A Blower Door Can't Tell You How Much Mechanical Ventilation You Need
With the help of people who know a lot more than I do, I'm slowly getting this stuff figured out. I wrote about my problems with using ACHnat a while back, and when I posted that article at Green Building Advisor (GBA) recently, Stuart Staniford wrote one of those comments that left me thinking, Dang, I should have known that.
Staniford, another physicist I've known since 2005 through my peak oil obsession, pointed out that there really is something we need ACHnat for when he wrote, "there's no other way to turn the infiltration rate into btus, right?" D'oh! Of course, and we need to turn infiltration rate into BTUs for energy modeling, something I'm in the middle of teaching to two new crops of home energy raters over the next two weeks (here in Atlanta and then in Toronto).
The issue came back into focus for me a couple of weeks ago when I wrote about the little ventilation dustup between John Krigger and Paul Raymer, two other folks who know more than I. When I posted that article at GBA this week, I added the following bit to it after going back and reading one of my favorite Joe Lstiburek articles.
Blower doors and mechanical ventilation
One problem in the great ventilation debate is that weatherization and home performance crews are trying to using blower doors to determine if they need to add mechanical ventilation. Joe Lstiburek wrote about this issue in his article http://www.buildingscience.com/documents/insights/bsi-053-just-right-and-airtight. After opening with praise for using a blower door to help make homes more airtight and to measure leakage, he then wrote:
But then they think that a blower door actually is a precise measuring tool for how air will leak across the building during service. Wrong. Even more serious an issue is to then take the leap that using a wrong assumption about the results of an approximate measurement can be used to decide that mechanical ventilation is not needed. Bad, very bad, and potentially deadly.
I wrote about the problems with ACHnat at GBA recently, and that’s what Joe’s referring to above. What he says they do at Building Science Corporation is, “To me, the ventilate right part is easy: put in a ventilation system and pick a rate." In footnote 4 of that article, he describes that their method is to install a system that’s capable of providing 1.5 times the ventilation rate recommended by the latest version of ASHRAE 62.2.
This is an important debate because we want homes to have less air leakage. We also want them to have good indoor air quality, and that means mechanical ventilation. Krigger makes some good points about all the resources going into training for ASHRAE 62.2, but the time has come to drop the Building Airflow Standard and stop pretending that blower doors can tell us how much mechanical ventilation a home needs.
Just Right and Airtight by Joseph Lstiburek