Good remodeling contractors are always looking for opportunities to help their customers save money and energy. When it comes to making significant improvements to the energy efficiency of a home, the attic turns out to be the low-hanging fruit.
Although heaping insulation into the attic has been the focus of weatherization rebate programs and tax incentives for decades, piling on more insulation only solves half the energy-loss problem for a typical home. To really reduce the amount of heat transfer through an attic, air-sealing must be part of any competent retrofit or weatherization project. A combination of air-sealing and insulation in the attic can reduce air leakage by up to 40 percent and drop the home’s space-conditioning costs by 20 to 40 percent.
Heat Loss and Gain
Attics are especially prone to heat loss for three reasons (see Image 1):
- Rising warm air escapes into the attic from conditioned spaces in the home.
- Homes are like giant chimneys: Temperature differences between the attic and the conditioned living space create a stack effect, which draws outside air through gaps into the building envelope and lets it escape through a multitude of air bypasses into the attic.
- Bypasses include ceiling/wall-framing intersections at upper wall plates, electrical and plumbing penetrations, HVAC ductwork, dropped ceilings, can lights and flues. These are rarely air-sealed during construction or when upgrading existing insulation, which leads to heat loss in winter and heat gain in summer. Bypasses also contribute to reduced thermal performance of wall systems.
Although most people believe a poorly sealed attic leads to heat-loss issues, heat gain also occurs from attic bypasses into living spaces, which increases cooling loads. Image 2 illustrates the heat gain around a can light into the living space. Note the light is not on.
In addition, because of concerns regarding attic ventilation, the perimeters of attics typically are poorly insulated and rarely air-sealed. Sloping roofs compress insulation and create lower depths of insulation at roof-wall intersections. This causes greater heat loss and cold spots at room edges.
Exfiltration of conditioned air occurs when gaps between upper wall plates and ceiling drywall, built-down soffits and dropped ceilings have been ignored. In addition, air flowing from soffit vents over the insulation, known as wind washing, scavenges away heat. Traditional insulation baffles may help keep loose-fill insulation in place during calm weather, but these flimsy protectors offer little protection to wind washing.
Sealing the Attic
There are three different ways to effectively air-seal an attic and reduce the uncontrolled transfer of heat to and from the conditioned living space:
- Spot air-seal the attic and re-insulate the attic space.
- Spray foam the “lid” or entire floor of the attic and reinsulate the attic space.
- Spray foam the entire underside of the roof creating a nonvented attic or install continuous ventilation baffles and create a vented roof deck but airtight attic.
1. Spot Air-sealing
The least expensive option is to use spray-foam insulation to selectively seal the attic. The existing insulation is temporarily relocated and closed-cell spray foam is used to seal all joints, bypasses and penetrations into the attic. Although spot air-sealing is labor-intensive, it is critically important that contractors seal all areas of the attic where air can migrate to and from the conditioned space. This includes carefully ensuring all wall plate to ceiling drywall seams, soffits, dropped ceilings and knee walls are sealed and insulated. Proper soffit baffles also must be installed at the perimeter. In addition, flues or chimneys should be appropriately air-sealed with methods that do not create fire hazards.