The search is on for an American home. The goal is a home with the look and feel of a traditional suburban house, which the buying public demands, yet one that is at least twice as efficient in its use of energy and other resources. And, it must cost no more than the competition—less, if possible.
Getting houses into some urban infill locations can be a delicate operation.
Photo courtesy Hickory Consortium Spurred by government grants, entrepreneurial vision, or straight environmental idealism, teams of engineers and architects across North America are pursuing this holy grail. While lobbyists for the National Association of Homebuilders are busy convincing state regulators that meeting the Model Energy Code (MEC) will price houses out of the range of first-time home buyers, these teams are leaving the MEC far behind.
And some large production homebuilders, collectively responsible for tens of thousands of new houses each year, are paying close attention.
Four teams have been working since 1994 in partnership with the U.S. Department of Energy’s
Building America Program, and all now have test houses under construction, with full-scale developments in the works. Each of these teams consists of engineers, architects, product manufacturers, and production homebuilders. Meanwhile, other builders, independent of any government incentives, have introduced cutting-edge homes directly to the buying public.
Given the wastefulness of conventional American houses, it’s not surprising that these innovators are confident of making dramatic improvements. This article takes a look at efforts to improve the conventional American house, without challenging its mass-market appeal. For the most part, these are pragmatic strategies, designed with the large-scale production builders in mind. Not addressed here are the radically different approaches to housing, such as straw-bale, cob construction, and rammed-earth. While such approaches may ultimately be more sustainable, as long as the vast majority of new housing is of conventional construction, it makes sense to optimize those systems as far as possible.
Strategies for improvement
Whether they’re test houses or market-ready developments, these designs share many of the same goals for optimization, yet each is taking a slightly different approach.
Described below are some of the trends that are emerging for various components of the home, as implemented by the four Building America Teams: the Consortium for Advanced Residential Building, the Hickory Consortium, the Building Science Consortium, and Integrated Building and Construction Solutions, Inc. (IBACOS). Each team and its current projects are briefly described in the boxes accompanying this article.
Envelope improvements
Heat loss (or gain) through the building envelope—roof, walls, and floor—is responsible for most of the heating and cooling loads in houses. This heat flow occurs in the form of conduction, radiation, and air infiltration, through the building envelope, so thermal improvements to the building must address one or more of these issues. The envelope also contains much of the structural material used in houses, making it an important realm for resource efficiency in material choices.
Nearly all of the designs aimed at improving energy performance have found ways to reduce the amount of structural material in the frame. Conventional houses are notoriously overbuilt, and leaving out the unnecessary framing both saves money and improves thermal performance by leaving more room for insulation. Ed Barbour of the NAHB Research Center, Inc., of Upper Marlboro, Maryland (an organization that is only peripherally affiliated with its less progressive namesake, the National Association of Home Builders) has championed the efforts to remove unnecessary structural members through his work with
optimum value engineering.
Among the many ways to reduce structural framing are spacing studs 24” (600 mm) on-center instead of the more common 16” (450 mm), sizing headers over openings appropriately for the load (and insulating them), and reducing the number of studs required to frame a corner from four to three or two (see
EBN
Vol. 3, No. 1 for drywall clips used with two-stud corners). The double top plates commonly used on walls can be reduced to a single top plate if the joists or trusses are aligned over the studs.
Structural insulated panels (SIPs—also called stress-skin or foam-core panels) reduce the framing material used by relying on the inner and outer oriented-strand-board skins and the foam core to provide the needed structural strength. A prototype now under construction by the Consortium for Advanced Residential Building in Frederick, Maryland, will test the viability of SIPs as an energy-efficient technology for production builders.
Constructing an airtight shell is important for three reasons: to save the energy that is lost through infiltration (or exfiltration) of conditioned air; to ensure that fresh air from the ventilation system is distributed throughout the house, rather than being short-circuited by air leakage; and to prevent possible moisture damage caused by humid air condensing inside of structural cavities. Many existing air-barrier strategies, such as the use of a polyethylene sheet as both an air barrier and a vapor-diffusion retarder, are both labor intensive and very difficult to perfect. Even small imperfections in an air barrier can become problematic as moisture accumulates at those points.
Taped seams on the foil-faced foam sheathing create a tight outer air barrier on Building Science Consortium Houses at Prairie Crossing.
Photo courtesy Building Science Corporation To address these concerns, the Building Science Consortium has developed a redundant, double air barrier strategy that is being used in over 300 units by the Shaw Homes at Prairie Crossing in Grayslake, Illinois.
As this team is led by engineer Joe Lstiburek and architect Betsy Pettit, long-time advocates of the airtight drywall approach, it is not surprising that sealed drywall, instead of poly, is the first air barrier. The second barrier is achieved by taping all the joints in the exterior foam sheathing. The team has even worked with manufacturers to develop appropriate tape sealants for these joints. To seal window flanges to the sheathing they’re using a stronger tape designed for sealing ducts. Besides increasing the R-value and making up the exterior air barrier, the insulating foam sheathing provides added insurance against moisture problems in the walls by keeping the temperature in the cavities above the dew point for any air that might leak out of the house.
Panelized and modular construction solves some air leakage problems by assembling building sections in a controlled, factory setting. But the joints between these large sections, when they’re assembled on site, can be especially problematic. Working with modular homebuilder Epoch Corporation, the Hickory Consortium has designed a recessed rim joist with a compression gasket between first and second floor modules. This detail effectively seals between the modules and provides an easily accessible cavity for insulation at the rim-joist. The cavity is sealed with a strip of sheathing after the insulation is installed.
In hot, humid climates, Building Science Consortium is promoting another departure from current practice. Conventional wisdom has it that venting the roof or attic space in a hot climate is the best way to reduce cooling loads. Lstiburek is challenging that convention, choosing instead to build unvented roof (and attic) systems, and to move the roof insulation from the ceiling plane to the roof deck (and gable-end walls). Although it requires insulating a larger surface area, Lstiburek and Florida Solar Energy Center researcher Armin Rudd argue that it will result in a better insulation blanket and air barrier, because there are many fewer penetrations and irregularities to accommodate.
More significant, however, is the advantage in terms of mechanical systems. Despite the energy penalty, it is still standard practice in many areas to locate air-conditioners and cooling ducts in unconditioned attics. Moving the insulation to the roof plane is a relatively simple way to extend the conditioned space, so that all ducts and mechanical equipment can operate more efficiently. In humid climates, another advantage to this approach is that in eliminating roof or attic ventilation, it prevents the introduction of hot, humid air to the attic, where the moisture it carries can condense on any cool surfaces and cause mold growth and other problems.
Along with airtightness and reduced thermal bridging, increased overall insulation levels are a common strategy. Low-e glazing and argon gas fill has become standard for windows in climates with significant heating loads, and most of the test production houses meet that standard.
HVAC innovations are big
The cost of upgrades to the building envelope is paid for, in most cases, by the resulting savings in heating and air-conditioning equipment. For many of the projects the heating loads are so small that properly sized heating equipment is not widely available, so they have taken to designing alternative systems. One direct advantage of the window and wall upgrades is that heating registers and radiators no longer need to be located on outside walls to counteract cold drafts. This improvement saves money, space, and fan power by allowing for much shorter duct or piping runs. Another benefit of having shorter duct runs and smaller mechanical systems is that it makes it easier to locate the entire system within the conditioned space.
All Building America consortia are creating houses with mechanical systems contained entirely within conditioned space. IBACOS is working with RGC Corp. in southern California to reduce energy consumption to 60% of the State’s relatively tight Title 24 requirements. By specifying low-e insulated glass windows and a tighter envelope, IBACOS is able to use an air-conditioner that is small enough to fit in a closet, and ducts that can easily fit within the conditioned space.
The Hickory Consortium has developed an innovative “Home Run Heating System™” that allows for affordable and efficient zoned air distribution for heating and cooling. The system uses a fan coil, or water-to-air heat exchanger, to heat or cool the supply air. From the fan coil, small variable-speed fans supply air to individual rooms. Each fan is independently controlled by a thermostat and can be adjusted to provide the appropriate air flow. Any source of hot or cold water can supply the fan coil.
The Building Science Consortium is taking a different approach, with integrated heating, ventilation, and domestic hot water. “We’re moving towards one fan and one heat-making unit per house,” says architect Pettit. An electronically commutated motor (ECM) allows for efficient operation at different airflow levels.
Several technologies are being employed to save natural resources (and money) in foundations. The Consortium for Advanced Residential Building is testing the use of 6”-thick (150 mm), poured-in-place concrete walls instead of the more standard 8” (200 mm) walls. IBACOS test houses in the Northeast have used precast concrete panel foundations, saving nearly 75% of the concrete in a typical poured foundation. The embodied energy savings of this approach would be partially offset if the precast panels are transported a long distance to the site, so the availability of locally produced panels is a factor not only for cost, but also for environmental performance.
Given the widespread availability of innovative foundation systems, it is surprising that more alternatives are not being tested. Numerous stay-in-place, foam-form products are available, as are shallow, frost-protected foundations (see
EBN
Vol. 4, No. 4). This lack of attention to alternate foundation systems may be because many of the projects are being developed in the South, where simple slab foundations are the most cost-effective option; or it may be that the manufacturers of foam-form systems don’t have the resources, or awareness, to join a Building America team.
Bringing it to the mainstream
When the Building America program’s second phase was rolled out by the U.S. Department of Energy’s National Renewable Energy Lab in 1994, its goal seemed ambitious—to get teams representing all phases of the construction process to fundamentally rethink the way houses are built. Along the way, these teams were supposed to dramatically improve the energy and environmental performance of the homes, make them more attractive to homebuyers, and reduce their cost. As part of this effort, all four teams are developing methods to assess and measure the less-easily quantified environmental benefits of alternative approaches.
“What we are doing has a real potential for changing the way we build,” claims George James, DOE’s program manager for Building America. The program has been funded for 1997 at the 1996 level of $3 million. Previous funding cuts forced an extension of the schedule, but all four teams are now on track with test houses and one full-scale development. Perhaps most significantly, each team includes at least one production homebuilder, with the potential to market the resulting technologies and strategies to the tune of hundreds or thousands of units per year. “We’re working with the builders, not telling them what to do,” notes James.
In competitive housing markets, criticism from the competition has been a significant problem. “This is a very important impediment to progress,” says architect Gordon Tully of Steven Winter Associates. “They are so afraid about what their competition will say about their product.” Previous efforts by homebuilders to use shorter duct runs by keeping the registers on interior walls, for example, have lost out to buyer concern, fueled by competing builders. Efforts to use more resource-efficient, thinner-profile wall studs met a similar fate. We can only hope, now that several major homebuilders are working toward similar ends through the Building America Program, that such sniping will cease.
For years we’ve been hearing from the homebuilding community that energy efficiency and environmental sensitivity cost too much. The projects described here are demonstrating that by rethinking the entire product and the process of making it, these homes need not be more expensive. Environmental and energy improvements conceived as “add-ons”—like a whirlpool tub in the master bathroom—do cost more. But fundamental improvements in the design don’t have to.
While the houses profiled here are taking great strides forward, there remains a lot more that could be done. With the exception of Hickory Consortium, for example, most are just beginning to address the tremendous energy use and pollution that current housing patterns are perpetuating through the transportation patterns they establish. Changing the tastes of American homebuyers is a task yet to be done. Short of that, these projects are making great inroads. And they have the potential to change the way houses are built all across the coutry.
