- Designer: Steve Baczek, Architect
- HERS Rater: Mike Duclos, Energy Efficiency Associates
- General Contractor: Byggmeister Design Build
What is HERS Rating
A home energy rating systems (HERS) rating is an analysis of a home's projected energy efficiency in comparison to a "reference home," based on the 2006 International Energy Conservation Code. A HERS rating involves both an analysis of a home's construction plans, as well as on-site inspections and testing by a certified home energy rater. Tests typically include a blower-door test (to test the leakiness of the house) and a duct test (to test the leakiness of the ducts).
The lower a home's HERS Index, the more energy efficient it is. A home built to current code should score a HERS index of approximately 100, whereas a net zero energy home should have a HERS Index of 0. Each 1-point decrease in the HERS Index corresponds to 1% reduction in energy consumption compared to the HERS reference home. Thus a home with a HERS Index of 85 is 15% more energy efficient than the reference home, and a home with a HERS Index of 80 is 20% more energy efficient.
Toward a Zero Energy Home
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What's Better Interior or Exterior Insulation?
Retrofitting an existing house for substantially better energy performance has its challenges, especially when the building is nearly a century old and home to both its owners and four student tenants. If the house is completely gutted from the inside, the project begins to resemble new construction in both cost and complexity. Staying in the house during construction can get tricky, and period trim and detailing could be difficult or impossibly to preserve.
An Energy Hog Worth Keeping
Cador Pricejones knew all that as he pondered energy upgrades for his 1914 home in Somerville, MA. Pricejones, the production manager at Byggmeister, a Boston-area remodeling and design company, had a better plan: Tackle the job from the outside and leave the house interior unscathed. The house had an unofficial HERS rating of 119, meaning the house used almost 20% more energy than a house built to current energy codes. Most of the windows, siding, trim, and interior were original.
Energy use for the 2,966 sq. ft. of living space in the house was about 88,000 Btu per square foot per year, slightly higher than the average for two- to four-family homes in the Northeast United States. A calculator from the Thousand Home Challenge recommended energy consumption of 18,000 Btu per square foot per year, a reduction of about 80%.
Pricejones has done the easy things:
- installing energy-efficient applications and lighting fixtures
- tracking down phantom electrical loads
- insulating the attic
but the house required more drastic intervention.
Adding Insulation to the Outside
Pricejones decided not to disturb either the siding or the existing windows. Instead, he attached new double-hung windows to the casing of the existing windows and then created new exterior framing by screwing 2x3 studs to the outside of the building through 3-in.-thick blocks of rigid foam insulation.
The studs created a new wall cavity on the exterior of the building, which was filled with 4 in. of closed-cell polyurethane foam. A small air space between the face of the new insulation and face of the studs made a shallow air space behind the siding, allowing the back of the siding and trim to dry should it get wet. When added to the cellulose insulation already in the existing walls, they achieve an R-value of nearly 40.
In the end, the house has two sets of windows, the original single-pane units and new double-pane units installed directly over them. The arrangement isn't as odd as it may sound. Pricejones says it's not unlike having storm windows, and the combination of the existing windows with new double-paned low-e units mean a much tighter building envelope.
Up In the Attic
In the attic, Pricejones sealed all plumbing and wiring penetrations and added more cellulose to bring the total R-value to 60. In the basement, he framed a wall with 2x4 studs on the flat, leaving a gap between the framing and the foundation wall, and sprayed in 4 in. of closed-cell foam against the foundation wall. Studs provided the backing for moisture-resistant drywall. The basement slab remained uninsulated, in part because it would have been a logistical hassle to move everything and also because of the limited value of insulating a below-grade area that is unconditioned.
Upgrading the Mechanical Systems
Pricejones replaced two older boilers and two water heaters in the building with a single high-efficiency condensing gas boiler with an indirect hot-water storage tank. The sealed-combustion boiler does not require a chimney. There is not central air-conditioning system.
Renewable energy systems, which Pricejones is wanting to add, will include three flat-plate solar hot-water collectors designed to produce 100% of the domestic hot water needs during the summer. During the winter, when the amount of sunlight falls dramatically, the system will produce a portion of the hot water.
Also on order was 5.25kW photovoltaic array for the roof. Thanks to low electrical consumption, Pricejones expects that the PV system will have enough capacity to meet all of his family's needs plus some of the tenants'.
The house's HERS is expected to fall from 119 to roughly 52, not including any of the renewable energy contributions. Once the solar hot-water and PV systems are factored in, he hopes to see the HERS rating fall to somewhere near 40 (a net zero house would have a HERS rating of 0).