Where to Insulate

My contractor recently installed insulation directly on the inside of my roof. Does this make good sense? Should have he installed the material on the floor of the attic?
murc98 - 10/15/06 11:44 AM

Well, that depends.  If you are finishing your attic, then roof rafters is really the only place you can install insulation.  If that is the case, it is critical you leave space between the insulation and the roof sheathing so that it can ventilate.  For example, if you have 8" deep roof rafters, you'd want to install 6" of insulation and leave the extra 2" as space that can ventilate.  If not, moisture will get trapped against the roof sheathing and potentially rot it out.  Make sure that you have both soffit and ridge ventilation to wash air through this cavity.

Now, if you are not finishing your attic, then insulating the roof is a big mistake.  Not only will this do little to keep heat in the house, insulating the roof tends to cause a very short life expectancy for your roof shingles as well as moisture problems, especially if it is over stuffed into the rafter bays as warned against above.

Insulation, properly installed in an unfinished attic is installed against the backside of the drywall ceiling, between the floor joists.  Most homes should have at least 12 inches.  If you have 2" x 8" ceiling joists, use 8" between the joists and then add another 4" to 6" above the joists using unfaced fiberglass batts laid perpendicular to the ceiling joists.  If the insulation has a vapor barrier, that should be installed against the "heated space," which in this example would be the ceiling below.

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