Measured Energy Savings From Foundation Insulation in Minneapolis Single Family Homes

The Operation Insulation (0I) program has insulated about 4000 homes and small apartment buildings in Minneapolis since 1983. The program does not usually recommend foundation insulation to homeowners unless they have already insulated their attic and walls or show a special interest in it, since its cost effectiveness has not been documented. About 115 homes have had this work done, 32 of which had foundation insulation as the only major retrofit through OI. The energy use of these homes was analyzed using the Princeton Scorekeeping Model (PRISM). Of the original sample of 32, 17 cases were dropped due to factors such as poor gas data, the existence of other confounding retrofits or a change in the volume of heated space.

The remaining 15 cases averaged gas savings of 12.5% ± 5.0% or 155 ± 80 therms; the median savings were 11.2% or 125 therms. The average retrofit cost was $1533, and the median payback was 19 years. Analysis was further broken down by interior versus exterior insulation application. Interior retrofits included framing, insulation and taped and mudded gypsum board, and exterior jobs included digging a trench deep enough to accommodate a 4' sheet of extruded polystyrene fitted to the top of the rim joist and covered with a protective finish. Two jobs used a combination of methods. Savings for interior insulation (n=8) were 15.0% ± 5.1%; savings for exterior jobs (n=5) were 10.1% ± 2.9% (a non-significant difference). Interior jobs were more expensive than exterior ($1820 vs. $1170), but an important additional benefit of interior insulation is a finished basement as a final product. With new construction costing as much as $70 per square foot (compared to $2.40 per square foot of basement area for interior jobs), this retrofit may be cost-effective when factors other than energy conservation are considered.

As an outgrowth of this research, the Energy Office is participating in a more intensively monitored and controlled study of foundation insulation conducted by Robinson Technical Services for Oak Ridge National Laboratory. This research is discussed in a poster paper presented at this conference. 

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Measured Energy Savings From Foundation Insulation in Minneapolis Single Family Homes