Our walls and ceilings are insulated with blown cellulose. There are no settling problems with cellulose as long as it's installed to the proper density. We handle vapor diffusion by using vapor-retarder paint on the interior walls.
For these homes, we used low-e triple- glazed R-5 fiberglass windows from Thermo-Tech (www.thermo-techwindows.com). We chose a somewhat lower R-value on the south in return for higher solar transmittance. The doors were fiberglass models from Therma-Tru (www.thermatru.com).
Good air-sealing is crucial for a high-performance home. These homes have a fully delineated air-sealing path from the foundation slab to the roof ridge. It includes gasketing at the base of the sheathing and spray foam around windows and doors, but for the most part the air barrier is provided by the Zip System, which consists of 4x8 sheathing panels with specially designed protective tape to seal the joints. You simply install the panels, tape the seams, and you're done. Our carpenters really like this sheathing. It installs like plywood or OSB and is an excellent air barrier when the seams are taped. We have used it on every home we've built in the last four years.
The key to getting the most from this approach is to make sure there are no interruptions in the air barrier. We do this by framing each home without overhangs at the soffits or rakes. After the Zip sheathing is in place, we have a sealed box with no protrusions. We then apply the overhangs. This adds time to the framing process, but the result is a very tight air barrier.
Cypress backers were screwed to the back of each tail, then the assemblies were screwed through the sheathing into the rim joist. A TimberLok screw driven diagonally through the top of each tail into the rim helps resist downward forces. Pine-board sheathing completes the open-tail look.
Note that at the roof, the air barrier is the outer surface of the roof sheathing. These homes are designed and built with "hot" roofs - that is, there is no structural ventilation. In our experience, good insulation and air-sealing will keep excessive heat and moisture out of the roof system, so there's no need for vents. In fact, we've used this roof system for 30 years; it wasn't part of the code until recently, but long ago we convinced our local building inspectors of its effectiveness. We have never had performance or moisture problems with hot roofs.
The way to test the effectiveness of an air barrier is with a blower door, and our company considers the blower door to be an important part of our carpenters' tool kits. We generally do three separate blower-door tests: before insulation, after insulation, and after drywall and mechanicals have been installed. The final tests on these homes ranged from 117 to 184 cubic feet per minute (cfm) at a pressure of 50 pascals - seven to 10 times better than the Energy Star standard. (Some of these homes meet Passive House airtightness standards.)
Optimizing the Mechanicals
A tightly built house needs good mechanical ventilation to keep the indoor air healthy. A home that's striving for zero energy can't rely on exhaust-only ventilation, which exhausts heated indoor air and replaces it with unconditioned outside air. So we installed heat-recovery ventilators (HRVs), which use heat from the exhaust air stream to temper the incoming fresh air supply. These homes are compact enough that we were able to reduce installed and operational costs by using small, relatively inexpensive single-speed units. Marc Rosenbaum, our building performance engineer, chose a Fantech model SH704, which cost less than $3,500 installed and draws just 36 watts.
We also helped keep costs down by installing each HRV without controls and setting it to run continuously at 50 cfm - a bit more than the ASHRAE 62.2 requirement of 45 cfm for a two- to three-bedroom home of less than 1,500 square feet. The fresh-air supply is evenly distributed to the homes' bedrooms and we have a measured exhaust rate of 25 cfm per bath (as opposed to relying on a fan rating to build a system that meets code but isn't tested).
Heating and cooling are provided by a Daikin RXS24DVJU single-zone air source heat pump. It's a ductless split system with an outdoor compressor and a single indoor unit. A conventional home would be more likely to use a three- or four-zone model with indoor units in each bedroom, but these small, high-performance homes can be conditioned with a single indoor unit.
Installing one indoor unit instead of three or four provided enough savings to offset the cost of the HRV. Natural convection is sufficient to carry heat to all the house's rooms, unless the occupants turn the heat way down or keep doors closed. In these homes, we addressed those situations by installing electric radiant ceiling panels for supplemental heat in the bedrooms. Based on past experience, we expect that some homeowners will never use these radiant panels.
At this writing (December 2010), the homes have been occupied for six months. We have energy meters installed in each house, and they're read monthly by one of our staff. Six of the eight households have achieved zero energy so far, which means they have used less energy than their PV system generated.
We also installed submeters for specific systems. In addition to giving us more data to work with, these meters have proved to be useful diagnostic tools. For instance, when solar electric production seemed very low on one home, the meter for the PV system told us immediately that the panels weren't supplying power to the house, a situation that would have taken time to diagnose if we had not been submetering. We quickly realized that one of the kids had switched off the AC disconnect.
As mentioned, the square-foot cost for these homes didn't include the 5-kilowatt Sun Power PV systems (www.us.sunpowercorp.com), which were paid for by a state grant. The construction details make for great energy performance, but it's the PVs that make it possible for these homes to achieve zero energy.
The market price for the PV system we installed was about $35,000, but actual costs to the builder or owner can be as much as 45 percent less because of tax and other incentives. If electric rates don't go up at all (an unlikely scenario), savings will equal installation costs in about 10 years. The system can reliably produce power for at least 25 years.
We offset the panels to one side of the roof in case the homeowners eventually want to install a solar hot-water system or more PV panels.
We're Always Learning
We continue to use most of the construction details from these homes on current new-home projects and have no plans to change them anytime soon. That's not to say that they won't evolve over time; of course they will. There are some things we already know we would like to do differently.
1. We're not satisfied with the thermal performance of available windows. We're looking for better choices that will meet the budget constraints of the average home.
2. We'd like to provide an affordable option for water heating. We're investigating heat-pump water heaters, which extract heat from the basement or utility room and use a backup electric resistance element to supplement the heat pump during periods of peak demand.
3. These houses are still too expensive. We would like to refine our designs for small homes so that we can cut the cost by 10 percent to 20 percent without sacrificing quality, performance, or aesthetics. We've just begun work on a major design project to try to accomplish this. We'll see where it goes - it's a tall order.
In the end, meeting the goal of building quality homes requires a blending of mind-sets. On the one hand, designers and builders need to think in terms of production - using building methods that keep costs down and projects profitable. On the other hand, we must be willing to push forward and experiment with promising new approaches - with the intention of incorporating successful experiments into the production system.
Ultimately, these are complementary, not contradictory, ways of thinking. More demanding buyers, stricter energy codes, and our own aspirations have made building technology a constantly changing practice. The degree to which we keep learning may be the key to our ability to thrive in an uncertain future. At the very least, it will keep us on our toes!
John Abrams is cofounder and CEO of South Mountain Co., a 35-year-old employee-owned design-build and renewable energy company in West Tisbury, Mass. Thanks to Marc Rosenbaum and Derrill Bazzy for their help with this article.