Simple Rules, Complex Systems
The physics are relatively simple. Here are the two most important rules: 1. Heated air rises. 2. Condensation occurs when warmer moist air meets a cooler condensing surface below its dewpoint. When you’re talking about condensation issues in houses, those are really the only two rules that matter. The complexity comes about when those two rules are applied throughout the entirety of a home’s building envelope. One of our consulting jobs was a house that was suffering from the effects of these two rules.
The house in question was fully renovated just a couple of years ago and had spray foam insulation applied to the underside of the roof deck. What had been the attic is now partly finished space with an open door to a mechanical room where you can see the foam insulation. Since renovation, each winter when temperatures get cold enough, there is a lot of water that comes dripping out from the top of the roof trim outside and stains its way down the trim and clapboards. Is the brand-new roof leaking? Where’s the water coming from?
Because of the basic shape of most houses and the fact that we heat them in the winter, there is usually something called stack effect in, well…in effect. When the air we heat rises, it travels up through the floors and stairways of the house and pushes against the attic. If the attic plane isn’t air-sealed, the air just keeps on going right through the attic and outside. Cold air is drawn in through the bottom of the house to replace it, is subsequently heated and continues to rise, and voila – a convection loop! Not only does this send valuable heat out of the house quickly, it also pushes a lot of water vapor up towards the roof (vapor from our cooking, showers, plants, breathing, etc.). If enough of this water vapor finds a cold enough surface, it will condense into liquid water on that surface.
After diagnosing with a blower door, thermal imaging camera and theatrical fog machine, we were able to figure out the cause of the water: An incomplete spray-foam installation is allowing warm moist air from the house to rise past the insulation in some places, where it will then condense on the roof plywood and roll down under the roofing itself to make its way out onto the trim. Understanding the cause of the problem enables us to properly devise a solution and understanding the simple physics involved helps us find the cause.
Nate Gusakov is a lead auditor for Zone 6 Energy. Zone 6 Energy is a home-grown Vermont company specializing in air leakage diagnostics and consulting. Zone 6 Energy offers commercial and residential blower-door testing, home energy audits, and AeroBarrier installations throughout New England and upstate New York.