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 author
2 Danish Roofing Advisory Board, Denmark. tbn@tor.info
INTRODUCTION
Air 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.
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 PRACTICE
Vapor 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. Blower door measurement where the original door
is replaced by a canvas door provided with a computer
controlled fan.
Figure 2 (from [3]). Airtight joints can be made by
overlapping combined with either tape (a) or sealing strip
(butyl rubber) (b).
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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 ADVANTAGES
An 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.
DISCUSSION
Experience 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] 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
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