Q. When you're doing a blower-door test, does it matter whether you're pushing air in or pulling it out? In other words, is the air infiltration rate the same at –50 pascals as it is at +50 pascals? And if so, when would you use one approach and when the other?

A. Mike Rogers of Green Homes America, a home-performance contracting company with locations across the country, responds: The short answer is yes, it does matter. You will often get different cfm50 results depending on whether you are depressurizing (pulling air out) or pressurizing (pushing air in). Because depressurization is the standard approach in the U.S. and Canada, you will generally want to depressurize so that you can better compare apples to apples, either across buildings or as "before" and "after" measures of one particular building. (Europeans prefer to take both pressurized and depressurized readings, and average the two.)

Why is depressurization used? The most cited reason is that it more accurately mimics natural leakage where mechanical ventilation backdraft dampers are present. Depressurizing tends to close the dampers, whereas pressurizing tends to open them. In cold climates, some people have also expressed concern about blasting concentrated cold air into one area (at the blower door) when pressurizing, potentially killing indoor plants. When a structure is depressurized, cold air leaks in from many diffuse points.

In terms of finding leaks, sucking and blowing have different advantages. Depressurizing makes it easier to feel the presence of leaks with your hands. Pressurizing makes it easier to find leaks with some sort of artificial smoke.

The one time pressurizing definitely trumps depressurizing is when there is concern about drawing pollutants into a home - fiberglass fibers from the attic, rat poop, mold, or mildew from a crawlspace, or ashes from the fireplace. Incidentally, as barroom tales illustrate, it is absolutely not a good idea to use a blower door in either direction with a fireplace or woodstove burning!