Saturday, December 4, 2010

How Important Is a Window's Air Infiltration?

Did you know that regardless of how well the components of a window functions in providing insulation, its air leakage or air infiltration will negate these insulating features?

I recently had a discussion with an architect about how most window companies fail to disclose the air infiltration ratings of their windows.  The reason for their failure to disclose is obviously meant to hide an issue that would render their insulation rating worthless. 

The best example of this would be the following calculation:

Air Infiltration Calculations:  This is the theoretical amount of air being exchanged based on the test data for the respective windows.  The calculation is based on 5 clad wood casement windows at 20 square feet each and an average wind velocity of 25 miles per hour.  The amount of air leakage is air that will actually have to be heated or cooled to maintain the inside temperature in the building.  (Note that the numbers below are for 100 square feet of windows only.)

Clad Casement Window vs. the H Window Awning - (Air infiltration --lower number = better performance):   
High performance clad casement window:   .20 cfm per sq.ft. = 1,200 cubic feet/hr or 28,800 cubic feet/day.
H Window Awning :   .01 cubic feet per minute per sq.ft. =  60 cubic feet/hr or 1,440 cubic feet/day

Based on this testing, here is an easy example of how this works.    If you have a small home or large room sized at 20’x40’x9’with 7200 cubic feet of space, with 5 average-sized (4’x5’), high-quality clad wood casement windows, facing a sustained wind of 25 MPH, you would have to heat or cool the air in the home or room 4 times each 24 hours.
Under these same circumstances, the H Window awning would exchange the heated or cooled air every other day.

The example above is based on a very high-quality clad wood casement window.  Other operating windows, such as sliding or double hung, would most likely leak even more than a casement window.


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