Deep Energy Retrofit

Cape Cod Insulation – A few strategies for carrying out deep-energy retrofits of existing homes in a northern climate


Insulate foundation walls on the interior

Spray polyurethane foam (SPF) can be used against existing foundation walls—even rough walls such as stone. A good approach is often to add a couple inches of SPF against the wall, then build interior frame walls with 2x6s that are insulated with cavity-fill cellulose or fiberglass. You have to deal with any moisture problems first, since moisture entry and mold can be very significant problems.

Add rigid foam insulation to house walls

cape cod insulation, sustainable building cape codGetting anywhere close to the R-40 goal for wall insulation isn’t possible by simply insulating existing wall cavities. (Insulating 2×6 walls with dense-pack cellulose or fiberglass achieves less than R-20.)

The best approach for dramatically boosting wall R-values is usually to add a layer of foam insulation. Whether to add this on the interior or exterior depends on what shape the exterior siding and interior walls are in and how much space you have to work with.

In a big house where the exterior siding is in good shape, insulating on the interior with 4″ of rigid foam, then adding new drywall and trim may make sense. But in most existing homes, adding a thick layer of foam on the exterior usually makes more sense. Adding four inches is a reasonable plan.

Extend roof overhangs and window openings

Assuming rigid foam is added to the outside of walls, the roof overhang may need to be extended to protect the walls and windows. This is a big job that can easily cost $10,000. Window and door openings also will have to be extended, with proper flashing, air sealing, and trim.

Replace or upgrade windows

cape cod insulation, sustainable building cape codWindows are a key component of deep-energy retrofits. Existing single-glazed or insulated-glass windows will probably need to be replaced with state-of-the-art triple-glazed windows with two low-emissivity (low-e) coatings and low-conductivity gas fill, such as krypton. If the existing windows are in good shape, adding double-glazed, low-e storm windows might be an option, though such windows will likely have to be custom-made, since no insulated-glass storm windows are currently on the market.

Add more attic insulation

If you have an unheated attic, more insulation (cellulose or fiberglass batts) can usually be added on top of what’s already there. If you have a cathedral ceiling and insulation in the roof, boosting the R-value can most easily be achieved by adding a layer of rigid foam insulation on top of the roof when re-roofing is done. In this case, detailing at the eaves and gable-end of the roof has to be carefully planned to keep the roof from looking clunky.

Cost of deep-energy retrofits, of course, is a huge challenge. For an average-sized house, the cost of this scale of retrofit could easily cost $50,000 to $75,000. I’m hoping that new loan funds will become available that will make this approach more feasible.


cape cod insulation

cape cod insulation

cape cod insulation







  • Weatherization

  • Green building

  • Sustainable building

  • Insulation

  • Energy Star Building

  • Blower Door Testing

  • Energy Audit

  • Duct Testing

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