You may have heard that before, but you can never hear it too often. Even here in warm,sunny California, just leaving the windows cracked is not a great strategy for the health of you or for your home. Here is a great post from my friend August Hasz in Colorado on the subject, lifted from an online conversation about straw bale buildings but relevant to all buildings.
First my bias: we engineer mechanical and structural systems for buildings. So it goes without saying see better results from adding a level of control and predictability. With that bias comes a lot of experience looking at under-ventilated homes. The discussion about building and occupant related pollutants is a big factor, but misses another crucial component- condensation due to moisture. The building envelope may be permeable enough to survive, but in cold climates condensation on windows is unavoidable without controlling moisture in the home. In cold climates what we see time and time again for a remotely tight home (homes with 2 or 3 times the air leakage we are talking about here) is significant condensation build-up resulting in damaged materials and mold growth on surfaces. This primarily at windows due to the cold temps, but if there is any other location were the insulation detail isn’t right problems will happen there as well.
We have had a number clients/friends decide to skip the ventilation system over the years, and every time what ends up happening is they are either forced to run bathroom fans continuously in the winter (heat loss) or find a way to retrofit an HRV or ERV in. That applies to standard construction, bales, anything you build that is somewhat well sealed. We all know retrofits are expensive and to be avoided, so why leave it to chance?
This probably is not too much of a surprise in the high-mountains of Crested Butte, Colorado, I assume the effect is worse in Chris’ backyard. I was however surprised to see it is a big problem even in Moab. Moab is where we go to thaw our bones out in the middle of winter, it isn’t cold right? We have been involved in the Community Rebuilds project happening there (If you aren’t familiar, make sure and check out what Emily Niehaus has started over there, we are now starting one in our valley as well) and 3 homes had significant condensation on windows this winter resulting in a mold problem and damaged finishes. The result is that all future CR homes will have HRV’s.
From my perspective there is no debate: build it tight, and ventilate right. Relaxing the tightness of the home just leaves it up to chance- did I make it leaky enough for the building envelope to dry? enough to get the moisture out of the air? enough to get the result of cooking (gas combustion and lots of other pollutants)? It isn’t’ worth the risk to building occupants or to the building. Do we want to have a realization 10 years from now about health problems from a homes that seemed leaky enough, but oops not quite… Daily life makes pollutants, even if we are all watching everything we do (breathing, showering, cooking). So now I have a choice of how to ventilate: crack a window? exhaust fan? or HRV/ERV? Only the heat/energy recovery ventilator allows for exchanging air efficiently. The others are fighting against all the great work you put into the building envelope.