0.35 K0.4 0.35 K0.4
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In the simulations in Simulink wind speed, temperature and fan characteristics are supplied together with topography of the surrounding and shielding conditions for the building. In addition to this, an initial indoor pressure is provided. From wind speed, temperature differences, the ventilation system and the guessed indoor pressure, air flows through leakages, vents and fans are calculated. If there is a balance between the air flows, the iteration is terminated and the resulting air change rate, change in heating demand and heat exchange in the walls are calculated. If there is not a balance a new indoor pressure is assumed until air flow balance is achieved. This is done for wind speeds ranging from 0 to 25 m/s in increments of 0.5 m/s.
4. Simulation results Please observe that the wind speed, on the x-axis, in the figures is the wind speed measured at a meteorological weather station. 4.1. Exhaust—only ventilation system The results from the simulations of the cases with an exhaust ventilation system are presented in Figs. 4 and 5. In the figures the dashed dark lines represent the house with an airtightness of 6 ACH. 4.5 4 3.5
ACH
3 2.5 2 1.5 1 0.5 0
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 4. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 and shielded by obstacles of equivalent height of the building. The ventilation system is mechanical exhaust-only. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
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7
6
5
ACH
4
3
2
1
0
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 5. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 without any shielding of the building by nearby obstacles. The ventilation system is mechanical exhaust-only. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
The solid light lines represent the house with an airtightness of 3 ACH and the dotted lines represent the house with an airtightness of 1 ACH. Each house is set in the four different surroundings according to Table 1. This will result in four different air change rates for each of the three different airtightnesses and is represented by the four lines for each airtightness. The shielding in Fig. 4 is by obstacles equal to the height of the building. In Fig. 5 there is no shielding of the house by nearby obstacles. The results in Figs. 4 and 5 show that for wind speeds above 5–7 m/s, there will be a substantial increase in ventilation rate if the house is leaky and positioned in open terrain, the upper most line of each airtightness. For a wind speed of 10 m/s, the leakiest building, with shielding by obstacles equivalent to the building height, experience a ventilation rate of 1.4 changes per hour. This is almost three times the designed ventilation rate, which is 0.5 ACH. In cold weather this will dominate the heating demand and with a temperature difference of 38 8C the increase in heating demand is close to 3 kW, due to the excessive ventilation rate. The same building in open terrain without shielding by nearby obstacles will experience an air change rate of 2.2 ACH at a wind speed of 10 m/s. This is equivalent to an additional heating demand of more than 5 kW. For a house with an airtightness of 1 ACH there will be very little influence by wind speeds below 10 m/s if the house is shielded by obstacles of equivalent height to the building. The sheltered tight house will require an additional heating demand of less than 0.5 kW at a wind speed of 10 m/s and the unsheltered will require an additional heating demand of about 1 kW.
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4.2. Exhaust—supply ventilation system The results from the simulations of the cases with exhaust–supply ventilation system are presented in Figs. 6 and 7. Each house is set in the four different surroundings according to Table 1. This will result in four different air change rates for each of the three different air tightnesses and is represented by the four lines for each airtightness. The dashed dark lines in the figures represent the house with an airtightness of 6 ACH. The solid lines are results of the house with an airtightness of 3 ACH and the dotted lines represent the house with an airtightness of 1 ACH. In Fig. 6 the house is shielded by obstacles equivalent to the height of the building and in Fig. 7 it is not shielded. The results presented in Figs. 6 and 7 show that even when there is no wind there will be an increase in heating demand due to infiltration. This is not the case for the extract–only mechanical ventilation system for the houses with an airtightness of 3 ACH or better. The reason is that the exhaust–supply ventilation system does not create a large enough underpressure to prevent air exfiltration through the building envelope. However, the mechanical exhaust–supply ventilation system is not affected by the wind to the same extent as the exhaust-only. This is due to the fact that there are no inlet vents on the facades in the house with the exhaust–supply system. Almost all the supply air is distributed by the mechanical ventilation system, which is not as influenced by wind as the inlet vents are.
4
3.5
3
ACH
2.5
2
1.5
1
0.5
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 6. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 and shielded by obstacles of equivalent height of the building. The ventilation system is mechanical exhaust–supply. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
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6
5
ACH
4
3
2
1
0
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 7. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 without any shielding by obstacles in the vicinity of the house. The ventilation system is mechanical exhaust–supply. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
For the leaky house, 6 ACH, the additional heating demand due to stack effect alone is 1 kW. At a wind speed of 10 m/s the additional heating demand is 2.5 kW for the sheltered house with an airtightness of 6 ACH and 4.2 kW for the unsheltered. 4.3. Heat recovery effects When air infiltrates, or exfiltrates, through a building envelope heat exchange takes place between the solid material and the air. Depending on the leakage path and material this heat exchange will reduce the over all heat loss compared to a case with the same air change rate but without interaction between solid material and the air. Fig. 8 shows the change in heating demand for the house when equipped with a mechanical exhaust– supply ventilation system. The building is surrounded by obstacles equivalent to the height of the building. The dark dashed lines show the change in heating demand for the leaky house, 6 ACH, with and without consideration to heat exchange effects when air infiltrates and exfiltrates through the building envelope. The light solid lines show the change in heating demand for the house with a medium airtightness, 3 ACH and the dotted lines show the change in heating demand for the tight house, 1 ACH. Without heat recovery effects the additional heating demand is at a wind speed of 10 m/s 2.8 kW for the leaky house if it is situated in open terrain and sheltered by nearby obstacles equivalent to the height
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Change in heating demand [kW]
10
8
6
4
2
0
–2
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 8. Change in heating demand for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 and shielded by obstacles in the vicinity of the house, equivalent to the building height. The house is equipped with a mechanical exhaust-only ventilation system. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid, lighter, lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH.
of the house. With heat recovery effects taken in to account, due to air in- and exfiltrating the walls, the change in heating demand is 2.5 kW.
5. Conclusions The results presented above show that for a tight house with a mechanical exhaust-only ventilation system, the ventilation rate is mainly influenced by the wind speed. The stack effect is too small compared to the indoor underpressure induced by the extract fan to produce an increase in the ventilation rate. However, for an exhaust–supply system, the stack effect is sufficient to influence the ventilation rate even for a tight house and for a leaky building it can be significant. The house equipped with exhaustonly ventilation system, on the other hand, is more sensitive to high wind speeds. The result of this is that both houses will experience more or less the same air change rate at higher wind speeds. The larger sensitivity to high wind speeds for the exhaust-only ventilation system is mainly due to the inlet vents. If they are open, they represent an air leakage of 1.1 air changes per hour when the building is pressurized to 50 Pa. The airtightness of a building is usually measured with the inlet vents closed. This means that when the extract fan is unable to maintain an indoor underpressure across all the walls of the building, the airtightness of the house will be less than the measure one. For the house in the simulations
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with an airtightness of 1 ACH due to leakages in the building envelope, the actual airtightness will be 2.1 ACH at a 50 Pa pressurization test, if the inlet vents are not sealed. The terrain, or surrounding, in which the building is situated, together with the shielding in the vicinity of the building, is critical regarding the impact of the wind speed on the air change rate. Even for a very airtight house, the wind can influence the air change rate and thus the heating demand, at high wind speeds. The heat exchange when air in- and exfiltrates the walls of the modeled building can compensate some of the increase in heat loss when the ventilation rate exceeds the intended one. But the potential heat exchange is too small to be able to recover all the heat loss due to an increased air change rate at higher wind speeds. It is therefore not advisable the relay on heat recovery effects when designing the heating device of a building. To be able to meet the goals regarding energy use in the future, it is necessary to construct airtight buildings. Even a building with an airtightness of 1 ACH might be too leaky if energy use in houses is to reach the goals set by the Swedish authorities concerning energy use.
References [1] Swedish Energy Agency, The energy status, facts and figures; 2003, http://www.stem.se. [2] BABS 1967, Svensk Byggnorm (The Swedish Building Code) 67, paragraf 32:21, Fo¨reskrifter ra˚d och anvisningar till byggnadsstadgan, Boktryckeri AB Thule, Stockholm; 1967 [in Swedish]. [3] SBN (The Swedish Building Code) 1975, Svensk Byggnorm, q 1977 statens planverk och LiberFo¨rlag/Allma¨nna Fo¨rlaget, Stockholm; 1977 [in Swedish]. [4] Boverkets Byggregler (The Swedish Building Code) BBR 94, 1993, Boverket, byggavdelningen, ISBN 91-38-12851-9, Norstedts Tryckeri, Stockholm; 1993 [in Swedish]. [5] SOU 2000:23, The Swedish Governments Public Investigations, http://miljo.regeringen.se/propositionermm/sou/pdf/ sou2000_23c.pdf. [6] AIVC. Technical note 44. Coventry: the air infiltration and ventilation centre 1994. [7] Peter R. Heat transfer through a well insulated external wooden frame wall. Lund: Department of Building Technology. Building Physics; 1997 [Byggnadsfysik, LTH, Box 118, 221 00 Lund, Sweden]. [8] Virtanen M. Thermal coupling of leakage air and heat flows in buildings and building components. Espoo: Doctoral Thesis. VTT Technical Research Centre of Finland; 1993 [P.O. Box 1803, FIN-02044, VTT, Finland]. [9] Arne E, Bertil F. The optima-house. Air quality and energy use in a single-family house with couterflow attic insulation and warm crawl space foundation.: Lund Institute of Technology. Department of Building Science; 1996 [Byggnadsfysik, LTH, Box 118, 221 00 Lund, Sweden]. [10] Johan C. Forced convective–diffusive heat flow in insulations, a new analytical technique applied to air leakage through a slit. Proceedings of the 3rd symposium, building physics in the nordic countries, Copenhagen. vol. 1 1993 p. 137–44.
The influence of wind speed, terrain and ventilation system on the air change rate of a single-family house Bjo¨rn Mattsson* Department of Building Technology, Chalmers University of Technology, 412 96 Gothenburg, Sweden
Abstract When the heat balance of a building is assessed, the heat needed for the ventilation air is usually calculated according to the intended ventilation rate. However, in order to calculate the air change rate accurately several aspects have to be considered. One important parameter is the ventilation system. Whether the building has a mechanical exhaust-only, supply-only, balanced exhaust–supply or natural ventilation system will influence the air infiltration rate through cracks in the building envelope. High infiltration rates lead to an increase in the heating demand and can result in an inadequate capacity of the designed heating installation. In this paper, computer simulations of the air change rate for a detached single-family house are presented. The house is simulated in different topographical surroundings, equipped with a mechanical exhaust-only, or a balanced exhaust–supply, ventilation system. In addition, the airtightness of the building is varied, from very tight, 1 air change per hour (ACH), to quite leaky, 6 ACH, when pressurized to 50 Pa. Results from the simulations show that the same house has quite different air change rates in different surroundings with different airtightness. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction Considering the possible influence of man on the climate, it is vital to use as little energy as possible in all sectors of society. The use of energy in Sweden is mainly attributed to services and heating of buildings, 154.7 TW h, transportation, 94.2 TW h, and the manufacturing industry, 151.8 TW h, annually [1]. After the energy crises in the mid 1970s, regulations on energy use in buildings were made more stringent [2,3]. The stricter rules concerned both insulation and airtightness. Today the building code of Sweden, BBR 94 [4], states that for the production of new residential buildings the overall thermal insulation of the building envelope, i.e. the U-value, should be w0.25 W/(m2 K), or less, including * Tel.: C46 31 772 1995; fax: C46 31 772 1993. E-mail address: [email protected]. 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.04.008
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windows and doors. The code also states that when a residential building is pressurized to 50 Pa, the air leakage shall be no greater than 0.8 l/s and square meter of the building envelope. This corresponds to an airtightness of three to four air changes per hour (ACH) for an average single-family house pressurized to 50 Pa. The regulations state also that half of the energy used to heat the ventilation air has to be recovered. The minimum ventilation rate is in the same code stated to be no less than 0.35 l/s and square meter of the floor area. With a U-value of 0.25 W/(m2 K) and a ventilation rate of 0.35 l/s and square meter floor area, the energy used for heating the ventilation air is about 40–50% relative to the transmission heat loss through the building envelope, without heat recovery of the ventilation air. However, for the construction of new buildings the goal of the Swedish government is to limit the energy use to 90 kW h per year and square meter floor area by the year 2010 and, if possible, reduce it down to 60 kW h/m2 [5] in a more distant future. The energy required to heat the ventilation air of the house in the simulations presented in this paper is about 42 kW h/m2 annually if the house is located in the south of Sweden. For a location in the north of Sweden it would be about 60 kW h/m2 or more. Thus, to be able to reach the goals set up by the government a large portion of the heat in the ventilation air has to be recovered. This signifies that the energy used to heat the ventilation air has to be strictly controlled. In a leaky house, this is very difficult. 1.1. Statement of the problem In Sweden, the majority of the production of new single-family houses has a structure of wooden frame-work. In order to get the construction airtight plastic foil is often used. Even so, many buildings frequently become quite leaky. This is due to difficulties in applying the plastic foil at joints between different construction details, poor workmanship or bad fitting between the vertical studs and the sill and top plate of the framework. Fig. 1 shows a possible leakage path due to poor workmanship or bad fitting between vertical studs and the sill plate.
Wind protection foil
Plastic foil 0.2 mm Leakage path due to poor workmanship or bad fitting between vertical studs and the sill plate Air infiltrating the wall
Vertical wooden studs s 600, 45x170 and mineral wool
Gypsum board 13 mm
Sill plate 45x170 (168)
Air infiltrating the wall
Fig. 1. Leakage path through a wooden frame construction.
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Whether a house will experience a large air infiltration rate or not, is not only dependent on the overall airtightness of the building. It depends also on what kind of ventilation system the building is equipped with and in which surrounding it is located. The surrounding has a large impact on the wind speed and thus on the wind-induced pressure on the building envelope. These conditions, together with stack effect, determine the driving forces behind air leakage.
1.2. Objective The objective of this paper is to investigate the influence of ventilation system and location and shielding of the building, on air change rate and heating demand for a detached single-family house. The house is modeled as having an airtightness of 1, 3 or 6 ACH at a 50 Pa pressurization test. This range of airtightness is commonly found in modern Swedish houses. The objective is also to investigate which impact heat recovery effects, when air infiltrates or exfiltrates through a building envelope, have on the change in heating demand for a single-family house.
1.3. Method In order to meet the objective of this paper, computer simulations of a house model, subjected to different conditions, have been performed. The impact on transmission heat loss of air infiltrating through the wooden frame-work walls of the building was computed in Fluent, a commercial Computational Fluid Dynamics (CFD) code. For documentation on Fluent, see their web page, http://www.fluent.com/. In Simulink, a toolbox in the mathematical software Matlab, a model of the house was assembled. For documentation regarding Matlab and Simulink, see web page, http://www. mathworks.com/. The results from the CFD simulations were used in Simulink to take into account heat exchange effects when air infiltrates through the walls of the house.
1.4. Limitations The house model used in the computer simulations has only two leakage paths in the building envelope. One path is between the sill plate and the wind protection, board or foil, and through the mineral wool, see Figs. 1 and 2. This leakage path is located 0.5 m above the ground. The other leakage path is at the top plate and located 2.9 m above the ground. Both leakages run all around the building and are thus present at all four walls of the building envelope. In addition to the leakage paths, there are, in the case of mechanical exhaust-only ventilation, four inlet vents. These are located one at each wall, 2.6 m above the ground. The walls are modeled as two-dimensional cross sections; see Fig. 2, when calculating the heat exchange effects. The intake and exhaust vents of the mechanical ventilation systems are not influenced by wind or stack in the computer models.
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Gypsum board 13 mm
2500
Plastic foil
Mineral wool 190 mm
Wind protection foil
Sill plate 45 x 190 mm
Fig. 2. Vertical cross section through an external wall of the building.
2. Theoretical background Roots [7] has investigated the effect of air infiltration on the transmission heat loss through a wooden frame wall. In his thesis, he studies, among other issues, the impact of air infiltration due to imperfections of the insulation material originating from electrical installations and the workmanship, on the transmission heat loss through the wall. Roots have made both measurements and computer modeling. His conclusions are that air infiltrating the wall leads to a decrease in the transmission heat loss due to dynamic, i.e. convective, insulation effects. Virtanen [8] has modeled and measured the impact on heat transfer through a wooden frame wall due to air infiltration through hypothetical cracks and the porous insulation material of the wall. His results show that the conductive heat transfer through the wall decreases with 4–10% when air infiltrates through it. Elmroth [9] measured the energy use for a detached single-family house with dynamic insulation. The part of the insulation that functioned dynamically was placed on the attic floor. A mechanical exhaust fan created an under-pressure in the indoor air relative to the air in the attic space. The pressure difference forced air to seep through the insulation and some of the transmission heat loss was brought back in to the house by the air. The decrease in the overall heating demand for the building was about 6%. One reason for this low energy gain was that only half of the ventilation air entered the house through the dynamic insulation. The rest entered through unintentional cracks in the building envelope. The CFD-program, Fluent, has been validated regarding coupled heat and air transfer through a porous insulation slab. This was done against an analytical solution, Claesson (1993) [10]. The results yield good agreement.
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3. The computational model The house model is a one-storey square building with a floor area of 10 by 10 m. The height of the walls is 2.5 m, and the air volume of the building is 240 m3. The walls of the building are made of wooden frames with mineral wool between the studs, see Fig. 2. The thermal conductivity of the mineral wool is 0.036 W/m K and the air permeability is 8!10K9 m2. The thermal conductivity of the wood is 0.14 W/m K. It is through the exterior walls that air may infiltrate the building. The leakages are modeled according to Fig. 1. Air can enter, or exit, the porous insulation material through joints between the wind protection foil and the sill and top plate, respectively, flow through the insulation and exit, or enter, via the joint between the gypsum board and top and sill plate. The house is equipped with a mechanical ventilation system, either exhaust-only or exhaust–supply. The mechanical ventilation system gives a ventilation rate of 0.5 air change per hour when there is no wind or stack effect. In the model with exhaust–supply ventilation, the mechanically supplied air volume is only 90% of the mechanical exhaust air volume. This is a common practice in order to avoid an indoor over pressure, which can lead to moisture problems in the building envelope. The model with mechanical exhaust-only ventilation has inlet vents, one on each facade. These are located 2.6 m above the ground. In theory, they are supposed to supply all the ventilation air. In reality, quite a large portion enters through leakages in the building envelope if the building is not very airtight. The flow characteristics of the leakages are modeled according to Eq. (1), and for the intentional inlet vents according to Eq. (2). The flow exponent in Eq. (1) is chosen from [6]. The exponent in Eq. (2) is obtained from the manufacturer of the vents. Ra Z C1 DP0:7
ðm3 =sÞ
(1)
Ra Z C2 DP0:5
ðm3 =sÞ
(2)
In Eqs. (1) and (2) above, Ra is the airflow, C1 and C2 are constants, and DP is the pressure difference, in Pa, across the wall. The exponents in Eqs. (1) and (2) are characterizing the flow. An exponent of 0.5 denotes fully turbulent flow and an exponent of 1.0 represents laminar flow. The transformed wind speed is modeled according to Eq. (3), which has its origin in British Standard BS5925: 1991 [6]. For the wind-induced pressure, Eq. (4) has been used. For the stack effect, i.e. pressure differences due to differences in indoor- and outdoor temperatures, Eq. (5) was used. vr Z um kr a Pw Z Cp
ðm=sÞ
rair v2r 2
ðPaÞ
Pstack Z ðrair ðTi Þ K rair ðTo ÞÞzg ðPaÞ
(3) (4) (5)
In Eq. (3) vr is the transformed wind speed at the location of the building, um is the measured wind speed at a meteorological station, r is the height (m) above the ground for which the wind speed is calculated, and k and a are constants according to the surrounding of the building. The constants, k and a, are presented in Table 1, [6].
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Table 1 Constants for transforming the measured wind speed Surrounding
k
a
Open flat country Country with scattered obstacles Urban City
0.68 0.52 0.35 0.21
0.17 0.20 0.25 0.33
In Eq. (4) Pw is the wind-induced pressure on the building envelope, Cp is a constant, see Table 3, rair is the density of the air (kg/m3) and vr is the transformed wind speed according to Eq. (3) calculated at the ridge of the roof of the considered building. For the modeled building the ridge is assumed to be 5 m above ground. In Eq. (5), Pstack is the pressure difference across the walls of the building due to density differences between the indoor and outdoor air caused by temperature differences. The variable z (m) is the distance from the neutral pressure plane, i.e. the plane, where the indoor air pressure equals the outdoor pressure and g is the constant of gravity, 9.81 m/s2. In the simulations the incident angle of the wind was chosen to 458 relative to the normal of the windward wall, see Fig. 3. The measured wind speed is modeled from 0 to 25 m/s. The measured annual average wind speed of Sa¨ve, Gothenburg, is 4.4 m/s. The outdoor temperature was chosen to be K16 8C, i.e. the temperature used when designing the heating installation system of a house in Gothenburg, Sweden. The indoor temperature is 22 8C. The simulations of the air change rate and heating demand were performed for four different types of terrain. In addition, two kinds of ventilation systems, exhaust-only and exhaust–supply, were modeled.
Facade 4
Facade 3
Facade 2
Facade 1
45°
Fig. 3. Orientation of the facades of the building and incident angle of the wind.
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Table 2 Disposition of the simulations Terrain
Mechanical ventilation system Exhaust
Exhaust–supply
Air changes of the house (h 1 3 Open flat country Country with scat-tered wind breaks Urban City
)
K1
6
1
3
6
x
x
x
x
x
x
x
x
x
x
x
x
x x
x x
x x
x x
x x
x x
This together with three different airtightnesses, 1, 3 and 6 ACH of the building resulted in simulations according to Table 2. Apart from the different situations presented in Table 2, each set up was simulated for three different shielding conditions of the house. These were: 1. The building is shielded by obstacles of equal height 2. The building is shielded by obstacles of half its height 3. The building is not shielded The shielding of the building is quite critical for the wind-induced pressure on the building envelope. In Table 3 the wind pressure coefficients, used in Eq. (4), of the four fac¸ades of the building is presented [6]. The incident angle of the wind is 458. For the orientations of the fac¸ades, see Fig. 3. The shielding of the building is normally attributed to other surrounding buildings. However, in rural areas it can be trees, bushes or other types of obstacles. Another issue that has to be discussed is the wind speed. The wind speed, which is measured at a meteorological station, is usually measured at a height of 10 m. In addition, the stations are situated in open terrain, e.g. at an airport or near the coastline. This gives wind speeds that in all cases in the simulations here presented are equal to or lower than the measured wind speed when converted to a local wind speed at the location of the building. Table 3 Wind pressure coefficients for different shielding conditions Facade
1 2 3 4
Cp Shielding 1
2
3
0.05 K0.3 0.05 K0.3
0.1 K0.35 0.1 K0.35
0.35 K0.4 0.35 K0.4
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In the simulations in Simulink wind speed, temperature and fan characteristics are supplied together with topography of the surrounding and shielding conditions for the building. In addition to this, an initial indoor pressure is provided. From wind speed, temperature differences, the ventilation system and the guessed indoor pressure, air flows through leakages, vents and fans are calculated. If there is a balance between the air flows, the iteration is terminated and the resulting air change rate, change in heating demand and heat exchange in the walls are calculated. If there is not a balance a new indoor pressure is assumed until air flow balance is achieved. This is done for wind speeds ranging from 0 to 25 m/s in increments of 0.5 m/s.
4. Simulation results Please observe that the wind speed, on the x-axis, in the figures is the wind speed measured at a meteorological weather station. 4.1. Exhaust—only ventilation system The results from the simulations of the cases with an exhaust ventilation system are presented in Figs. 4 and 5. In the figures the dashed dark lines represent the house with an airtightness of 6 ACH. 4.5 4 3.5
ACH
3 2.5 2 1.5 1 0.5 0
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 4. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 and shielded by obstacles of equivalent height of the building. The ventilation system is mechanical exhaust-only. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
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7
6
5
ACH
4
3
2
1
0
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 5. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 without any shielding of the building by nearby obstacles. The ventilation system is mechanical exhaust-only. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
The solid light lines represent the house with an airtightness of 3 ACH and the dotted lines represent the house with an airtightness of 1 ACH. Each house is set in the four different surroundings according to Table 1. This will result in four different air change rates for each of the three different airtightnesses and is represented by the four lines for each airtightness. The shielding in Fig. 4 is by obstacles equal to the height of the building. In Fig. 5 there is no shielding of the house by nearby obstacles. The results in Figs. 4 and 5 show that for wind speeds above 5–7 m/s, there will be a substantial increase in ventilation rate if the house is leaky and positioned in open terrain, the upper most line of each airtightness. For a wind speed of 10 m/s, the leakiest building, with shielding by obstacles equivalent to the building height, experience a ventilation rate of 1.4 changes per hour. This is almost three times the designed ventilation rate, which is 0.5 ACH. In cold weather this will dominate the heating demand and with a temperature difference of 38 8C the increase in heating demand is close to 3 kW, due to the excessive ventilation rate. The same building in open terrain without shielding by nearby obstacles will experience an air change rate of 2.2 ACH at a wind speed of 10 m/s. This is equivalent to an additional heating demand of more than 5 kW. For a house with an airtightness of 1 ACH there will be very little influence by wind speeds below 10 m/s if the house is shielded by obstacles of equivalent height to the building. The sheltered tight house will require an additional heating demand of less than 0.5 kW at a wind speed of 10 m/s and the unsheltered will require an additional heating demand of about 1 kW.
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4.2. Exhaust—supply ventilation system The results from the simulations of the cases with exhaust–supply ventilation system are presented in Figs. 6 and 7. Each house is set in the four different surroundings according to Table 1. This will result in four different air change rates for each of the three different air tightnesses and is represented by the four lines for each airtightness. The dashed dark lines in the figures represent the house with an airtightness of 6 ACH. The solid lines are results of the house with an airtightness of 3 ACH and the dotted lines represent the house with an airtightness of 1 ACH. In Fig. 6 the house is shielded by obstacles equivalent to the height of the building and in Fig. 7 it is not shielded. The results presented in Figs. 6 and 7 show that even when there is no wind there will be an increase in heating demand due to infiltration. This is not the case for the extract–only mechanical ventilation system for the houses with an airtightness of 3 ACH or better. The reason is that the exhaust–supply ventilation system does not create a large enough underpressure to prevent air exfiltration through the building envelope. However, the mechanical exhaust–supply ventilation system is not affected by the wind to the same extent as the exhaust-only. This is due to the fact that there are no inlet vents on the facades in the house with the exhaust–supply system. Almost all the supply air is distributed by the mechanical ventilation system, which is not as influenced by wind as the inlet vents are.
4
3.5
3
ACH
2.5
2
1.5
1
0.5
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 6. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 and shielded by obstacles of equivalent height of the building. The ventilation system is mechanical exhaust–supply. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
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6
5
ACH
4
3
2
1
0
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 7. Air change rate for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 without any shielding by obstacles in the vicinity of the house. The ventilation system is mechanical exhaust–supply. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH. The upper most line of each airtightness represents the building in open terrain. Successive, lower, lines represent open terrain with scattered obstacles, urban terrain and larger cities.
For the leaky house, 6 ACH, the additional heating demand due to stack effect alone is 1 kW. At a wind speed of 10 m/s the additional heating demand is 2.5 kW for the sheltered house with an airtightness of 6 ACH and 4.2 kW for the unsheltered. 4.3. Heat recovery effects When air infiltrates, or exfiltrates, through a building envelope heat exchange takes place between the solid material and the air. Depending on the leakage path and material this heat exchange will reduce the over all heat loss compared to a case with the same air change rate but without interaction between solid material and the air. Fig. 8 shows the change in heating demand for the house when equipped with a mechanical exhaust– supply ventilation system. The building is surrounded by obstacles equivalent to the height of the building. The dark dashed lines show the change in heating demand for the leaky house, 6 ACH, with and without consideration to heat exchange effects when air infiltrates and exfiltrates through the building envelope. The light solid lines show the change in heating demand for the house with a medium airtightness, 3 ACH and the dotted lines show the change in heating demand for the tight house, 1 ACH. Without heat recovery effects the additional heating demand is at a wind speed of 10 m/s 2.8 kW for the leaky house if it is situated in open terrain and sheltered by nearby obstacles equivalent to the height
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Change in heating demand [kW]
10
8
6
4
2
0
–2
0
5
10
15
20
25
Measured wind speed [m/s] Fig. 8. Change in heating demand for a house with an airtightness of 1, 3 or 6 ACH positioned in surroundings according to Table 1 and shielded by obstacles in the vicinity of the house, equivalent to the building height. The house is equipped with a mechanical exhaust-only ventilation system. The dashed lines represent the results for a building with an airtightness of 6 ACH, the solid, lighter, lines represent the results for a building with an airtightness of 3 ACH and the dotted lines represent a building with an airtightness of 1 ACH.
of the house. With heat recovery effects taken in to account, due to air in- and exfiltrating the walls, the change in heating demand is 2.5 kW.
5. Conclusions The results presented above show that for a tight house with a mechanical exhaust-only ventilation system, the ventilation rate is mainly influenced by the wind speed. The stack effect is too small compared to the indoor underpressure induced by the extract fan to produce an increase in the ventilation rate. However, for an exhaust–supply system, the stack effect is sufficient to influence the ventilation rate even for a tight house and for a leaky building it can be significant. The house equipped with exhaustonly ventilation system, on the other hand, is more sensitive to high wind speeds. The result of this is that both houses will experience more or less the same air change rate at higher wind speeds. The larger sensitivity to high wind speeds for the exhaust-only ventilation system is mainly due to the inlet vents. If they are open, they represent an air leakage of 1.1 air changes per hour when the building is pressurized to 50 Pa. The airtightness of a building is usually measured with the inlet vents closed. This means that when the extract fan is unable to maintain an indoor underpressure across all the walls of the building, the airtightness of the house will be less than the measure one. For the house in the simulations
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with an airtightness of 1 ACH due to leakages in the building envelope, the actual airtightness will be 2.1 ACH at a 50 Pa pressurization test, if the inlet vents are not sealed. The terrain, or surrounding, in which the building is situated, together with the shielding in the vicinity of the building, is critical regarding the impact of the wind speed on the air change rate. Even for a very airtight house, the wind can influence the air change rate and thus the heating demand, at high wind speeds. The heat exchange when air in- and exfiltrates the walls of the modeled building can compensate some of the increase in heat loss when the ventilation rate exceeds the intended one. But the potential heat exchange is too small to be able to recover all the heat loss due to an increased air change rate at higher wind speeds. It is therefore not advisable the relay on heat recovery effects when designing the heating device of a building. To be able to meet the goals regarding energy use in the future, it is necessary to construct airtight buildings. Even a building with an airtightness of 1 ACH might be too leaky if energy use in houses is to reach the goals set by the Swedish authorities concerning energy use.
References [1] Swedish Energy Agency, The energy status, facts and figures; 2003, http://www.stem.se. [2] BABS 1967, Svensk Byggnorm (The Swedish Building Code) 67, paragraf 32:21, Fo¨reskrifter ra˚d och anvisningar till byggnadsstadgan, Boktryckeri AB Thule, Stockholm; 1967 [in Swedish]. [3] SBN (The Swedish Building Code) 1975, Svensk Byggnorm, q 1977 statens planverk och LiberFo¨rlag/Allma¨nna Fo¨rlaget, Stockholm; 1977 [in Swedish]. [4] Boverkets Byggregler (The Swedish Building Code) BBR 94, 1993, Boverket, byggavdelningen, ISBN 91-38-12851-9, Norstedts Tryckeri, Stockholm; 1993 [in Swedish]. [5] SOU 2000:23, The Swedish Governments Public Investigations, http://miljo.regeringen.se/propositionermm/sou/pdf/ sou2000_23c.pdf. [6] AIVC. Technical note 44. Coventry: the air infiltration and ventilation centre 1994. [7] Peter R. Heat transfer through a well insulated external wooden frame wall. Lund: Department of Building Technology. Building Physics; 1997 [Byggnadsfysik, LTH, Box 118, 221 00 Lund, Sweden]. [8] Virtanen M. Thermal coupling of leakage air and heat flows in buildings and building components. Espoo: Doctoral Thesis. VTT Technical Research Centre of Finland; 1993 [P.O. Box 1803, FIN-02044, VTT, Finland]. [9] Arne E, Bertil F. The optima-house. Air quality and energy use in a single-family house with couterflow attic insulation and warm crawl space foundation.: Lund Institute of Technology. Department of Building Science; 1996 [Byggnadsfysik, LTH, Box 118, 221 00 Lund, Sweden]. [10] Johan C. Forced convective–diffusive heat flow in insulations, a new analytical technique applied to air leakage through a slit. Proceedings of the 3rd symposium, building physics in the nordic countries, Copenhagen. vol. 1 1993 p. 137–44.