Hey Mr. Green,
My attic has part rock-wool insulation and part fiberglass, to the depth of the ceiling joists. I'd like to throw a "blanket" over the existing layer, preferably one that won't release bits of fiberglass when I walk around up there. Is there a nontoxic, nonflammable, ideally encapsulated insulation that doesn't cost an arm and a leg?
--Anna in Grass Valley, California
You need at least another six inches of insulation up there, as do millions of others, according to the U.S. Department of Energy. That's because 40 percent of a typical home's energy use goes toward heating, and a lot of it is lost through the ceiling. (The DOE's Web site offers customized insulation recommendations.) Encapsulation insulation (in which the fiberglass fibers are sealed behind a protective film) is available.
Yes, fiberglass can irritate the body, but I think its dangers have been exaggerated. If you're concerned about formaldehyde, you can use formaldehyde-free fiberglass insulation, and all fiberglass is made with up to 40 percent recycled glass. Other options include insulation made of recycled paper.
Whatever you choose, avoid tromping on it: You'll squish out its R-value, and with the joists concealed by the new layer, one misstep could send you plunging through the ceiling (truly costing you an arm and a leg). Installing insulation can be tricky, however, especially in an older home, so I strongly recommend consulting a reliable installer before you begin.
Finally, don't forget to insulate and weather-strip the door that leads to the attic, and remember that if your house has leaks around windows, doors, pipes, vents, ceiling lights, or even electrical outlets, you can lose 30 percent of your heat. The DOE provides excellent information about finding and dealing with these mini-escapes here.