The attic includes everything from the finished ceiling up to and including the underside of the roof. Many attics have air leaks from the house, insufficient insulation, poorly installed heating systems, and inefficient skylights. All of these can waste your energy and money, and leave you feeling too hot or too cold, so it’s critical to confront these problems head-on.
Because warm air rises, the air high in the house is usually warmer and at higher pressure than the air in the rest of the house. During cold weather, this stack effect constantly drives indoor air up and out the top of the house—especially through attics. In hot weather, colder, denser air sinks to the ground floor and migrates outward, causing a reverse stack effect that can pull in hot air from above. So it’s crucial to address attic leaks for the sake of both comfort and energy use. If you plan to convert your attic into a living area, you can improve its performance as part of your remodeling.
First Decision: Go Unvented?
For centuries, homes have been built with ventilated attics. Screened vent holes under your eaves, a long slot vent under the overhang, or louvered openings at the gable ends of your exterior walls are part of the venting system that allows air to move through your attic. This ventilation can keep the house cooler in summer and discourage moisture buildup in the attic year-round. However, it can also allow conditioned air to escape, which wastes energy.
The alternative is to have an unvented attic, which means including the attic in the conditioned area of your home and keeping moisture out. Openings between the attic and the outside are sealed, and the underside of the roof is insulated. This allows airflow between the attic and the house, and blocks continuous, uncontrolled airflow through the roof structure, which conserves energy.
Should you change your vented attic to an unvented attic, and if so, how? That depends on several factors, including your climate zone, the location of your heating and cooling system, and the geometry of your attic. This is a good time to call in a building professional knowledgeable about building science.
If you choose to convert your vented attic to an unvented attic, air sealing is your first and most cost-effective step. Seal any openings, gaps, cracks, or vents at your new air boundary. Take special care when sealing complicated areas, such as knee walls or odd angles.
Next, move the thermal boundary to the plane of the roof deck. With attic spaces often defined by knee walls, sidewalls, dormers, sloped rooflines, and flat ceilings, deciding where to locate the new thermal boundary can be difficult. Once this boundary is determined, you can proceed with sealing and insulating.
In some areas of the United States, ductwork and HVAC equipment are often run through attics. Building contractors and building code developers have recently realized that installing this equipment within the insulated space can improve energy efficiency. If you must have the HVAC equipment in a vented attic, pay special attention to duct insulation (Chapter 9).
Codes for Unvented Attics
Requirements for unvented attic assemblies are provided in Section 0806.4 of the International Residential Code (2009 IRC). This section addresses ways to prevent condensation on the inner surface of the roof deck if temperatures are significantly lower on the inside than on the outside of the roof. The code offers two options, both of which use air-impermeable insulation to increase overall R-value. In colder climates, a vapor retarder must be part of the assembly. Check this code to make sure your project meets its requirements.
Precautions for the Vented Attic
It may be okay to leave your attic vented if
One advantage of a vented attic is that it keeps the roof cold in winter, reducing the potential for creating ice dams. For this approach to work effectively, it is crucial that the attic floor and any of its sidewalls be properly insulated and sealed to block heat and moist air from the adjacent conditioned space.
It may pay to hire a home energy professional to evaluate whether your attic has sufficient airflow to ventilate properly. In some situations, you may need a powered attic fan to rid the attic of excessive heat or moisture. Attic fans aren’t appropriate for all situations, however; a fan won’t help, for example, in a poorly sealed home. Your energy professional can help you make the best choice for your circumstances.
If your home has HVAC equipment or ducts in the attic, carefully seal all penetrations through thermal or air boundaries. Otherwise, mechanical attic ventilation can pull in conditioned air from living spaces, creating drafts and wasting energy. It’s difficult to seal complex attic spaces properly; bear this in mind when considering whether to keep your attic vented.