Insulation between roof rafters is the most important factor in heating and cooling the attic. Choose the highest possible R-value (a material’s ability to block heat moving through it), and install it correctly to prevent heat loss in winter and heat gain in summer. Also leave a couple of inches between insulation and rafters to allow airflow from soffit vents to roof vents.
Fiberglass batts: Six-inch-thick, high-density fiberglass batt insulation with an R-value of 21 (40 cents/sq. ft.) fits snugly in most 2x8 rafter space but also leaves a couple of inches of airspace. In extreme climates, add more insulation to your attic ceiling by increasing the depth of each rafter by nailing 2x2s on the bottom edges of the rafters. The added depth can accommodate R-30 (43 cents/sq.ft.), 8.5-inch-thick high-density fiberglass batt insulation.
Rigid foam: One way to bump up the R-value of your batts is to attach 1-inch-thick sheets of rigid foam to the rafters over fiberglass insulation, then drywall over it. That combination adds an R-value of about R-6. Expect to pay about $1.30/sq.ft. Of course, adding depth to your rafters lowers the ceiling, so make sure your new ceiling height doesn’t violate code restrictions for attic ceilings—typically 7 feet 6 inches.
Spray foam: A third heating and cooling option is to have professionals spray foam into the area between the rafters. The R-value of spray foam is approximately twice that of fiberglass of similar thickness. Prices for spray foam installation vary widely by market—expect to pay a minimum of $3/sq.ft.
Be sure to insulate interior walls between the living area and any unused portion of the attic, such as the space behind knee walls. Also, plug any heating or cooling escape routes at the ends of gables or around existing windows.
Extending heating and cooling systems
Insulation keeps heating and cooling IN your new attic space, but your HVAC system delivers the heat and cool TO the space. Consult with an HVAC professional to determine if your existing system has the capacity to handle the extra load. Even so, you may have to find a path for new ductwork. In some instances, you can “borrow” space from a closet in the living area below your attic.
If installing new ductwork isn’t feasible, consider installing electric baseboard heaters. Two 1,000-watt baseboard heaters, plus a small baseboard heater for the bathroom, will warm the attic for about $150.
Small air conditioners also can supplement or replace your central air conditioning system. Small window units and portable air conditioners that provide between 7,000 and 10,000 BTUs of cooling power will adequately cool a 15x15 attic room. Expect to pay $300 to $600 for a small air conditioner.
Benjamin Allen has edited dozens of home improvement, remodeling, and home repair books published under the Better Homes and Gardens, Home Depot, Stanley, and Ortho brands.