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What Kind of Exterior Insulating Sheathing Can I Use on an Existing Wall for Climate Control?

Many older houses are not well-insulated. It was not until the 1940s and 1950s that insulation became a major consideration in house construction. Adding insulation to an existing attic is usually fairly simple, but adding insulation to existing walls can be a major task. You can spray expandable foam into cavities, but that requires cutting holes between every stud set. One alternative is rigid foam board sheathing.

  1. Foam Sheathing Types

    • Three types of rigid foam board insulation are available: expanded polystyrene, extruded polystyrene and polyiso or polyisocyanurate. Each is similar in appearance but differ in heat resistance or R-value. Polyiso has the highest R-value, expanded polystyrene the least. Extruded polystyrene is best suited for use on walls. All three types generally come in thicknesses from 1 to 4 inches.

    Installation Options

    • One way to add foamboard to an exterior wall is to remove the siding or cladding, install the foamboard and reinstall the siding. This can be labor-intensive and costly. It's best done if the siding is in poor condition and going to be replaced anyway. A second option is to install foam board over the existing siding and add new siding on top. This works best if the siding is smooth, not overlapped clapboard, so the foam board goes on a smooth surface.

    Installing Foam Board

    • While rigid foam board sheathing can be installed with adhesive or galvanized nails, nails work best. Foam board is placed about an inch above the bottom of the existing wall covering and nailed in place to studs. Wide-headed galvanized nails need to be long enough to penetrate the foamboard, any wood siding and sheathing into the studs. Seams need to be sealed with tape from the foam board manufacturer.

    Adjusting Doors and Windows

    • Adding foam board sheathing to an existing wall may require adjusting door and window frames. Exterior foam board on walls is usually 1 1/2 inches thick. Even installed over basic wood sheathing, it may require adding to the window and door framing. It certainly will be needed if foam board is added over existing siding because the wall will be thicker.

    Moisture Barrier

    • Foam board requires a vapor or moisture barrier between the sheathing and the warm side of the wall. In new houses, this is often provided with a house wrap. On an existing wall, it may be necessary to install a barrier or use foam board that has a moisture facing. Make sure any existing insulation on the exterior of the wall does not have a barrier; two barriers in the same area create condensation problems.

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