Ask the Architect

The properly sealed house, Part III

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Q. We interviewed three contractors for a spring project, and one kept talking about switching to using spray foam insulation. Is it better because it costs a lot more? We’re not sure it matters enough for the cost. Also, he said we need a beefed-up water barrier because we live within a mile of the ocean. Is this true? Our neighbors all used fiberglass.

A. In the past two weeks I divided the answer into four parts of wall systems. It doesn’t matter if you live near water. Wind-driven rain, humidity and moisture is everywhere. Think of it this way: Every day you live and breathe in an ocean many times larger than the one near your home. Our atmosphere is an ocean above our heads, with currents and waves of water slowly rising and falling around us all the time. In a desert, the currents are still present, but they’re carried aloft by high pressure from the heated air closer to the surface. Survivalists can still gather moisture on the bottom side of a plastic sheet in the early-morning humidity change.

The interior of your wall is affected by temperature transfer from within as a weather change, just like warm and cool air masses meeting in the sky, only on a much smaller scale. Closed-cell foam is the best insulator, and worth the cost over the life of a building, because it acts as an air barrier by lightly expanding to fill the wall cavity as a temperature barrier, forming tiny closed air spaces that slow the transmission of heat or cool and as a vapor barrier limiting or stopping the movement of water vapor. The only part of the four “control layers” it is not meant to reduce is liquid, like rain. If you constructed a wall using the closed-cell foam and an exterior synthetic stucco system over thick insulation boards with a corrugated back (air channel), you’d have one of the most complete residential barrier systems available.

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