Insulating your attic is often the most important insulation project you can undertake. First, determine the R-value of any insulation you already have.
To find out how much insulation to add, subtract your finding from the recommended R-value. You may put new insulation over old, and you need not match types.
Before beginning, isolate any light fixtures between joists. When working in an unfinished attic, step only on joists, never between them. Better yet, place 1 by 6s, 1 by 8s, or strips of 3/4-inch plywood next to each other atop three or four adjacent joists to serve as a work platform.
Measuring Attic Insulation (shown below right)
Kneeling safely on the plywood platform, slip a ruler between the joist and insulation to measure its thickness. Average several measurements taken from different parts of the attic, and then multiply the average by the R-value per inch of insulation:
Fiberglass (batts/blankets): R-3.0/in.
Fiberglass (loose fill): R-2.5/in.
Cellulose (loose fill): R-3.4/in.
Rock wool (loose fill): R-2.8/in.
Installing Batts and Blankets (shown below left)
If the attic floor is not insulated, use insulation with a vapor-retardant face, and place the facing next to the ceiling. In hot, humid climates—or if the attic is already insulated—use unfaced fiberglass.
Start at the perimeter, and work toward the attic access door. Place the insulation between joists, cutting it to length. For higher R-values, lay additional insulation across the joists.
Adding Loose-Fill Insulation (below right)
Rent or borrow a loose-fill insulation blower from a home center, making sure you have enough hose to reach all corners of your attic.
With a helper to load the blower with insulation as needed, start at the perimeter of the attic, and work toward the attic access door. Fill every joist space completely and evenly with the insulation, leaving attic vents uncovered. Level uneven spots with a rake.
TIP: Controlling Moisture
Water vapor that condenses inside insulation ruins it. The solution is to install a vapor barrier. Some types of rigid foam insulation serve as their own vapor barrier.
Polyethylene sheeting works well with most insulation. Fiberglass batts and blankets are available with a vapor-retardant facing on one side. Vapor barriers are most often placed on the interior side of the insulation, but in hot, humid climates, they go on the exterior side.
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