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Q.I'm renovating an uninsulated cape built with concrete-block walls. The plans call for gutting and reframing the interior and installing shingle siding on the exterior. Because the building needs to be insulated to comply with local (Long Island, N.Y.) code, I was considering adding a layer of foam insulation on the outside, then covering that with plywood as a nailing base for the shingles. Is there a better way?

A.Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: In a warm climate, it would make sense to use impermeable rigid foam to insulate the exterior of the building, because it would place the vapor barrier in the right position and make the mass of the block wall available to help modulate internal heat gain.

But in a cold climate such as yours, the most economical approach would be to insulate on the inside, particularly since you're reframing anyway. It would be a good idea to apply masonry dampproofing to the inside surface of the block. Then space a 2x3 wall away from the inside of the masonry wall by 3 inches (more or less depending on your insulation and thermal goals; R-19 would be a good target in your climate).

The best option would be to install blown-in-place fiberglass or cellulose insulation so that the fiber completely fills the void between the block and frame walls as well as the stud bays. This application will reduce thermal bridging through the studs and increase the overall performance of the walls.

But the job could certainly be done with fiberglass batts, too, with the first layer oriented horizontally between the framing and the block wall, and the second layer conventionally installed in the stud bays.

Check the insulation specs to ensure that the density and depth of the insulation you choose achieve your target R-value, and be sure to install the vapor barrier on the warm side as usual.

On the exterior, an efficient low-cost option — which would eliminate the expense of layers of foam and sheathing — would be to install horizontal strapping on the outside surface of the wall in such a way that the on-center spacing matches the required nailing pattern of the shingles. You will have to calculate the exposure for each course of shingles and then install the strapping accordingly; window and door openings will have to be strapped out around their perimeters to receive trim.

Next, cover the walls with felt paper or housewrap, positioning the wrap on top of the strapping and extending coverage into the door and window openings. (Windows and doors will need to be either replaced or reinstalled to match the newly established exterior plane of the wall.)

As with any type of wall, good water management is essential, so pay attention to the details as you install the felt paper (or housewrap) and flash door and window openings. (For more on water-managed wall assemblies, see "Detailing Rain-Screen Siding," 3/06, and "Water-Managed Wall Systems," 3/03.)

Installing the strapping horizontally rather than vertically shouldn't cause problems. If the wrap is installed and flashed properly over the strapping, very little liquid water should get behind the wrap. Any water exposure that does occur would be occasional and the wall design as described should easily dry toward the outside.