In most homes in the southern United States, the level of insulation found is adequate for achieving an energy efficient home. However, air leakage and duct leakage are typically high, resulting in high energy bills, comfort problems, and poor indoor air quality. Most people assume that air leakage occurs primarily through the windows, doors, and outside walls of a house.
The reality is that the majority of air leakage through a home’s building envelope occurs between the house and the attic. Typical leak paths include light fixtures (especially recessed can lights), bathroom exhaust fans, chases, HVAC boots, gaps between drywall and framing, and attic kneewalls. The air that leaks into the house from the attic can be 140 degrees or more in the summer, with high humidity and airborne insulation fibers. Ductwork that is located in the attic transports 55 degree air (during cooling mode) with only R-6 insulation protecting it from the 140 degree attic air. Supply leaks into the attic obviously waste energy, and return leaks suck attic air directly into the system and distribute it throughout the house. In a Manual J equipment sizing calculation, it is typically assumed that 14% of all the cooling provided by an attic duct system will be lost to the attic.
An effective way to improve the performance of a home is to eliminate the hot, vented attic space by air sealing and insulating the roofline of the house. Since the roof is already water-tight, air sealing it is very easy – typically all that is necessary is to eliminate attic vents. Insulating the roofline is not easy unless using a spray foam insulation such as Icynene open cell. Since the attic will now be unvented, it is important that the roofline insulation be in continuous contact with the roof decking to prevent condensation on the underside of the roof decking. For this reason, batt insulation or any insulation that sags is not recommended. Once the attic roofline is sealed and insulated, the attic temperature stays within 5 to 10 degrees of the rest of the house, even without a supply register, as long as exiting attic floor insulation is removed. Since the attic space is cool and no insulation fibers are present, leaks between the attic and the rest of house are no longer of concern. In addition, ducts can deliver air much more efficiently.
For instance, many builders in the metro Atlanta area that have used Icynene open cell spray foam insulation to create an unvented attic with great success. In some cases, energy consumption is 50% better in a home with unvented attic sprayed with Icynene than a well-sealed home insulated with fiberglass batts. While the initial investment of spray foam insulation is higher, there is always the potential of a reduction in heating and cooling equipment size when unvented attics are incorporated into the homes not to mention the ongoing, long-term savings the homeowner will achieve in their month heating/cooling bills. In many remodeling jobs, the cost of air sealing all the leak paths in a typical attic is higher than the cost of the Icynene spray foam insulation.