Insulating a block wall

Q: Is it okay to insulate a hollow block wall with loose fiber insulation? The exterior walls of our 90-year-old house are made with stucco-covered cinder blocks. Will there be any problems with internal condensation if I have insulation blown into the 4-inch wide cavity inside the blocks?

A: Although you probably won't have trouble with condensation occurring inside the newly insulated cavities of the block wall, there are several other reasons to consider a different approach. And the first is the block cavities themselves. It's not uncommon for hollow block walls to have all kinds of rough bits of hardened mortar protruding inside. And depending on what you've got at your place, this could greatly impair the ability of insulation to fill all the spaces properly. Another problem is water infiltration from the outside. Concrete blocks are quite porous, and water can easily make its way inside, hopelessly soaking your loose-fill insulation. And even if all this weren't enough, there's 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 foam 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.