An Energy Retrofit in Stages

After dismantling the old gable-end chimney above the roofline, the crew enclosed the lower part in a framed, insulated bump-out. The vertical strapping over the bump-out — not yet complete on the rest of the wall — is a rain-screen nailing base for horizontal fiber-cement siding.

While located in a pleasant residential neighborhood, the original "farmer’s widow house" was drafty and poorly insulated.

A 1970s addition off the back added much-needed square footage but suffered from the same poor energy performance. In addition to leaving the siding exposed to rain and weather, the absence of eaves overhangs in the front created a visually awkward transition to the broad eaves in the rear.

The original 2x4 rafters left little space for insulation at the eaves.

The area was filled with medium-density spray foam, which blocked air leaks.

The foam insulation provided a continuous dam for the cellulose that would be blown into the attic. Where excess foam bulged beyond the eaves extensions, the crew cut it away with a sharp handsaw.

At the interface between the addition and the original attic, warm, moist air flowing through the batts in the cathedral ceiling had been condensing on the sheathing, rusting the nails and staining the plywood.

Along the addition's ridge, air leaking from inside had left a dirty deposit in the batt insulation.

Foil-faced polyiso board, screwed to the addition roof deck, supplements the original batts, which were left in place between the rafters. Joints between panels — and the intersection with the board deck of the original house, visible at rear — were sealed with spray foam.

The gable-end rake of the original house was extended to meet the addition overhang, providing improved weather protection and a cleaner appearance.

With siding and sheathing removed, potential leaks in the plates were easily sealed with foam. Also, as a preventive measure, the exterior bathroom wall was foamed.

While nailing up the new sheathing, the carpenters took the time to bore holes over each framing cavity — much to the appreciation of the insulation sub who would later fill them with densepack cellulose.

The perimeter of the slab-on-grade addition was insulated with two layers of 1 1/2-inch extruded polystyrene.

The extruded polystyrene matched the thickness of the polyiso on the walls.

Rather than go to the expense of moving the electric meter away from the wall to accommodate the full thickness of the polyiso board, the crew boxed in the area around the meter and filled it with a thinner layer of XPS.

Once stripped of carpeting and tested for moisture, the slab floor of the addition was insulated with XPS foam board fit tightly between lengths of 2-by furring.

Including the insulation, the finished floor, consisting of 1/2-inch plywood and 5/8-inch prefinished oak, raised the room’s threshold by almost 3 inches. A gently sloped transition was added later.

The ERV that supplies the tight retrofitted house with fresh air is installed at head height in an often-used area, making filter changes easy.

The ceiling opening where the flex duct from the new bathroom fan transitions to rigid duct in the attic was enclosed in a plywood box and sprayed with foam for air-sealing and protection from foot traffic.

The thin, uneven layer of rock wool covering the floor was later covered with 18 inches of loose-fill cellulose. An inexpensive combination thermometer- hygrometer makes it easy to keep an eye on temperature and humidity in several locations, indoor and out.

While natural-gas consumption was dramatically reduced as a result of tightening the house, electrical consumption did not follow the same curve. This was at least partly due to unforeseen moisture buildup in the home, which necessitated the use of a basement dehumidifier — thus the spike in electrical use in July 2010. Basement moisture will be dealt with in the second phase of the project.

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