Moisture in Houses
Paul Kando
After a hefty snow storm 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’s going on?
Reading insulation levels from melting snow
photo credit: mycozyhome.com
After a hefty snow storm 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’s going on?
In a heated house moisture – from basements, kitchens, showers, 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 the heat-energy 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 BTU/lb. Research has shown that through a tiny, 1 mm (< 1/32") 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 mm 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 Passivhaus certified.
We may not be able to upgrade to Passivhaus 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, wrong or partial information. Follow the audit report’s recommendations. Seal all air-leaks, including electrical outlets, 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 your house understands the basic physics and follows your 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 BIN) will add an inexpensive air/moisture barrier to otherwise leak-free walls and ceilings.