Where You Should Have Insulation
Last updated in March 2015
All structural elements around your home’s living spaces that are heated and cooled should be insulated. It’s most practical to insulate many of these areas when the home is built or as part of a major renovation. Otherwise, accessibility drives costs and often determines what’s worth doing.
For most homes, the most cost-effective way to install or add insulation is to target attics. The physics are simple enough: Because warm air rises, your attic is the frontline in the battle to conserve energy in the winter and the first place to check for sufficient insulation. If your attic is unfloored, installing insulation is quite easy, and there is no limit to the thickness to which insulation can be added. If you have a floored attic, you can still add insulation, but installing it usually entails removing and replacing floorboards, which adds to the price, and the amount of insulation will be limited by the depth of the joists below the attic floor. Insulating between exposed roof rafters isn’t as effective, as you’ll still lose energy between the conditioned space and the unconditioned attic.
In finished attics without cathedral ceilings, insulation can be installed between the attic ceiling and the roof if the area is accessible. If your attic has a finished cathedral ceiling, insulation can be installed above the ceiling in the spaces between the roof joists, but it generally makes sense to do this work before the attic is finished, during remodeling, or during re-roofing, when it’s easier to access stud walls and roof joists. The same is true for homes with flat roofs and no attics.
If your home was built in the 1970s or later, its interior walls probably have adequate insulation. If your home is older, it might be worthwhile to install insulation. Doing so is most likely to make sense if you are already planning other major renovation that will involve refinishing the exterior walls. To insulate finished exterior walls, installers drill access holes between each pair of wall studs. The holes can be made from the interior—through drywall or lath/plaster—or from the exterior, depending on the configuration of the house and type of siding. Installers blow in insulation through the access holes, and then patch and reseal the openings and do any needed refinishing of walls. As you can imagine, this is much more time-consuming, messy, and costly than insulating an open unfinished attic.
Underneath Floors Above Crawlspaces and Basements
Areas underneath ground floors above unheated spaces like crawlspaces and basements are also excellent candidates for insulation. Crawlspaces should be dry year-round (moisture causes insulation to deteriorate), and a vapor barrier should be placed on the floor of the crawlspace.
Basement and Other Masonry Walls
The most practical way to add insulation to masonry walls is to build finished stud walls inside the perimeter of the masonry walls and insulate between the studs before finishing. Because basement walls require special attention to moisture control, consult a building contractor before considering adding insulated interior walls.