Ask the Architect

The properly sealed house, Part II

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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. 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. Last week I covered the exterior “liquid control barrier,” the most exterior of the four layers of control of moisture and temperature. Going from outside in, the next is the air barrier. Most insulation, except foam, isn’t a very good air barrier, because it does little to stop air flow, mainly because air seeps around fiberglass batting. If you see fiberglass insulation in the wall or ceiling cavity that has been stapled to the inside of the wall studs, moving air will get through. On a really cold day, you’ll wonder where that chill is coming from, turn up the heat, waste energy, and still feel uncomfortable. Instead, the paper or foil face should extend side to side and cross over the wall stud face to form a complete barrier, face stapled. As I’ve often written, don’t let insulators try to convince you that side attachment is best for the sheetrock installer because the staples take a few extra minutes to tap in with a hammer and your home will then have a complete air barrier.

In addition, the exterior joints of the plywood need joint taping with a self-adhering waterproofing tape made just for this purpose. This tape is also used to seal the exterior frame of the windows and doors. So often I stand by windows in new construction before the insulation is finished and can see outside through gaps because there is no water and air sealing tape. I once read to a contractor his truck license plate number with my hands cupped like blinders by peering through a window frame gap because he didn’t believe me. He had to look for himself before agreeing it needed to be done. Problem was, he never used the sealing tape, and six months later was served papers when he wouldn’t fix the damage this caused. The windows and doors leaked, and ruined everything around them. He guaranteed he had done it, but closed it in before anyone could inspect.

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