Q: Is it okay to add wall insulation to a hollow block wall by adding a loose-fill fiber product in the block spaces? The exterior walls of our 90-year-old house are made with stucco-covered cinder blocks. Is it a good plan for me to have wall insulation blown into the 4-inch wide cavity inside the blocks?

A: The short answer is no, it is not a good idea. One reason is that it’s common for hollow block walls to have all kinds of rough bits of hardened mortar protruding inside, and this would probably stop the wall insulation from filling all the cavities properly. A bigger problem is water infiltration from the outside. Concrete blocks are quite porous, even when covered with stucco, and water can easily make its way inside, hopelessly soaking your loose-fill insulation. There’s also the issue of thermal bridging. Those portions of the block wall that extend from the exterior surface all the way through to the inside will conduct huge amounts of heat, regardless of the insulation levels between them. Even if the cavities were completely insulated and dry, you’d probably gain little thermal benefit. A better approach involves applying rigid sheets of insulation on the interior or exterior surfaces of the wall. This creates a continuous barrier of insulation that’s also impervious to water and condensation. It’s more troublesome to install around windows and doors, but very effective.