To reduce energy consumption, look for a furnace with a blower powered by an electronically commutated motor (ECM). Such motors use significantly less electricity than conventional permanent split capacitor (PSC) motors.
A gas water heater’s EF includes thermal standby losses but not electrical power usage. Studies have shown that power-vented water heaters draw between 100 and 200 watts for an average of 84 minutes per day (about 76 kwh per year); high-use families have water-heater run-times of up to 240 minutes per day (about 219 kwh per year).
Although annual electricity use attributable to power-vented water heaters is relatively low, one Canadian researcher concluded that “it appears that the power-vented water heaters deliver very little energy savings when you factor in the use of the power-vent motor” (Energy Design Update, January 2004).
“Spray polyurethane foam is a vapor retarder.”
This is a half-truth. Closed-cell spray foam — also called “2-pound foam” because it has an average density of 2 pounds per cubic foot — is an effective vapor retarder. Installed at a thickness of 21/2 inches, closed-cell spray foam has a permeance of only 0.8 perm.
On the other hand, open-cell spray foam (average density, 1/2 pound per cubic foot) is not a vapor retarder. Installed at a thickness of 3 inches, open-cell spray foam has a permeance of about 16 perms, making it fairly permeable to water vapor.
When installed directly against wall or roof sheathing in a cold climate, open-cell spray foam needs to be protected on the interior side with a vapor retarder. In most cases, painted drywall provides enough vapor resistance to avoid problems.
However, when open-cell spray foam is installed in a cold climate between rafters to create a so-called “cathedralized” attic, the roof sheathing can accumulate moisture. Though rare, this problem is most likely to occur in homes with elevated indoor humidity. The solution is to cover the attic side of the insulation with a vapor retarder — vapor-retarder paint, for instance.
“Air-conditioned homes don’t need a dehumidifier.”
In a hot humid climate, air conditioners make a home more comfortable by lowering the temperature of the air (sensible heat removal) and by dehumidifying the air (latent heat removal). When the thermostat detects that the indoor air temperature is too warm, the air conditioner switches on; when the thermostat is satisfied, the air conditioner switches off. While the equipment is operating, some dehumidification occurs. However, the ratio of latent heat removal to sensible heat removal is a function of equipment design and weather conditions; it is out of the control of the homeowner.
When an air conditioner runs flat out for hours at a time, it’s usually pretty good at dehumidification. But in an energy-efficient house with low-solar-gain windows, the typical air conditioner runs for fewer hours. Although the equipment easily cools the house, it may not lower indoor humidity levels to comfortable levels.
As reported in Energy Design Update (January 2003), researchers in Houston were called to investigate high levels of indoor humidity plaguing a group of energy-efficient homes participating in the U.S. Department of Energy’s Building America program. They discovered that “improvements in window performance and envelope tightness … lowered the buildings’ sensible cooling loads to the point that existing air conditioners [were] unable to handle the latent load.” The recommended solution: Each house needed a stand-alone dehumidifier in addition to a central air conditioner.