Passive Solar Success Set in Concrete
by Judy Niemeyer
Passive solar design combined with thermal mass construction is a proven technique for energy and cost efficiency. At Tierra Concrete Homes, we use this technique in our passive solar concrete homes. The National Renewable Energy Laboratory (NREL) has provided design analysis to help reduce the homes' heating and cooling requirements as much as possible.
Tierra homes are typically three-bedroom, one-story ranch-style houses with about 1,500 ft2 of living space. We recently built the first of 17 homes in a passive solar subdivision in Fowler, Colorado.
Concrete provides high thermal mass and is adaptable, attrac-five, and easy to construct quickly. Our flit-up concrete construction method for precast walls is similar to commercial concrete construction, except for the way we insulate the walls and foundations. We use a frost-protected shallow foundation. The foundation is 2 ft deep with 2 inches of rigid insulation outside the concrete around the perimeter.
We have a patent application on the wall method. The walls are constructed at our precast manufacturing plant in Pueblo and transported to the building site intact. They are poured horizontally, so they can be as little as 4 inches thick without sacrificing strength. Poly-isocyanurate insulation forms the base of the exterior wall form. When the walls are poured, the insulation is secured to the outside surface with connectors 16 inches on center. The exterior insulation, rebar reinforcement,top plate for attaching trusses, block-outs for windows and doors, and the electrical conduit and boxes are all in place before the concrete is poured over the rebar.
The walls are lifted by crane, trans-ported to the job site, stood on the foundation, and welded together. To prevent thermal short circuits, we insu-late the joints with 2-inch rigid foam and fill any cracks with spray foam. The partition walls between the rooms are also concrete, with the exception of the plumbing walls, which allow for pipes. An average home can be raised in one or two days.
Tierra Homes precasts concrete walls for their buildings. Their construction method for walls (patent pending) involves putting exterior insulation, rebar reinforcement, a top plate for attaching trusses, block-outs for windows and doors, and electrical conduit and boxes in place before the concrete is poured over the form.
The rest of the home is conventional, with wood trusses, a stucco exterior, and drywall ceilings. We install shingle, metal, or tile roofing, depending on the homeowner's budget and preference. The floors are usually slab on grade to provide greater mass for solar heat storage. However, we can build with a basement or second story.
We place extra emphasis on insulation, with R-11 in the foundation, R-14 rigid panels on the walls, and R-38 in the attic. Due to the Pueblo climate, R-I4 provides adequate insulation when used in conjunction with high mass walls. Note that the walls are 100% insulated, whereas framed walls with cavity insulation have thermal bridges at the studs (see Wall R-Values That Tell It Like It Is, HE Mar/Apr '97, p. 15).
We sometimes negotiate an owner finish arrangement, with Tierra providing the enclosed shell and the homeowner finishing the interior. We also license and train builders who want to use our technique.
Passive Solar Design
The whole building is designed to harness the sun in winter, breezes in summer, and natural daylight all year long. During the winter, the insulation on the outside surface traps solar gains in the walls. In summer, the walls absorb heat from the room during the day. The design strategies we use are
Increasing south-facing windows for direct gain from the sun in winter. This involves selecting the site and orienting the building so that the long wall of the home faces south, with no obstruction of the winter sun coming through the windows. We try to have south-facing glass area equal to at least 15% of the floor area of the home, and we recommend that the occupants shade the windows at night with insulated curtains or window quilts to keep in heat.
Providing shading from the summer sun. This is done by proper sizing and placement of north, east, and west windows (2%-4% of the floor area of the home), and by designing overhangs based on latitude and local climate. Landscaping can also play a role. We recommend evergreen trees on the northwest to protect from winter winds, deciduous shade trees on the east and west, and low shrubbery immediately south of the home (see Remodeling with the Sun, p. 11).
· Planning for natural night ventilation to allow the night air to cool the home. We plan the use of interior space--furniture, appliances, and room arrangement--with an open floor plan to allow air to circulate freely. This also lets the sun penetrate deep into the house in winter.
· Designing the windows and interior walls to provide enough natural daylight so that occupants can use any room during the day without having to turn on a light.
We use double-pane, low-emissivity windows, with a shading coefficient of 0.84 and whole-window U-value of 0.38 (about R-2.6). South-facing clerestory windows provide better distribution of solar gains to the back haft of the house and provide additional daylighting as well. Although casement windows are best for both airtightness and maximum ventilation, homeowners often opt for sliders for the cost savings.
We use overhangs over the south-facing windows to shade the sun in the summer and allow maximum solar gain in the winter. However, the large mass of the house prevents unmanaged solar gains from having a large effect on indoor temperature, so we use shorter overhangs than those prescribed by common rules of thumb. Our overhangs do not shade the glass for the entire summer, but they are sufficient to protect occupants from uncomfortable radiant heating on hot summer days.
Clerestory windows near the ceiling contribute passive solar heating in winter, year-round daylight, and summer-time ventilation to this Tierra concrete home.
A backup heating system is required by code and by financial institutions. This may be a gas fireplace set on a thermostat, radiant electric heat panels, or hydronic radiant heat in the floor. A central furnace is not necessary. The houses are mechanically cooled only by ceiling fans in every room.
In addition to passive solar design and good insulation, we install compact fluorescent lighting fixtures throughout the home. We try to find ceiling fan/lights that are large enough to accommodate compact fluorescents. We also put in low-flow toilets, faucets, and showerheads, and we recommend xeriscaping the property--landscaping with native plants that do not require much water (see Xeriscape: Winning the Turf War over Water, HE July/Aug '94, p. 31).
Selling Passive Solar Homes
Selling passive solar concrete homes is a challenge. Many people think of concrete as cold, gray, and ugly. Passive solar design isn't well thought of either--people imagine the houses built 20 years ago with walls of glass, strange rooflines, and summer overheating. However, once they see and feel our homes, they change their minds. The insulated concrete homes are warmer in winter and cooler in summer; the inside walls are textured and painted to look like drywall and the exterior walls have a stucco finish just like a conventional home.
Our marketing is necessarily based on education. I give public seminars quarterly to discuss passive solar design and our concrete construction method. Response to the seminars has been favorable. One of our homes in Pueblo West was in the Parade of Homes in 1996. We also displayed full-sized sample walls at the 1996 and 1997 Pueblo Home and Garden Shows so people could see and feel them.
We have not been able to collect utility bill data on the five homes built in Colorado so far. However, the homeowners who live in passive solar concrete homes really enjoy them. One of them told us, I found that the temperature does not vary much inside. When it is cold outside the house stays warmer and when it gets hot outside the house stays cool longer. During the winter the house stays so warm we just throw a few logs in the wood stove and that is the only heat we need.
Another said, Besides saving on heating and cooling, the house is so quiet. There is no noise from the outside, and it is also quieter between the rooms.
These homes are cost-competitive with custom homes constructed with conventional techniques. Costs for insulation (roof, wall, and frill slab) and for the concrete walls are higher than equivalent costs in typical wood-frame construction, but there are significant savings from reduced construction time, limited drywalling, and the elimination of central heating or cooling systems.
In addition, Tierra is an Energy Star Builder (affiliated with the U.S. Environmental Protection Agency's Energy Star Program), which qualifies the homes for Energy-Efficient Mortgages. EEMs give home buyers access to a lower interest rate, lower closing costs, and a higher debt-to-income ratio, so they can qualify for more house. We're building two homes during spring 1997 for homeowners who are working with their banks to get EEMs.
Low utility bills should be the best economic selling point for these homes. The homes are 85% more energy effi-cient than required by the Model Energy Code. One home in Pueblo West won an Energy Value Housing Award from the National Associa-tion of Home Build-ers for the most energy efficient house in the moderate cli-mate zone for 1997. The homes have received a Five Star Plus rating on the Energy Rated Homes of Colorado Program.
We also participate in NREL's Exem-plary Buildings Pro-gram. Paul Torcellini of NREL reported in an analysis of a sam-ple home that no mechanical cooling is recommended, and conventional heating equipment is not needed. The home NREL studied has a propane fireplace for backup heating. NREL's initial analysis predicts that the house should use less than $100 per year for heating, assuming a 70°F thermostat setting with night setback to 65°F in the winter and a comfort level below 78°F in summer. (Pueblo has about 5,400 heating degree-days and 970 cooling degree-days.) NREL is currently monitoring the house to obtain detailed performance data, so they can make recommendations for the next generation of Tierra homes.
The monitoring has already provided some good feedback. The house was not getting the level of solar gain NREL had expected. They found that insect screens on the windows were blocking a significant amount of solar heat, and recommended that the occupants take these offfor the winter.
Completed walls are transported to the job site and lifted onto the founda-tion by cranes, then welded together. Partition walls between rooms are also concrete, with the exception of plumbing walls, which need allowance for pipes. An average home's walls can all be erected within one or two days.
Conventional home building methods in this country are not conducive to a sustainable environment and do not take advantage of renewable resources. We only transport the walls within a 100-mile radius of our casting facility, which cuts down on transportation energy use. The life span of concrete is exceptionally long. It is not subject to termites, fire, rot, or angry tenants who try to punch holes in walls (although occupants do have to drill holes instead of just using nails to hang pictures). In our method of concrete construction there is minimal use of lumber and very little waste. Compared to conventional walls, concrete walls require less maintenance, have lower heating and cooling needs, and last much longer.
Passive-solar techniques are so simple and cost-effective that there is no reason why more homes cannot be built this way. The trend in home building has been for greater energy efficiency in appliances and insulation, which is good. However, energy efficiency does not make up for poor design. Architects, builders, and land use planners need to change their designs to consider solar energy. Houses, like any other product, must be engineered as a total system.
Tierra Concrete Homes can be reached at (800) 373-9930.
Judy Niemeyer is the president of Tierra Concrete Homes in Pueblo, Colorado.