Christine and I recently attended two educational events, and have some tidbits of wisdom to share.
The first was a general class in reducing moisture problems in new homes, taught at the PG&E Energy Center
by Steve Easley , a nationally renown building scientist. The Energy Center is a great resource, open to both laypeople and professionals. All the classes, which cover a range of energy and building science topics, are free, and the morning coffee and danish are great!
Highlights of Steve's class:
Keep it simple, stupid: Many roof leaks are exacerbated, if not caused, by overly complex roof design, with multiple intersecting hips, gables, ridges, and horizontal valleys. He showed several examples of designs that would almost have to leak.
Don’t tuck your raincoat into your underwear: Many problems arise when layers of waterproofing materials are incorrectly lapped – like shingles, the lower rows of materials have to go on first, with the upper rows lapped on top. Doing the reverse lets water in…like tucking your raincoat into your underwear. Sounds obvious, but he showed numerous examples of builders doing it wrong.
Stucco is a reservoir: it doesn’t look like Lake Mead, but stucco (as well as concrete, wood and many types of insulation) act as water reservoirs. They absorb moisture, whether from vapor in passing steamy air or from liquid rain water, and they need to be able to dry out to the exterior (not the inside) of structures. If your exterior finish material acts as a reservoir, it is best to keep it separated from the interior layers of the building envelope with an air gap. Materials can take a long time to dry out, particularly in dark, cool areas.
Know your materials: There is a big difference between various house wraps and building papers. Steve explained one egregious example: there is a perforated polyethlene sheeting product, essentially plastic wrap with small punched holes meant to let vapor escape. Instead, he described how moisture from wet exterior stucco would be sucked through the holes into the dry plywood sheathing inside the wall; the plastic (otherwise used as a vapor barrier) would then act to keep the water from evaporating. Products like Tyvek and good old traditional building paper do the opposite. Their holes are microscopic - so small they don't allow large liquid water molecules to pass but do let smaller vapor molecules pass.
Don’t build your house in a hole: Also sounds obvious, but again Steve showed several examples of grades and patios that sloped towards foundations instead of away from them.
Reduce air leaks: Between leaky ducts, gaps in finishes, ungasketed electrical boxes, and sagging insulation, houses are letting a lot of air movement happen that shouldn’t. When the air is moist and the building materials are cold, the moisture will condense out and leave your walls and ceilings dripping. Steve seems to be a big fan of spray foam insulation, for its ability to thoroughly seal structures while providing excellent insulation and even structural rigidity.
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