Q: Our home was built with uninsulated cinder block in the 1940s. I would like to insulate the interior walls on the north side, but the rooms are small. What’s our best option for an environmentally benign, cost-effective insulation? Are breathability and condensation issues? What’s the best way to attach the material to the walls? —Ben Mates, Salt Lake City, Utah
A: Though you can insulate your wall from the inside or outside, if space is of utmost concern, your best option is outdoor insulation. According to the Building Science Corporation, one of the nation’s leading building science firms, basement walls with exterior insulation outperform those with interior insulation. No vapor barrier should be used on the interior side of externally insulated walls.I recommend two options: above-grade polyisocyanurate (“polyiso”) or below-grade extruded polystyrene (XPS), commonly called Styrofoam (a trademark of Dow Chemical Company). Polyiso and XPS aren’t environmentally friendly options in terms of material composition and manufacturing, but they do offer an affordable way to keep a home’s temperature comfortable while reducing energy bills. I’m currently unaware of any environmentally benign product available for outdoor insulation. Given that, polyiso and XPS offer the best performance and can help create an energy-efficient home.
Polyiso is a rigid insulation with an extremely high R-value, though it can’t be used if it will come in contact with water. Ideal for use above grade, polyiso can be glued to cinder block, then you can add additional siding to the exterior. Siding options are almost limitless, but “fiber-cement” siding (from James Hardie or CertainTeed) is a good choice. Made primarily of portland cement combined with fillers, fiber-cement siding is easy to install, holds paint well, and is fireproof. To use fiber-cement siding, attach nailing strips to the cinder block so you can more easily nail siding to the house. Keep in mind that you can use a maximum of 1 inch of polyiso with 3.5-inch nails, according to the installation requirements from fiber-cement manufacturers.
Below grade, XPS is a good bet because of its resistance to water damage. It can also be glued onto existing cinder-block walls, but installation isn’t easy. To properly insulate below grade, you’ll need to dig down to the footing, which should be relatively easy because it’s backfill. Once you’ve dug to the footing, glue the XPS to the cinder block. You can install it to whatever thickness you desire—three inches will give you an R value of 15. The next step is to backfill, which will snugly hold the insulation to the cinder block. It’s also a good idea to install flashing—material that prevents water from seeping into the building and causing a leak—on the area that transitions to below grade to ensure that water doesn’t penetrate behind the insulation.
Pete Nichols has an extensive background in green building materials and technologies and is the founder and marketing manager of Sustainable Flooring, a bamboo and cork manufacturing cooperative.