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...
to read the full article.