Although I used to be a builder, I now work as a reporter for an energy-efficiency newsletter. Joining the tribe of energy nerds has altered my perspective, making me single-minded and opinionated. Watching new-home builders in action, I can often be heard to mutter, "Why do they always build it that way?"
Of course, I realize that many of the builders I grumble about are actually familiar with energy-efficient construction techniques — they just can't convince their clients that energy efficiency is worth the extra investment. Most builders are accustomed to juggling several balls at once: They need to satisfy their clients, keep the local building inspector happy, and make a profit.
Sometimes, however, a builder gets lucky and lands a client who insists on a high-performance home and is willing to pay for it. To help you get ready for that day, here's a list of dos and don'ts from an energy nerd's perspective — starting with the don'ts.
Don't Design a Complicated Roof
For those who espouse the principle "form follows function," the ideal roof is a simple gable over an unheated attic, much like the roof on the house we all drew in kindergarten. Unfortunately, designers these days are fond of complicated roofs — ones with enough valleys, dormers, and intersecting planes to make the home look from a distance like an entire Tuscan village.
Such roofs are difficult to insulate without resorting to spray polyurethane foam. Though spray foam is effective, it's also expensive. In most cases, simple roofs are easier to insulate, easier to ventilate, and far less prone to ice dams than complicated roofs.
Don't Install a Hydronic Snow-Melt System
Snow can be removed from a driveway with a shovel, a snow-blower, or a plow. It can also be removed by burning great quantities of fuel to heat water circulating through buried pipes.
In rare cases — for example, at the home of a handicapped client — a hydronic snow-melt system makes sense. In most homes, however, such systems are uncalled for.
In 60 years, when global climate change has made snow rare, history books will explain to our grand-children how hydronic snow-melt systems used to work. Our descendants will shake their heads, astonished that their ancestors burned fossil fuels so wantonly.
Don't Build a Poorly Insulated Slab
In a hot climate, an uninsulated slab in contact with cool soil can lower cooling costs. In a cold climate, though, slabs should be well-insulated.
Some cold-climate builders, having learned that heat rises, install thick attic insulation while leaving their slabs uninsulated. But heat actually moves from warm to cold in all directions. While it's true that in winter the soil beneath a slab is warmer than the outside air, a slab can still lose a significant amount of heat.
In cold climates, a basement slab should be insulated with at least 2 inches of extruded polystyrene (XPS) under the entire slab. For a slab-on-grade home in a cold climate, specify 3 or 4 inches of XPS under the entire slab, with additional vertical foam at the slab's perimeter.
Foil-faced bubble pack (R-1.3) is no substitute for adequate insulation; under a slab, it's virtually useless.
Don't Insulate Rim Joists With Unfaced Fiberglass
Although fiberglass insulation is a thermal barrier, it is not an air barrier. If unfaced fiberglass is used to insulate a rim joist, moist indoor air can filter through the batt, leading to condensation at the cold rim joist. The result, eventually, is mold and rot.
There are several acceptable ways to insulate a rim joist. Rigid foam insulation can be installed on the exterior of a recessed rim joist; small pieces of rigid foam can be inserted in each joist bay from the inside; or spray polyurethane foam can be used to seal the entire rim-joist area.
Don't Install Recessed Can Lights on the Top Floor
Despite their tendency to cast strange shadows on people's faces, recessed can lights retain an inexplicable popularity. Ignoring the pleas of lighting experts — who note that it makes more sense to light the ceiling than the floor — many customers still request recessed cans.
When installed in an insulated ceiling, these fixtures are an energy disaster.
Some builders have switched to "airtight" cans. But airtight cans are not completely airtight. The amount of leakage depends on the care exercised when installing the gasketed trim kit, and any future trim changes can affect the fixture's airtightness.
It is much easier to air-seal electrical boxes installed for surface-mounted fixtures than to air-seal a recessed can. Just say no to recessed cans.
Don't Install Oversized Hvac Equipment
Compared with homes built 30 years ago, today's houses are more airtight and better insulated, so their heating and cooling loads are smaller.
Yet many hvac contractors continue to use old rules of thumb to size furnaces and air conditioners, often throwing in a generous safety factor for good measure.
Oversized furnaces and air conditioners cost more than right-sized units. Oversized equipment frequently operates less efficiently, too, because it suffers from short cycling. An oversized air conditioner often shuts down before it's had a chance to wring much moisture out of the air, compromising comfort.
Although hvac contractors usually claim to have performed detailed load calculations, you should insist on seeing written evidence. Heating and cooling loads should be calculated for each room and must be based on accurate specifications for window sizes, orientation, and U-factor, and for the installed glazing's solar heat coefficient. Don't let your contractor talk you into adding a safety factor to a calculated load.
Experience has shown that builders who want right-sized hvac equipment need to educate themselves on this issue and double-check the work of their hvac sub.
If you don't feel qualified to verify your sub's calculations, at least specify two-stage equipment that can operate at partial load most days of the year.
Don't Install Hvac Equipment Or Ducts in an Attic
An attic is almost as cold as the exterior in winter, and can be much hotter than the exterior in summer. While attic floors are often insulated to R-38, attic ducts are usually insulated to a measly R-4 or R-6.
During the summer, the difference in temperature between the cool air in the ducts and a hot attic is much greater than the difference in temperature between the indoor and the outdoor air. So why does attic ductwork have so much less insulation than a wall or a ceiling?
Moreover, the air in a supply duct is at a much higher pressure than the air inside a house. Since most duct seams leak, a significant portion of the volume of air passing through attic ducts usually leaks into the attic. Any leaks in return ducts allow the blower to pull hot, humid attic air into the air handler.
Installing a furnace or air handler in an attic causes even more problems than merely installing ductwork there. A recent study found that the leakage of a typical air handler, coupled with the leakage at the air-handler-to-plenum connection, amounts to 4.6 percent of the airflow on the return side. If the air handler is installed in an attic, a 4.6 percent return-air leak can produce a 16 percent reduction in cooling output and a 20 percent increase in cooling energy use. Any duct leakage would make the situation even worse.
In most homes, hvac equipment and ductwork belong in the basement or crawlspace. If it's absolutely necessary to build on a slab, include a utility room for hvac equipment and install ducts in air-sealed interior soffits.
Don't Install a Powered Attic Ventilator
Many builders assume that hot attics are a problem. If soffit and ridge vents don't keep an attic cool, they may decide to install an exhaust fan in the attic to improve attic ventilation. This is almost always a mistake.
If an attic has no ductwork or hvac equipment and its floor has a deep layer of insulation, high attic temperatures don't matter much. In fact, high attic temperatures can help lower winter heating bills.
Several studies have shown that a powered attic ventilator often draws its makeup air from air leaks in the attic floor, pulling conditioned air out of the house instead of in from the soffits. This, of course, increases the homeowner's energy bills.
Don't Use a Standard Furnace Fan To Distribute Ventilation Air
Most new homes include some type of whole-house mechanical ventilation system — for example, a passive outdoor-air duct connected to a furnace's return-air plenum. Some builders provide ventilation by connecting a heat-recovery ventilator (HRV) to the home's forced-air ductwork.
Both methods have an Achilles heel: They depend on the furnace fan to distribute ventilation air. In homes equipped with air cleaners, homeowners may leave the furnace fan running continuously.
This can carry a substantial energy penalty. Furnace fans are designed to move a lot of air — up to 1,400 cfm — yet most homes require only 50 or 100 cfm for ventilation. In fan-only mode, certain furnaces can draw as much as 700 to 800 watts.
One solution is to specify a furnace with a blower powered by an electronically commutated motor (ECM) that draws 200 to 250 watts in fan-only mode. Another is to choose a different type of ventilation system — a simple exhaust-only system or an HRV with dedicated ventilation ductwork.