Top 10 Building Science Secrets

The primary purpose of housewrap is water protection, as shown here where it serves as a drainage plane to keep wind-driven rain and snow out of the walls. Housewrap doesn’t make a good air barrier. While the seams can be taped to improve its air resistance, it should never be taped at the bottom, otherwise it can trap water. You’re better off air sealing with a combination of peel-and-stick tapes, sealants, and gaskets.

Power ventilators, even solar-powered ones, can cool the attic only if the fan pulls outside air into the attic. This air cannot come from the house, or you’re just sucking away conditioned air. This means the ceiling plane must be airtight. But if you have an airtight ceiling, you don’t need an attic fan. Bottom line: Power ventilators are a waste of money and a liability.

An attic pull-down stair, all 10 square feet worth, doesn’t seem like much area. But in a 1,000-square-foot attic insulated to R-38, that 1% increase in area can result in a 27% decrease in the ceiling insulation R-value.

Unprotected knee-wall insulation is prone to falling out and allows air to flow through it, as shown here.

To solve the problem of unprotected knee-wall insulation falling out and allowing airflow, the exterior side of the knee wall should be sheathed, as shown here. Rigid insulation works well as a sheathing to cover the knee-wall insulation because it adds R-value to the knee-wall assembly. Be sure the floor below the knee wall is blocked off, as well. Otherwise airflow will short-circuit through the floor framing.

The duct shown in the photo at left fails on two fronts: 1) It runs through the attic, which may be the most stupid place you can put ducts in an air-conditioned home. 2) The duct run is pinched where it passes through the truss webs. This constricts airflow, increasing duct pressures so the conditioned air carried in the duct flows out of the leaks faster.

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