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Airtightness

Why check airtightness?

Controlled airﬂows, having adequate ﬂow rate and passing at the appropriate

locations are essential for good indoor air quality. Leakage, allowing the

uncontrolled air to follow inappropriate paths, should therefore be reduced

as much as possible. This requires an airtight building envelope and airtight

ductwork: building and ductwork airtightness is a prerequisite for eﬃcient

natural or mechanical ventilation. Envelope leakage is not an appropriate way

for airing buildings.

In buildings with hybrid ventilation, the indoor air quality is controlled

partly by a mechanical ventilation system and partly by a natural ventilation

design. The share between these two systems could be seasonal (natural venti-

lation in mild seasons and mechanical ventilation with heat recovery during hot

or cold seasons) or spatial: small rooms with an external wall with natural venti-

lation, and large rooms or rooms located inside the building with mechanical

ventilation. In any case, both ventilation systems should be controlled.

While Figure 0.5 shows the exﬁltration ratios (i.e. the part of the supplied

air leaving the building by another way than the exhaust duct) for several build-

ings, Figure 4.1 shows the inﬁltration ratios (i.e. the part of outdoor air that is

not supplied by the mechanical ventilation unit) measured in 11 spaces that are

equipped with full mechanical (not hybrid) ventilation. Out of these ten spaces,

seven have an inﬁltration rate signiﬁcantly diﬀerent fr om zero, and in two of

them more than 30 per cent of the outdoor air is not controlled!

Exﬁltration has a negative eﬀect on heat recovery, since the heat in the air

leaving the building through leakage cannot be recovered (see Chapter 5,

‘Eﬀect of leakages and shortcuts on heat recovery’). In cold climates, warm

and humid indoor air going through the external envelope through leaks encoun-

ters increasingly colder surfaces. The water vapour of this air eventually

condenses on the coldest surfaces within the cracks, thus creating dramatic

condensation problems at leakage locations. Inﬁltration has a negative eﬀect on

indoor air quality, since inﬁltrated air is neither ﬁltered, dried, cooled nor heated.

A survey performed by Carrie et al. (1997) in France and Belgium has

shown that, on the average, 40 per cent of the supplied air is lost through duct-

work leakage before reaching the user. This obviously reduces the eﬀective

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Sharp JX-9400 Technical Information Seite 79