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top of the opening to observe the ﬂow direction. The neutral level is located

between the ingo ing and outgoing ﬂow directions. Sensitivity can be increased

or decreased by reducing or increasing the width of the opening.

In an airtight building, the neutral height will be located at about mid-

height of the measuring opening (see Figure 4.8, left). Cold air enters the

building through the lower half of the opening, and warm air leaves the

building through the upper part.

Using the Bernoulli equation and mass conservation, it can be shown that

the mass ﬂow rate through the upper or lower half of the opening is:



ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

gH T

ð4:28Þ

where (Van der Maas et al., 1994):

is the discharge coeﬃcient of the opening (for example, 0.6),



is the density of air outdoors (assumed to be the cold zone at temperature

A is the area of the opening of with W and height H,

g is the gravitational acceleration (about 9.81 m/s),

T is the indoor–outdoor temperature diﬀerence.

If some air enters or leaves the enclosure through another opening or through

leaks, part of the incoming air will leave or enter the building through it and will

not, therefore, pass throu gh the upper or lower part of the test opening. The

neutral height rises or goes down to balance the two airﬂows (see Figure 4.8,

right). Using mass conservation, the net mass ﬂow rate is given, assuming

Figure 4.8 Principle of the neutral height method for assessing

leakage area

Note: The left shows no leak (except the test opening), while on the right, the leakage is

above the test opening.

Source: Roulet, 2004.

Airtightness 73

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Sharp JX-9400 Technical Information Seite 94