"Innovative" New House Construction, 1989
By Albert Presgraves and Jenny Yasi
We moved into our new house on September 1, 1989. Construction started 5 months earlier, in April, although planning and design had started in 1987, when we bought the property. The house is basically a two story Cape Cod style, with a full attic above the second floor. It is located on a 1.5 acre parcel in the middle of Peaks Island in Casco Bay. Peaks Island is served by regular ferry service from Portland.
There were many design elements that Jenny and I easily agreed on. We wanted the house to be passive solar -- or, at least, solar oriented. We agreed on the approximate size, which was a result of our desired budget. But the number of details involved with designing a house is much more complex than can at first be imagined. And in addition to the design effort, we acted as the general contractor for the construction.
This article describes two of the elements that make this house different from most other houses being built today. First, the house was constructed as a "low-toxic" house. Second, the house uses an underfloor radiant heating system supplied by an electric hot water storage tank.
The reason for building a "low-toxic" house is because of the growing recognition that house building materials are a direct cause of toxic chemical sensitivity, or environmental illness. These ailments cause a variety of symptoms, from mild allergies to major nervous system and respiratory problems. According to different sources, the problem affects from 1 to 20 percent of the U.S. population, although only a fraction of these have been clinically diagnosed. Some of these people have such severe problems that they have to live in purified environments with a filtered air source.
Two modern trends combine to make environmental illness an increasing problem: first, the large amount of chemicals used in building materials (and furnishings), and the goal of building tight, energy efficient houses. One authority has identified some 1500 hazardous compounds that are used in residential building materials. The most pervasive chemical is formaldehyde, which is used in some 3000 products, from plywood to carpeting to paper bags. The basic idea of a low-toxic house is to minimize the use of building materials that contain hazardous chemicals. Each person must make their own decision about the degree that this principle should be followed. For instance, some people believe that fiberglass insulation is as carcinogenic as asbestos fibers, and should not be used. Drywall and joint compounds are also considered to be hazardous to some degree.
In our house, we tried to find a reasonable balance between the risk of chemical use and the cost (and availability) of the alternatives. We used plywood for sheathing on the outside of the house, and for the first floor subfloor, but we used almost no plywood inside of the vapor barrier system. The second floor deck and the attic floor are constructed of wood planking. The kitchen cabinets are solid pine (including the shelves and sides), instead of veneer over chipboard or plywood, which is common with most cabinets. We used solid wood or tile for the finish floors, instead of wall-to wall carpeting which is loaded with chemicals. The paint used on the drywall was a special non-toxic type that does not contain solvents, mildew-cides, fungicides or similar compounds that are generally found in interior house paints. Walls and floors are important considerations in a low-toxic house because of the sheer area involved.
The insulation system provides somewhat better than CMP "Good Cents" requirements, but it does not qualify as "super-insulated". In the walls there is 6 inches of fiberglass insulation covered with a layer of foil-faced "bubble-pack" polyethylene material. This was covered with horizontal strapping, to reduce the conductive connections of the studs, and to provide an air space between the foil and the drywall. This space provides a radiant barrier for added insulation, which is considered to be equivalent to about an R-4 insulating value, although it is not the same type of heat transfer. In the few places where the bubble pack could not be installed, polyethylene film was installed and taped to the bubble pack to provide a continuous membrane.
Great care was taken to provide a tight vapor barrier on the inside of the insulation system. In order to prevent air from entering the house from the basement, we did not build a stairway to the basement, and all penetrations through the floor were sealed. This reduces the amount of moisture and/or radon in the air that can enter the house.
Because of the tight house construction and to provide ventilation, we installed a heat recovery ventilation (HRV) system, also known as an air-to air heat exchanger. These units exhaust stale inside air, and transfer the heat to incoming fresh air. Our HRV is independent of the heating system, and provides an air flow of 150 cfm, with a heat transfer efficiency of about 80 percent. At this rate, the air change rate for the two stories is about 0.75 air changes per hour. We have found that operating the system a few hours per day will maintain a feeling of fresh air in the house.
The heating system is unusual in terms of both the distribution system and the heat source. The distribution is by radiant underfloor heating using a hydronic system (water in pipes). The heat source is an off-peak electric hot water storage tank. We selected the radiant underfloor distribution system because of its anticipated comfort factor. By maintaining a floor temperature of about 80 degrees F., the air temperature can be maintained at about 70 degrees F. The exact temperatures depend on the outside temperature, and the system requires a well insulated house. The close temperature differential is adequate to heat the house because of the large area of the floor. The distribution of air temperature between the floor and the ceiling is almost constant.
The system we used is one of the least expensive systems available. It simply consists of 3/4 inch plastic tubing (cross-linked polyethylene) attached under the subfloor with aluminum plates to help distribute the heat. The whole subfloor is insulated with 1-inch foil-faced rigid insulation over the bottom of the joists to prevent heat loss into the basement. The temperature of the water in the plastic tubing must be limited to about 120 degrees F. so that the floor temperature does not exceed about 85 degrees for comfort in bare feet. The system is designed to heat the whole house with the heat output from just the first floor. This eliminates the need for installing the tubing under the second floor deck, which has beams and planking exposed to the ceiling of the first floor. It also keeps the second floor temperature about 5 degrees cooler than the first floor, which we prefer, anyway.
The electric-storage heat source was selected for several reasons, including economics, low-maintenance, and compatibility with a solar collection system, if we ever want to install one. How can electric heat be economical? Because Central Maine Power will sell electricity for $.043 per kwh between the hours of 9:00 p.m. and 7:00 a.m. on a separate meter, compared to a standard residential rate of about $.09 per kwh. At 70 percent efficiency in an oil burner, this electric rate is equivalent to a cost of $1.27 per gallon of oil. This electricity is still more than the cost of oil delivered to Peaks Island, which is about $1.00 per gallon (December 1, 1989), but considering other problems associated with oil, we decided the electric storage heat system would be worth the additional fuel cost. The idea of the storage heat system is that a storage medium is heated during the "off-peak" hours, and used to heat the house during the remainder of the day. The storage medium in our house is 500 gallons of water in a polyethylene tank, rated for continuous duty at 160 degrees F. Allowing the tank to operate between 160 and 90 degrees F. provides about 290,000 Btu's of available heat. This is enough for 10 to 12 hours of the house heat load on the coldest days. After that, the house may not maintain a temperature of 70 degrees, but it will certainly not freeze. This 10 or 12 hours of available heat does not include a number of other factors, such as heat stored in the mass of the house, and solar gain, which is substantial on sunny days. There is also a woodstove in the living area of the house.
The method of heating the tank is an "innovative" design, maybe even experimental. It consists of four electric hot water tank immersion heating coils mounted out from a tube that is inserted into the tank. It has been working fine to date. For numbers, there is 15 kw of electric heat, which produces about 50,000 BTU/hour; the maximum heat load of the house (at -20 degrees F.) is about 34,000 BTU/hour; the difference heats up the tank for use during the next day.
There are plenty of other details that could be described, such as the control system and piping details but this covers the major points. We also had some difficulty in getting CMP to install two meters and then bill them correctly, but that's another story. Overall, everything has exceeded our expectations. The air in the house seems clean and fresh, and warm. The source of the heat is not detectable, and the temperature stays steady at the thermostat setting. It is a very pleasant environment.