AIR PERMEABILITY OF THE BUILDING ENVELOPE IN PRACTICE

E. Brandt1, T. Bunch-Nielsen2

1 Danish Building Research Institute/Aalborg University, Denmark. ebr@sbi.dk, Corresponding author2 Danish Roofing Advisory Board, Denmark. tbn@tor.info

INTRODUCTIONAir permeability is an important part of the requirements

for the building envelope as a lot of energy escapes through leakages in the building envelope. For permeable buildings the air exchange rate is dependent on actual wind conditions. The idea of setting requirements to air permeability/air tightness is that we shall be able to control where and how much air penetrates in and out of the building thereby making it possible e.g. to reuse the energy from the outlet air to preheat the intake air.

Requirements in building code. The requirement to air permeability in the Danish Building Code is max 1.5 l/(s m2) (heated floor area) [1]. This means that the requirement is more severe for complex buildings which may be seen as a way to promote simple building bodies.

Determination and control of air permeability. The air permeability of a building is determined by measuring how much air needs to be supplied or removed to maintain an air pressure in the building at 50 Pa above or below ambient air pressure respectively [2]. The air permeability is found as the mean of the infiltration and the exfiltration for a negative and a positive interior pressure respectively.

Figure 2. Blower door measurement where the original door is replaced by a canvas door provided with a computer controlled fan.

To find the location of the leaks in a building thermography made during infiltration, i.e. low pressure in the building, has shown to be the most effective method. Where the conditions do not allow thermography leakages might instead be detected by smoke or airflow measurements with thermo anemometer.

Indoor climate. The indoor climate is important when we make our buildings more airtight as air quality easily is impaired, e.g. by CO2 or radon, if the air exchange rate in the building is not sufficient. For very airtight buildings a mechanical ventilation system is normally needed to provide an air exchange rate of min 0.5 times per hour. This is especially the case for passive houses where air permeability down to 0.2 l/(s m2) has been found.

ACHIEVING AIR TIGHTNESS IN PRACTICEVapor barriers/vapor control layers. In a cold

climate, as in Denmark, the humid indoor air is normally prevented from entering the building envelope with an air tight vapor barrier which also is used to control the air permeability of the building. Heavyweight building components such as concrete wall elements are normally considered inherently airtight provided that the joints are sealed.

Air tightness at details. Achieving good air tightness is difficult and especially the details i.e. joints between the individual lengths of vapor barrier and to adjoining building elements are critical. As a general rule all connections must be made on a firm substrate ensuring that the materials to be joined are not moving mutually and that tapes or sealant strips can be rubbed firmly to achieve a good and durable connection, cf. figure 2. Experience from testing shows that a joint made with a squeezed overlap is not sufficiently airtight to fulfill current requirements to air permeability [3].

It is crucial that accessories like sealants, tapes prefabricated corners and collars with prefabricated penetrations for electrical cables or pipes, cf. figure 3, are suited for the purpose and are durable. This must be documented by the supplier of the vapor barrier.

Figure 2 (from [3]). Airtight joints can be made by overlapping combined with either tape (a) or sealing strip (butyl rubber) (b).

1

Figure 3. Penetration of vapor barrier by a pipe using a special prefab collar (available in different dimensions).

The vapor barrier is normally installed behind 50 mm insulation (from the warm side) in order to protect it and to allow installations on the inside of the vapor barrier. Clamps are placed parallel to the direction of the studs to avoid tension around the ends in connection with movements from wind pressure. The air tightness of vapor barriers in roofs is more important than in walls as there is a constant overpressure on the ceiling due to the chimney effect.

EXPERIENCE WITH MEASUREMENTS The blower door test is widely used for (required)

documentation of the air permeability of new buildings according to the national Danish building code.

Large buildings. For buildings more than 50000 m3 one blower door cannot keep the necessary pressure difference, with the expected air leakage. Determination of air permeability in such buildings have been done by dividing it into smaller sections or by using a number of blower doors connected into one large system.

Testing of watertightnes of windows. The blower door has been used to test windows on site. With an alternating under pressure of 0-50 Pa on the inner side of the window and water running over the window from a tube with drilled holes placed at the top of the window the effect of driving rain is simulated. In the actual case water penetration was observed on the inside after 20 minutes.

Testing of roof elements. The blower door equipment has also been used to find leaks in unventilated roof elements. It is a prerequisite in such elements that the vapor barrier is tight as otherwise more moisture penetrates up in winter than can be removed in summer. The blower door is connected to a smoke generator and an overpressure is established inside

the roof element. Any leaks will be visible as smoke is forced out, cf. figure 4.

Figure 4. Smoke forced out by overpressure in unventilated roof element. Note that the joints are not with a firm substrate and therefore it has not been possible to make them airtight.

SUPPLEMENTARY ADVANTAGESAn air tight vapor barrier is important to avoid moisture

transport by convection which is the most significant reason for moisture problems in the building envelope.

Other problems may also be solved using a (modified) blower door test. Even though an entire building meets the requirements for air permeability, there can still be unacceptable openings between dwellings in the building so fire requirements are not met or causing smell problems. Such problems may be detected establishing overpressure with a blower door combined with a smoke generator.

DISCUSSIONExperience from testing shows that it is practically

possible to improve the air permeability from 3–5 l/(s m²) to less than 1.5 l/(s m²) with simple means. In low energy houses it has proved possible to achieve values as low as 0.2 l/(s m²). An air exchange rate of 0.5 to 1.0 per hour is still needed in order to have a good indoor climate. However, with low air permeability mechanical ventilation with heat regain is normally needed (also to fulfill other requirements to energy saving).

Requirements for air permeability will also improve the buildings function with regard to a number of other items such as fire and penetration of radon and chemical pollution from the ground.

REFERENCES[1] Building regulations 2008, (also available in English), Erhvervs- og Byggestyrelsen, Denmark, 2008

2

[2] DS/EN 13829 (2001), Thermal performance of buildings. Determination of air permeability of buildings. Fan pressurization method [3] Brandt, E. et al., (2009) Moisture in buildings, Danish

Building Research Institute, Hoersholm, Denmark

3