Permeance

Permeance

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Building envelope technology has improved considerably in the last decades. During the 1950s, air changes per hour (ACH) measured about 10 ACH, while the 1990s saw values less than 5 ACH. So it still remains the area where the greatest improvements can be achieved in

The quality of the building envelope's 'airtightness' has immediate and long-term effects on the system's performance.

Knowing that for every part of cold air coming into a building, there is an equivalent amount of warm air going out, any air leakage reduction will directly impact energy consumption. Of course, in summer, the same holds true but in reverse, it is the warm air that has to be kept out.

The other very important consequence of air leakage relates to movement of humidity through the building envelope. As the air progresses towards the exterior, it will be conditioned according to the gradient temperature across the wall assembly.

As cold air cannot carry as much humidity as warm air, there is real possibility of condensation in the envelope along with a very significant risk of mold growth.

In certain conditions, for example during the winter when the indoor temperature may be 70°F and 95°F outdoors, the water vapor will reach dew point within the cavity unless there is significant insulation on the outer portion of the wall. However, if spray foam insulation is installed, the is no air in the cavity, and no condensation can occur regardless of temperature differential.

regards to energy consumption. Only with spray foam Insulation will this value drop below 1 ACH (as low as 0.05).

Insulation Science

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