Air infiltrating or ex-filtrating causes significant problems.  Air leaking into a building can carry moisture that can later condense in wall or roof cavities.  The second problem is heat loss. Richard T. Bynum writes in his  Insulation  Handbook, “Infiltration can comprise almost 50% of all heat loss during the winter months.”  Common sources of heat loss from air infiltration:

Capabilities

A blower door test is used to quantify building air leakage.   In general, the more data that can be gathered specific to the building (i.e. that details its unique characteristics), the more precisely the data from the blower door test can be interpreted.  The point here is that accurate execution of the blower door test is the first step; the second is to gather supporting data specific to the building itself.
The standard blower door test is a depressurization test.  This means that air will be blown out from the building, creating a negative pressure in the building.  The forces created by the blower door fan for this test (up to at least 50 Pascals) are quite powerful and will tear material from walls that is not securely fastened.  If the building is not finished (i.e. test is performed during construction), then the walls, floors, and ceiling should be of a material that can withstand the negative pressure.  Sheet rock and rigid foam will hold, Tyvek, poly, and aluminum vapor and air barrier materials probably will not.

Air sealing and insulation make a difference

One of our clients had blower door tests conducted on his 2 1/2 story house both before and after air sealing and insulating his home with urethane foam.

Before air sealing and insulation: April 2002

15.45 air changes per hour
.99 estimated natural air changes per hour

After air sealing and insulation: July 2002

3.66 air changes per hour
.30 estimated natural air changes per hour

            The owner has since reported a significant reduction in heating expenses. He went from a $200.00 a month heating and air conditioning bill down to less than $60.00.

Results from blower door test after air sealing

Effective air leakage: The estimated size of the hole (in square inches) if all of the individual leaks were combined into one hole.

Leak Ratio: This ratio is derived by dividing the equivalent leakage area by the total surface area of the building. This ratio is a good way to compare the air tightness of buildings that have different volume and surface areas.