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Virtual air change rate

By dividing the airﬂow rate at conventional pressure by the internal volume of

the tested enclosure gives a virtual leakage air change rate at that pressure. For

this ﬁgure, 50 Pa is the most used pressure diﬀerence, and the ﬁgure is then

noted n

50;

in [h

1

]. This value is less than 1 h

1

in airtight buildings but,

depending on the climate and building habits, buildings may have ﬁgures

larger than 10 h

1

. This ﬁgure does not indeed characterize the quality of the

envelope, since it depends on the volume of the enclosure. It provides an

indication of the importance of inﬁl tration in relation to building ventilation.

Speciﬁc leakage rate

The airﬂow rate at conventional pressure divided by the area of the envelope of

the tested enclosure provides a ﬁgure characterizing this envelope. For such

application, the most common pressure diﬀerential is 4 Pa, and this parameter

is then v

, or speciﬁc leakage rate at 4 Pa. It is expressed in m

/(h m

). It is also

the average air velocity through the envelope. This ﬁgure is less than 1, even

0.5 m

/(h m

) for airtight envelopes.

Equivalent leakage area

An equivalent leakage area, i.e. the area of a circular hole with sharp edges that

would have the same airﬂow rate at a given pressure diﬀerential, is:

¼ C

ﬃﬃﬃ



p

ðn 1=2Þ

ð4:19Þ

The uncertainty of the leakage area resulting from uncertainties on coeﬃcients

C and n is:

A

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ



C



þðn ln pÞ



ð4:20Þ

Speciﬁc equivalent leakage area

The equivalen t leakage area can be divided by the area of the envelope of the

tested enclosure to provide a speciﬁc leakage area. At 4 Pa, this ratio, expressed

in cm

, is close to the speciﬁc leakage rate expressed in m

/(h m

Airtightness of buildings

The main reason for conducting building airtightness measurements is to

characterize the leak age of the building envelope in the absence of climatic or

other variable parameters inﬂuencing the results. Therefore the building (or

part of the building or a particular component) is pressurized or depressurized

Airtightness 67

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