Attics: Easy Energy Efficiency
authors Steve Easley
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.
While spot air-sealing, contractors must pay attention to can lights. (See Image 3.) Many are not rated for insulation contact, so there must be adequate clearance. Covers can be constructed from metal coil stock, or prefabricated covers can provide a surface to which to air-seal. It’s also important to make sure the bulb wattage won’t cause the can lights to thermally overload. Some retrofitters routinely replace traditional can lights with airtight can lights, CFLs or LED lamps or can lights.
Finally, after all obvious (and not so obvious) areas are sealed, the entire perimeter of the attic is sealed with a minimum of 3 inches of closed-cell foam. This also is a great time to make sure ducts are sealed and in good shape. When the foam is in place, the insulation should be laid and brought to R-38 or higher.
2. Foam the Lid
The second approach to air-sealing an attic is to foam the lid. Similar to spot air-sealing, this approach requires removing or relocating all existing insulation and then applying 1 to 2 inches of closed-cell spray-foam insulation over 100 percent of the attic floor; a minimum of 3 inches of closed-cell foam should be sprayed at the attic floor perimeter. All attic bypasses, knee walls, existing ductwork, dropped ceilings and soffits passing through the attic space also should be sealed.
After the spray foam has been installed, insulation is replaced to bring the attic lid up to at least R-38 levels.
This approach is more expensive than spot air-sealing because you are spray-foaming the entire attic floor, which requires more material.
3. Create a Nonvented Attic
Many contractors may not realize a nonvented attic is surprisingly effective at preventing heat gain and loss. Although this solution can seem a bit counterintuitive, the process is fairly straightforward. Essentially, you are taking the thermal barrier (insulation) that was at the ceiling plane and moving it to the underside of the roof deck while insulating and sealing the gable ends.
With unvented attics, the air seal is at the roof deck so all the labor-intensive air-sealing of can lights and other attic bypasses is eliminated or minimized. The reason you do not need attic ventilation is because the attic is now conditioned, so moisture does not condense because it does not reach its dew point. Large-volume attics typically are not good candidates for the nonvented approach because the attic space would add considerable space-conditioned volume to the home.
To successfully create a nonvented attic space, all attic intakes and exhaust vents are blocked to prevent spray foam from exiting the attic during application. Existing insulation is usually removed or moved to increase R-value elsewhere in the home. The code-required amount of spray-foam insulation is used to insulate and seal 100 percent of the underside of the attic roof deck. One-half inch of foam is sprayed around roof framing members to reduce thermal bridging. Next, spray foam is applied from the roof sheathing onto the attic floor. Six inches of spray foam should be applied at the attic perimeter to provide air-sealing and insulation at perimeter walls.
R-13 to R-30 spray foam is then applied to the vertical gable ends and knee walls, etc. The final step is to apply an appropriate ignition barrier as required by local and state codes. If the attic has many can lights and dropped ceiling soffits that require a lot of air-sealing at the attic floor line, a nonvented attic approach can be less expensive than the first two options.
Another approach to nonvented attics is to apply continuous baffles from the eve to ridge. Spray insulation to the underside of the baffles like you would apply to the roof sheathing. This approach ventilates the underside of the roof sheathing while air-sealing the attic. Image 4 shows a baffle system that will satisfy code officials concerned about moisture in unvented roof decks.
There is no difference in energy performance whether you install continuous baffles or not; however, continuous baffles are an option if you’re not comfortable with an unvented roof deck or codes won’t allow it.
Attics represent one of the largest, easiest and most cost-effective opportunities to save energy in American homes. A combination of meticulous air-sealing and insulation can reduce air leakage and space-conditioning costs. On your next project, consult with the homeowner and inspect the attic to determine how to help your clients grab the low-hanging fruit of energy efficiency.