HERS=2 and ACH50=0.75 at Just $85/Sq. Ft.!

Apr 19, 2017 5:07:31 PM By Stacy Fitzgerald-Redd
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Paul Torcellini.jpgWhen Paul Torcellini, principal engineer in the commercial buildings research department at the National Renewable Energy Laboratory, decided to build his own home he admittedly had a bit of an advantage. As a building scientist and recognized expert in methods to achieve substantial wh\e building energy savings, Torcellini undoubtedly leveraged his experience to construct his 3,600 sq. foot Connecticut home to achieved a HERS score of 2 with a 0.75 ACH 50. The result: A finished product that garnered Torcellini a 2016 DOE Housing Innovation Award (read the full home profiled here).  Paul’s was also the lowest cost house among all the award winners. His home demonstrates that builders who fully leverage solid building science practices can achieve high-performance, air tight construction while keeping costs in check.

To be certified as a DOE Zero Energy Ready Home, a builder must certify the home to the Energy Star Certified Homes Version 2.0 and the U.S. Environmental Protection Agency’s Indoor airPLUS program. In addition, the home must meet the hot water distribution requirements of EPA’s WaterSense program and the insulation requirements of the 2012 International Energy Conservation Code (IECC). Finally, the home must have solar electric panels installed or have the conduit and electrical panel space in place for future photovoltaic (PV) panel installation. This innovation award winner’s home meets all of these criteria. It also required careful project management and contractor selection based on their ability to meet the Zero Energy Ready goal, as well as frequent quality checks by the builder and the HERS rater to assure that the details regarding careful product installation were observed.

Good Design Produces Great Results

As designed, Torcellini’s home would have achieved a HERS 35 rating without the PV panels, but the addition of this technology reduced his final HERS score to 2. Utilizing good design principles, such as the placement of the home to maximize the sun’s radiant heating and a very efficient wall construction structure, Torcellini achieved the results he intended.

Double wall construction including 2 x 4 24-inch on-center wood stud walls built side by side and set 5 inches apart, allowed the sheathed walls to form a 12-inch wall cavity dense-packed with fiberglass insulation for an R-52 wall. The insulation between the walls significantly reduces thermal bridging and without the need for exterior insulation.

The exterior surface of the OSB includes a water-resistant coating with seams taped with a proprietary sealing tape and the OSB acting as a sheathing, air barrier, moisture barrier, and drainage plane all in one, without the use of house wrap. Vertical furring strips were installed over the sheathing to provide a drainage gap behind the fiber-cement lap siding.

The same coated OSB was used for the roof decking and was topped with the furring strips installed vertically to provide a ventilation gap above the cathedral ceilings. The furring strips were topped with ¾-inch plywood decking that was completely covered with ice-and-water shield for the region’s typically cold climate. The attic was insulated along the roof line with R-60 to R-80 of blown fiberglass dense packed insulation in the roof rafters and vents were clustered on the roof to leave most of the south side of the home free for solar panels. 

The home also incorporates poured concrete walls insulated with R-10 rigid foam, double-pane, double-hung windows and a heat recovery ventilator to bring fresh air in an exhaust stale air.

In achieving the 0.75 ACH 50, the home is more than three times as tight as the 2015 IECC requirement of 3 ACH 50. This was achieved partly by taping the seams of the OSB so it would act as sheathing, air, barriers, moisture barrier and drainage plane without the need for housewrap.

Budget Friendly High Performance

Torcellini says that one of the biggest challenges in zero energy home construction is educating consumers that it doesn’t have to be more expensive. The total construction cost for the home was less than $85 per square foot, according to Eversource, the local utility. The project’s HERS rater developed a brochure to promote the home as an example of what is possible on a limited construction budget.

While the conventional thinking might be that such lofty energy performance targets are difficult to achieve on a reasonable budget, this award-winner’s home dispels that myth.



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