- 1). Measure the sill space. The sill is boards, placed on edge, that run the perimeter of the house. The plate rests on top of the foundation. The sill sits on top of the plate, with the floor joists resting on the plate and butting up to the sill. The view of the sill, from in the basement or crawl space of the house, will be between the floor joists. Measure the distance between the joists, usually 16 inches on center, and the depth of the space above the plate.
- 2). Caulk any gaps between the plate and foundation and the plate and the sill. This step will result in energy savings even is the sill space isn't going to be insulated.
- 3). Insulate the sill space with fiberglass insulation. Cut pieces from rolled or batt fiberglass insulation and stuff it into the space between the floor joists; this is the easy part. To complete the process, the area should be closed off with a plastic vapor barrier. Cutting pieces of roll plastic and stapling them into the gap between the joists is difficult, time consuming and, in all likelihood, not going to result in an airtight seal.
- 4). Insulating the sill space with a spray in foam is another option. Polyurethane foam forms when the two components of the foam, transported and applied from separate containers, are mixed as they are sprayed onto the sill between the joists. This forms a closed-cell foam with a high insulation value. It also provides an air seal--the foam works its way into all the crevices when it is sprayed onto the sill.
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