Insulating and Parging Foundations
Covering concrete walls with rigid foam insulation and troweling on stucco requires experience with the materials.
Synopsis: A builder explains his method of insulating concrete foundations by applying a layer of rigid foam insulation and a coat of stucco over the insulation above ground. He experiments with a few approaches before settling on a single coat of cement stucco over a substrate of expanded metal lath.
If you’ve got the idea that a builder’s skill is an unchanging body of knowledge passed down through the generations, think for a minute about insulating a foundation from the exterior. Even in cold climates, what you used to see between the bottom of the siding and the grade was the bare concrete foundation wall. But these days, with estimates of heat lost in a house through the foundation running as high as 30%, what looks like concrete is more likely parging, or stucco, applied over rigid foam insulation.
Insulating the outside of foundations has been a problem for a lot of builders, including me, because many of the materials and methods are new. Although rigid foam-board insulation doesn’t look like much of a problem, it isn’t as simple as it first appears. Polystyrene is the insulating material most often used. It comes in 2-ft. wide panels and handles like plywood, but it’s a lot lighter. You can cut it with anything from a knife to a table saw. But polystyrene foam is produced in two forms: expanded and extruded. Expanded polystyrene (EPS), also known as headboard, is more susceptible to soaking up moisture than its extruded cousin, say the researchers on one side of this controversy. This could lead to a considerable loss in R-value. Although I used expanded polystyrene on the job shown here, I think the extruded version is probably the better bet despite its higher cost.
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