Insulation R-values explained

Insulation slows the flow of heat through the walls, roof and floors that separate a house from the outdoors. The bigger the difference between indoor and outdoor temperatures, the more energy it takes to keep the house comfortable, and insulation cuts that demand. Its R-value is a measure of how well it resists heat flow. The higher the number, the better it insulates.

What sets the number

An R-value depends on two things, the kind of insulation and how thick it is. The same material laid twice as deep resists more heat. Some materials pack more resistance into less thickness, which is why spray foam is used in exterior walls and cathedral ceilings, where there isn't much room. The R-value is printed on the insulation's packaging, or on the paper backing of rolled insulation.

The common kinds

• Rolls and batts. Flexible blankets of fiberglass, rock wool or recycled fibers such as denim, sized to fit between standard wall studs and attic or floor joists.
• Loose-fill. Loose fibers or pellets of fiberglass, rock wool or cellulose, blown in with special equipment. It fills odd-shaped spaces and attics crowded with wires, ducts and pipes.
• Rigid foam. Boards that usually cost more than batts or loose-fill, used in exterior wall sheathing, on basement walls and over attic hatches.
• Foam-in-place. Sprayed into walls, onto attic surfaces or under floors, where it also cuts air leaks. It comes in closed-cell and open-cell types, and installing it takes protective equipment and ventilation, so it's usually a job for a professional.

How much is recommended

Recommended insulation levels depend on climate zone, from Zone 1 in the warmest places, including Hawaii and Puerto Rico, to Zones 7 and 8 in the coldest. They're set at the levels that pay off for local fuel costs and weather, which can differ from local building codes.

Continuous insulation is a layer applied to the outside of the wall, just inside the siding. When siding comes off an uninsulated wall, insulation can be blown into the empty wall cavity before the new siding goes on.

Sealing comes first

Insulation works best once the air leaks are sealed. Sometimes a house still feels drafty or too warm after the attic is sealed and insulated. Then the heating or cooling system may not be working properly, or the walls may need insulation. A home energy professional can tell which.

An annotated example

What to do next

Find your climate zone, then compare the insulation you have against the recommended levels for each part of the house. Have any air leaks sealed before insulation is added, and keep a record of the R-value of whatever goes in. Having the attic air sealed and insulated covers the attic job itself.