House orientation is the first thing a builder should look at when attempting to increase energy efficiency in his homes. Situating a house to take advantage of sun and shade can make an enormous difference in the energy required for heating and cooling. Then, leaky walls, inadequate insulation, improperly flashed windows and doors, thermal conductivity, inadequate housewrap, and a host of similar mistakes bear investigation as causes of energy-porous construction.
FOAM HOME: This extremely energy-efficient house by architect Joseph Vigil and artist Brandy LeMae features an insulated concrete form foundation wall (R-32), structural insulated panel wall (R-23) and roof (R-38), in-floor radiant heating, engineered-wood structural framing, low-solvent construction adhesives, and recycled cotton insulation.
Creating a tight shell and building better wall assemblies are top priorities in constructing a high-performance house. First steps in that direction include two interrelated areas, insulation and wall framing.
Generally, say building scientists, builders use too much wood in their walls. Miller, who builds his homes to the Department of Energy's Building America standards, uses 2x6 studs set 24 inches apart instead of 2x4s set 16 inches apart. This simple move accomplishes several things: It decreases the amount of wood in the wall but creates a stronger house, and it reduces heat loss and gain due to thermal bridging through the studs. But more important, less wood in the wall assembly frees up space for the builder to use more insulation, especially the type that completely fills the wall cavity. Using this type of insulation might cost more money, but it offers better performance. “We generally use cellulose insulation and use Icynene [sprayed foam] occasionally,” says Pam Sessions of Hedgewood Properties, a Cumming, Ga.–based company that builds to EarthCraft House green building standards.
Completely filling the wall prevents air from seeping into the conditioned spaces, says Locke, so the manner in which the insulation is installed is very important. “Small errors in insulation installation make a big difference in the overall performance,” he says. For example, if the installer leaves voids or gaps, or if the insulation slumps just a little, an R-13 product may yield only an R-6 performance.
A gap in the insulation is only one of the areas that can affect the performance of a house. A typical house, new or existing, has myriad areas where airflow and heat loss can occur. Areas to focus on include gaps in framing and places where the shell has been penetrated, such as around electrical wiring and vent pipes. Other trouble spots include the transition points from the basement wall to the first floor and from the second floor to the roof; chimneys; and dormers, says Peter Yost, a principal at 3-D Building Solutions, a high-performance–building consulting firm with offices in Brattleboro, Vt., and Bexley, Ohio. Sealing these gaps prevents air infiltration, and as an added bonus, it also stops moisture intrusion, curbing mold growth. When building a tight shell, Yost says, “you have to pay attention to moisture flow as much as you pay attention to energy flow.”
Other key steps include properly caulking and flashing all four sides of the high-efficiency windows you're (hopefully) using, installing adequate housewrap that allows the house to breathe, and using a properly installed drainage plane. “Drainage planes are water-repellent materials (building paper, housewrap, sheet membranes, etc.) that are located behind the cladding and are designed and constructed to drain water that passes through the cladding,” writes Joseph Lstiburek, a principal of Building Science Corp. in Westford, Mass., on his company's Web site. Controlling the flow of rainwater is the single most important factor in the design and construction of durable buildings and in controlling mold, he believes.
But even if you use advanced framing and reduce the amount of wood in your walls, heat conduction can still occur through the studs. Chris Briley, principal of Green Design Studio in Yarmouth, Maine, minimizes this thermal bridging by using horizontal straps. He places the straps between the drywall and the studs on the interior and between the studs and the exterior sheathing. Miller, meanwhile, uses a double air-barrier system consisting of ¾-inch polyisocyanurate foam sheathing installed over the structural sheathing to reduce heat loss and heat gain.A MODEL SYSTEM
The down side of tighter building envelopes is that they can result in a higher risk of poor indoor air quality and mold growth than in previous eras. In the old days, the typical house had adequate fresh air and few, if any, mold problems. Unfortunately, that same house was also drafty and wasted tons of energy. Today we know better: A tight shell is an improvement for energy efficiency, but the key to indoor air quality is controlling how you bring fresh air into the house. The best way to do this is with an effective HVAC system.
The reality is that today's HVAC systems are poorly designed, building scientists say. As a result, most people are uncomfortable in their homes and have to run their systems longer to get comfortable; typical ducts leak up to 20 percent of the air they move; and the systems bring in an inadequate amount of fresh air and offer poor humidity control and filtration. In addition, most are oversized.