Insulating Attic Spaces
Attic spaces are a very important area to insulate. During heating season, warm air rises to the top floor of a home, and the heat in it is lost as it transfers through the ceiling up into the colder attic space.
Colder climates require insulation values from R-38 to R-49 in the attic. That's 12" to 15” of fiberglass.
In the unlikely situation that there is no insulation at all in your attic, by all means crawl up there and lay the thickest batt you can into the joist spaces.
(However, if there is no insulation, you’ll also want to take advantage of the situation and seal any spots where air might be leaking up through holes in the top plates, or around the tops of partition walls.)
Depending on the age and construction style of the home, you might have joists that are spaced 24” on center, so you’ll need batts that are sized properly.
Insulating an attic is also a situation where you might find it easer to use rolls of insulation instead of batts.
If there is already a layer of insulation between the joists, install a second layer running perpendicular to the joists.
When insulating an attic, there are a couple of places to be careful of. Make sure you leave space around the eaves so that air can flow from the soffit, past the layer of insulation and exit through vents in the roof. (See the “Ventilation” section.)
Also, leave a 3" space between metal flues, chimneys and recessed lighting fixtures unless fixture is marked "I.C." (Insulated Ceiling).
If you have home with roof trusses (vs an older, “hand-framed” roof) you may find that it makes more sense to use “loose fill” or “blown-in” insulation.
Fiberglass and cellulose insulation are good choices for this. Do-it-yourselfers can usually buy the material and rent the equipment to do this job.
If you’d prefer to leave this job to the professionals, check out the "Insulating an Attic" video to see how it’s done.