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Air Permeance
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Even though the performance of air permeance of the building envelope has improved considerably in the last decades, (from 9 changes of air -CAH- in the 1950s to less than 5 in the 90s), it remains the area where the greatest improvements can be achieved in regards to energy consumption.
The quality of the shell '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 would directly impact the energy requirements to maintain a comfortable inside environment. 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.
In certain conditions, for example during the winter when the indoor temperature may be 20°C and -15°C outdoors, the water vapor will reach dew point within the cavity unless there is approximately R-15 insulation on the outer portion of the wall.
Ice will form and accumulate in the cavity as long as those temperature conditions persist. The degree of water vapour permeability of the intermediate sheathing (which directly impacts the drying capacity of the cavity) proves its usefulness when the exterior temperature rises.
National Building Code A-9.25.3.1.(1)
Air Barrier System for Control of Condensation
The majority of moisture problems resulting from condensation of water vapor in walls are caused by the leakage of moist interior heated air into these spaces rather than by diffusion?
"...air leakage must be controlled to a level where an occurrence of condensation will be sufficiently rare, or the quantities accumulated sufficiently small, and drying sufficiently rapid, to avoid material deterioration and the growth of mould and fungi."
The 1995 National Building Code introduces a measure of performance for products that will provide resistance to air leakage;
NBC 95 5.4.1.2
Air Barrier System Properties
"1) ...sheet and panel type materials intended to provide the principal resistance to air leakage shall have an air leakage characteristic not greater than 0.02 L / (s. m²) measured at 75 Pa."
Although the NBC states that this material can be installed anywhere in the building assembly, it also advises to be careful in selecting the location when the air leakage material also has low vapor permeance properties.
National Building Code A-9.25.3.1.(1)
Air Barrier System for Control of Condensation
"...Generally the location in a building assembly of the airtight element of the air barrier system is not critical; it can restrict air leakage whether it is located near the outer surface of the assembly, near the inner surface or at some intermediate location."
However, if a material chosen to act as an airtight element in the air barrier system also has the characteristics of a vapor barrier (i.e. low permeability to water vapor), its location must be chosen more carefully in order to avoid moisture problems.
A-9.25.4.2.(2)
Increased Vapor Diffusion Resistance
Air Barrier Systems
"...Any moisture from the interior air...carried by air leakage...is likely to be trapped at such an air barrier. This will not cause a problem if the air/vapor barrier is located where the temperature is above the dew point of the indoor air...otherwise; the trapped water vapor will condense or freeze. ...Moisture that remains in a building assembly into warmer weather can allow the growth of decay organisms."
The details of Air Permeance Systems are if PDF format. To view the detailed systems click on the links below.
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