Energy Design Update
September 2002
Industry News: Solar in the City

Solar in the City
For custom homes, three parties usually collaborate on the design and specifications: the client, the architect, and the builder. When Pat Hanson of Burlington, Vermont, was ready to build her house, she was fortunate to connect with architect W1lliam Maclay and builder Chuck Reiss. All three share an interest in energy efficiency and green construction, and the convergence of their common interests produced an elegant result. Hanson's three-bedroom, 2,018-square-foot house, built in 2000 on an infill lot in a residential neighborhood of Burlington, Vermont, includes both active and passive solar features (see Figure 1).

A Client Committed to Solar
Hanson wanted Maclay to design an energy-efficient house equipped with solar hot water and photovoltaic (PV) panels. "I just believe strongly that it's important to do what we can to reduce our fossil fuel use," said Hanson. "Even if photovoltaic panels aren't quite cost effective yet, the more people are willing to install them, the more likely they will be cost effective in the future."

Maclay's design is rooted in standard passive solar principles, with careful attention to orientation and window placement. Maclay and Reiss both have extensive experience building energy-efficient homes, and Hanson readily accepted most of their recommendations. She did, however, resist their advice to use pultruded fiberglass casement windows from Accurate Dorwin. "In other houses I've lived in, I've had some bad experiences with the wind blowing against open casement windows," said Hanson. "I wanted to have the double-hungs, so I ended up going with Marvin windows."

Reiss's close attention to air sealing paid off at the Hansen house. The blower-door results showed 480 cfm @ 50 pascals (Pa), equivalent to about 0.10 cfm per square foot of building envelope surface area @ 50 Pa -less than half the EEBA standard of 0.25. "We're real careful with any penetration," says Reiss. "We foam everything in -pipes, wires, and windows. We're also careful with the poly on the walls. We don't go to the level of taping the seams, but we overlap each seam by a foot." Reiss uses ordinary electrical boxes. "Since we have 5 1/2 inches of cellulose behind all of our electrical boxes, hardly any air gets though," he says.

Insulation
The Hanson house has a slab foundation, since site conditions did not permit a full basement. "I usually put 2 inches of rigid foam under a slab," says Reiss. "But on this house, the architect called for 4 inches." The vertical edge of the slab perimeter is insulated with two inches of rigid foam.

The 7-inch thick wall cavities were insulated with damp- spray cellulose. Reiss usually allows about a week for the cellulose to dry before putting up the wall poly. "The cellulose installers complained a lot about the horizontal strapping, since it's hard for them to run their rollers over it," said Reiss. On his next house, Reiss modified the wall details to accommodate his cellulose contractor. He installed 2 inch wide by 1 1/2 inch thick strips of rigid foam insulation on top of each stud, between the courses of horizontal strapping. This created a coplanar checkerboard-like grid, making it easier for the insulators to trim the cellulose flush (see Figure 3). Reiss was pleased that he was able to cut almost all of these rigid foam strips from scraps left over from insulating the foundation.

Although the attic of the Hanson house is unfinished, the rafters are insulated, creating a partially conditioned space that is available for future remodeling into living space. The 2x12 rafters were packed solid with dry cellulose, installed behind cross-linked polyethylene held in place by lx3 strapping. Although a ridge vent was installed, Reiss did not install any vent channels in the rafter bays.

Conventional framing usually dictates that rafters receive a bird's-mouth cut for installation on the top plate of the wall. In order to provide more room for insulation at the eaves, energy-conscious framers usually install the rafters above the attic rim joist, bearing on a 2x6 plate. Maclay provided a detail that goes one better: he specified the installation of four stacked 2x6 flat plates above the attic sub floor (see Figure 4 Note: opens in a new window, large file size). This raises the heel of the rafter an additional six inches. "We prefer to install R-50 insulation at the eaves, where ice damming can be a problem," says Maclay. "If possible, we try to get more than that."

Heating andVentilation
Hanson chose to install a hydronic radiant floor heating system. " A radiant floor heating system costs thousands of dollars extra, just to deliver a few hundred dollars of heat," says Maclay. "It clearly is not an efficiency measure, it's a comfort measure." But after living with hydronic heat for a year, Hanson has second thoughts. "If I were to do it over, I don't think I would go for the radiant heat," says Hanson. "It turns out that I don't like warm feet, although I know that most people do. And the radiant heat complicated the choice of flooring material."
Although Hanson, Maclay, and Reiss would have all liked to include a heat-recovery ventilator (HRV), budget constraints forced Hanson to settle for an exhaust-only ventilation system. The system consists of two 17-watt Panasonic bath exhaust fans controlled by programmable timers, and several passive inlet vents. "I try to encourage all customers to go to an HRV but it's a cost thing," says Reiss. "For smaller houses, an exhaust system with the fans on timers is barely functional."

Photovoltaics and SolarThermal
During the summer months, two 4 ft by 8 ft roof-mounted solar thermal panels supply most of the home's domestic hot water needs. The solar panels are backed up by an indirect water heater operating off of the gas-fired boiler. The antifreeze solution in the panels is circulated by a DC pump powered by a dedicated PV panel. The solar-heated water is stored in a 70-gallon tank which acts as a pre-heater for the water flowing into the indirect water heater. Since the Burnham boiler features cold starting, the boiler water stays at room temperature until the controls sense a demand for heat. During the summer, as long as sunshine is adequate to meet the domestic hot water needs, there is no need for the boiler to fire.
The grid-connected solar electric system consists of 12 PV modules with a total peak output of 1.4 kW and a utility- tied 1,000-watt sine-wave inverter. There is no battery storage.
The installed cost of the solar thermal system was about $4,600, while the PV system cost about $11,500.

Utility Bills
The house received a HERS rating of 89.6, earning it a 5 star Energy Star label. In 2001, Hansen used only 622 ccf of natural gas (see Table 1 below). According to Michael Russom of Vermont Gas Systems, the home's gas usage was 42% less than that of a typical Vermont home (that is, the "base case" home in the demand-side management modeling program used by the gas utility). The modeling software assumes that a typical 2,OOO-square-foot Vermont home uses 1,080 ccf of nah1ral gas for heating and appliances.
From March 27, 2000 (when the electrical net metering equipment was installed), through August 15, 2000, the house used 613 kWh of electricity. The cost of the power was offset by the value of 313 kWh of solar electricity fed back to the grid, resulting in net electrical bills totaling 300 kWh (2.2 kWh/ day).
An energy-efficient house offers not only lower utility bills, but also less tangible benefits. "Living in a house with a lot of insulation is much more comfortable, because your body isn't radiating heat to the cooler walls," says Maclay. "You don't have hot and cold spots in your house, or drafts, or temperature stratification. And when the electricity goes off, your house doesn't immediately get cold."

Table 1 - Natural Gas Consumption, Hanson House