After a hefty snowstorm, houses reveal their flaws: snowy roofs that show the framing underneath like an X-ray; huge, growing icicles along the eaves, even though it’s too cold for snow to melt; and indoors there may be water-stained or drippy ceilings. What is going on?
In a heated house, moisture — from basements, kitchens, showers, and potted plants — warms and evaporates. The vapor is absorbed by the warming air, which holds more and more as it warms. This air-held humidity contains all of the heat-energy that the water absorbed by evaporating. Warming air expands, becomes lighter, and rises, to be replaced by cooler air from below. If the rising warm moist air can leave the heated space through electrical outlets, switches, cracks, and holes, it will end up in the attic or the wall cavity.
Fiberglass insulation does not stop air flow, so the warm, moist air can reach a cold surface – the outer sheathing of the wall, the underside of the roof — where it cools, and the vapor it can no longer hold condenses. The condensate can cause damage. It also releases the heat it absorbed when it evaporated. The heat of evaporation of water is 970 BTUs per pound. Research has shown that through a tiny, one-millimeter hole, 360 grams of air-held water passes per day. That’s a lot of water and some 746 BTUs of heat. One leaky duplex outlet comprises a 20-millimeter hole into the wall cavity, and a house may have as many as 40 such outlets. Do the math: that’s a lot of heat to lose daily — and plenty to melt snow on the roof.
As we tighten our houses by air-sealing, we often overlook the need to deal with the accumulating moisture that will now be trapped indoors, causing mold, mildew, and rot damage to the structure. The need to find answers spawned “building science,” a whole new engineering discipline to design and build houses for maximum energy performance. The best of these are passive-house-certified.
We may not be able to upgrade to passive-house standards, but we can apply the key principles involved: improved insulation, air-sealing on the warm side of the insulation, and heat-recovery ventilation, to both conserve heat and remove excess moisture. Heat, air, and moisture interact as a system so we must deal with all three.
Start with a thorough, independent energy audit. Avoid acting on guesswork, anecdote, custom, or wrong or partial information. Follow the audit report’s recommendations. Seal all air leaks, including electrical outlets and switches; around pipes, wires, and chimneys; and access hatches that lead to the attic or behind knee walls. Only an energy audit can identify all leaks. Don’t skimp on installing heat-recovery ventilation. Make sure that anyone working on the house understands the basic physics and follows instructions. It is better not to have an air/vapor barrier than to have one on the cold side of an assembly. A base coat of shellac-based primer such as Zinsser’s B-I-N will add an inexpensive air-moisture barrier to otherwise leak-free walls and ceilings.
(Paul Kando is a co-founder of the Midcoast Green Collaborative, which works to promote environmental protection and economic development via energy conservation. For more information, go to midcoastgreencollaborative.org.)