Rigid Panels

What they are: Boards made from extruded polystyrene (XPS) or polyisocyanurate foam ("iso board"). These panels can go on the exterior of a house, over the studs (typically a weak spot in an insulation system) but under the siding, a technique that works well in hot, humid climates, as the boards become an effective vapor barrier. In colder climates, the boards can go inside the walls, where you want the moisture barrier to keep warm air from escaping. Before switching to spray foam insulation, Tom used to use polyisocyanurate panels that way—he would build a 2x4 wall, insulate between the studs with batting (paper face removed), then cover the whole thing with foil-faced panels and seal them with foil tape before putting up wall board. The combined R-value of the batts and board on the 2x4 wall was more than batts alone on a 2x6 wall, and he would gain a little extra space in the room.

Performance: Extruded polystyrene provides about R-5 per inch. Foil-faced polyisocyanurate delivers from R-7.2 to R-8 per inch.

Best use: During siding replacement or an interior gut job to add insulation on a 2x4 wall.

Issues: To meet fire rules, you must cover interior boards with drywall at least ½ inch thick. Polystyrene will break down if exposed to sunlight, so it can't be left uncovered too long.

Cost: One 4-by-8-foot sheet of 1-inch-thick expanded polystyrene foam costs about $10. A 2-inch-thick panel of foil-faced polyisocyanurate costs a little under $30.

Tom Silva's Advice on Handling Half-Insulated Walls

"If your house has some insulation in the finished walls but not enough to keep out the cold, you'll need to remove it before you can add more, because the old stuff can get in the way and lessen the effectiveness of new insulation. Figure out where you need to beef up by hiring an energy auditor with an infrared camera, or by peeking in around electrical outlets or behind trim pieces that you carefully remove. If you find some insulation, cut out a 12- to 16-inch horizontal band of drywall or plaster midway up the wall. Then just pull out the old insulation. If you choose loose fill or expanding foam, it can go in through the same hole—though you'll need to make more holes along the top of the wall to complete the job."
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