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Q.I’m rehabbing an old brick house in Charleston, W.V. I want to insulate the exterior walls, which are balloon-framed with 2x4s, unsheathed, and finished with brick. There’s a 1- to 2-inch air space between the 1 1/2-story-high frame wall and the brick siding, and a weep system at the base of the brick. I see evidence of some water leakage over the years but no rot. Would it be okay to blow cellulose into the wall cavity?

A.Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: According to the National Oceanic and Atmospheric Administration (NOAA), your house is in a climate that averages 4,700 heating degree days, 1,000 cooling degree days, 44 inches of rainfall, and 34 inches of snow annually. And according to the 2009 International Energy Conservation Code (IECC), you’re in climate zone 4. Therefore, your goal should be to insulate the walls with at least R-13 wall insulation. It’s reasonable to assume that water penetrates the brick weather barrier at least occasionally, but because the wall hasn’t been insulated in the past, the wall cavity has been able to dry to the indoors with little permanent damage. However, the new insulated wall system should resist water intrusion altogether, so filling the cavity with blown cellulose alone wouldn’t be adequate.

The existing balloon framing will make it difficult to install an exterior housewrap to protect the cavity wall. Instead, strip the interior wall surface covering so that you can spray closed-cell urethane foam directly onto the back of the brick. To keep the bricks from absorbing rainwater, first install polystyrene cathedral roof vent chutes between the studs against the brick wall, then spray a 1- or 2-inch layer of foam over the chutes against the brick wall. This will create an airtight layer around the exterior of the house while allowing water to drain down to the weep system at the base of the wall. Place the chutes from the top to the bottom of every bay. You can hold them in place temporarily with caulk or canned spray foam until the insulation contractor arrives.

Once this system is in place, you can fill the rest of the wall cavity with R-13 fiberglass batts (to meet minimum code), cellulose insulation (for a wall with an R-value of about 20), or foam (for an R-30+ wall). You probably won’t need a warm-side vapor barrier if you fill the cavity with foam or dense-pack cellulose, but be sure to follow local codes and the advice of your building inspector.