Concrete Block Insulation
From: This Old House
How do I insulate basement walls with stud bays only 1½ inches thick?
I have a 1950s brick Colonial with uninsulated concrete-block walls. Inside, the drywall is nailed to studs fastened flat against the block. How do I insulate walls with stud bays only 1½ inches thick?
—David Caruso, Springfield, Pa.
You can’t. And there is no effective way to fill the voids in an existing concrete-block wall. Your only option is to cover the inside of the exterior walls with a continuous layer of insulation, as though you were insulating a basement wall. In fact, this would be a perfect application for InSoFast foam panels (insofast.com), which I installed in the basement of the 2012 TOH TV project in Cambridge, Massachusetts. To see me using them, go to Cambridge 2012, Episode 3: Modernizing a Balloon Frame.
These tongue-and-groove panels are made of expanded polystyrene (EPS)—the same stuff that’s in the cups that keep your coffee hot—and are glued directly to the concrete block with a polyurethane adhesive. Drywall can be attached to them directly without the need for studs.
Your walls, in their present state, have an R-rating of about 3. According to the International Energy Conservation Code (IECC) for your area, mass walls—ones made of thick block, brick, stone, or adobe—should be at least R-10. Adding 2½-inch-thick InSoFast panels will bring your walls to R-13, 30 percent more than the IECC minimum.
Before you can install these panels, you’ll have to do some demolition. Remove all the trim from the exterior walls—window and door casings, baseboards, crown molding, you name it—and rip the drywall and studs off the block walls. Then, to account for the foam’s additional thickness, add jamb extensions to the windows and doors and fur out around their openings.
Now, as you glue the panels to the exposed concrete blocks, leave a ½-inch gap at the floor and ceiling, and cut holes in the panels to fit around outlets and wall switches. After the adhesive cures, add extension boxes to the outlets and switches, and spray foam into every gap—top, bottom, and around each box. Install the drywall over the panels, finish it, then replace the trim.
When you’re done, I bet you’ll see a welcome drop in your heating and cooling bills.