Start by checking to see what you have, and how much of it.

Insulation is rated by its resistance to heat flow—or R-value. The higher the R-value, the greater the resistance. If you double the R-value by adding insulation, you halve heat loss in the insulated area.

How Much Insulation Do You Have?
What you see Probable Material Estimated R-value
Loose fibers—lightweight yellow, pink, or white Fiberglass 2.5 times inches of depth
Loose fibers—dense gray or near-white, may have black specks Rock wool 2.8 times inches of depth
Loose fibers—small gray, flat pieces or fibers resembling newsprint Cellulose 3.7 times inches of depth
Lightweight granules Vermiculite or perlite 2.7 times inches of depth
Batts (laid-out rolls of insulation) Fiberglass 3.2 times inches of depth

To determine whether your attic is properly insulated, measure what’s there. Then use the table above to estimate its current R-value. Compare your estimate of what you already have with Department of Energy (DOE) recommendations for the area, shown on the table below. The DOE’s recommendations are broad, but they should give you a general sense of cost-effective insulation levels according to the area’s climate, material costs, and common fuels.

Recommended Insulation Levels
for the Boston Area
Attic R-38 to 49
Cathedral ceiling R-38
Wall cavity R-15
Floor over uninsulated space R-30
Walls underneath siding R-5

To check insulation in exterior walls, choose an electrical outlet, and—after turning off the power to the outlet—remove the cover plate. You should be able to see whether or not insulation is in the wall. Check separate outlets on the first and second floors, and in old and new parts of the house; insulation in one wall doesn’t mean insulation is everywhere. (You can check in frame houses by temporarily removing a small section of the exterior siding.) If your walls already have some insulation, it’s usually not necessary—and too difficult—to add more.

Also, look at the underside of any floor over an unheated space, like a garage, unfinished basement, or crawlspace. Measure the thickness of any insulation you find, which is likely to be fiberglass batts. If you see foam insulation board, check the label for an R-value; for sprayed-on foam insulation, multiply the thickness in inches by five to obtain its R-value.