How to compare different types of insulation
On each type of insulation, a label states the R-value per inch, a measure of resistance to heat transfer. The bigger the number, the more effective the insulation. Where space is tight, such as within wall cavities, you need a high R-value per inch. In an attic or under a floor, where there is more room, you can boost the insulation value of a lower-rated material simply by using a thicker layer.
As a rule, the more insulation you add, the more money you’ll save. But there is a point beyond which you can spend more on materials than you’ll recoup in lower energy bills. The tipping point varies depending on where you live. Consult the Department of Energy’s zip-code specific recommendations for the right amount of insulation for your climate.
Adding insulation in the attic
The attic is a great place to start, because adding insulation there is quick, easy, and cost-effective. (To make any insulation upgrade more cost-effective, it’s a good idea to seal air leaks first.) In the Northeast, for example, upgrading attic insulation from R-11 to R-49 would cost around $1,500 if you hire a pro—half as much if you do it yourself—and, depending on the type of heat you have, save about $600.
To determine how much to add, look up the recommended amount for your area, then subtract the value of your existing insulation. If you don’t know, you can figure it out using the Home Energy Saver online energy audit tool.
There are two ways to improve attic insulation. In unfinished space, you can simply add layers to what is already on the floor. Or, if you’re thinking about finishing the attic, you can put the insulation against the roof. Insulating the roof is the better method if heating and cooling ducts pass through the space, or if you live in a humid climate and want to cut down on musty smells coming from the attic.
If you’re doing the job yourself, blanket-type material is easiest to work with. Just be careful not to compress it or it won’t be as effective. If you’re hiring a contractor, go with loose-fill cellulose or fiberglass, which fills crevices better. You’ll pay a pro around $1 a square foot to blow in material; DIY batts cost about half that.
If you’re insulating the roof, sprayed foam polyurethane works best because it molds to rafters, blocks water vapor, and has a high R-rating per inch. Expect to pay about double the cost of loose-fill insulation.
No matter which method you choose, federal tax credits of up to $500 are available to defray the cost of materials.
Adding insulation to walls on main floors
It’s fairly easy to add insulation in stud bays where none exists. (To check, cut the power to a few outlets on exterior walls, then unscrew and look behind the cover plates.) A contractor drills small holes through the inside or outside wall and blows in material. Costs range from around $1.25 per square foot for loose-fill fiberglass, cellulose, or rock wool to $4.40 for polyurethane foam, which insulates about twice as well.
If your walls already have some insulation, you probably can’t add more without tearing into the drywall or plaster. That’s not cost effective unless you’re remodeling, so the best strategy may be to wait until you need to replace siding. Then you can add insulating sheathing underneath it.
Basements and crawl spaces
Even though hot air rises, homes lose heat in all directions. So besides insulating the top and sides of your house, you also need to insulate the bottom, where as much as 30% of energy loss can occur. As with the attic, you have two choices: Insulate under the bottom floor and treat the crawl space or basement as outdoor space, or insulate the walls and treat the area as indoor space. In that case, you would close off all exterior vents except those needed for combustion air or exhaust.
Though floor insulation is more common, wall insulation has many advantages, including cost—it takes about a third less material to insulate the walls of a 36-by-48-foot basement as to insulate the subfloor above. A key detail, not understood by all builders, is to place a layer of rigid foam insulation against the foundation to keep moisture from condensing against the cold walls. If you want to finish the basement, you can cover the foam with a stud wall, fill it with unfaced fiberglass insulation, and cover with drywall.