Wrapped Is (Usually) Bad
I recently was walking the insulation aisle of a big home store when to my dismay I saw wrapped fiberglass insulation is back. This is batt fiberglass insulation that is in a thin, perforated plastic sheathing so the fiberglass is contained in a tube of sorts. It’s supposed to be easier to handle. I’m not so sure about that, but I am sure that this type of insulation is usually bad news.
When I first saw this kind of insulation about seven years ago, its maker made the claim that the insulation was safe to install over existing insulation (even though it’s wrapped in plastic, or, one could say, “faced”). The manufacturer said because of the perforations in the plastic wrapping, it would still allow water vapor to pass through. It doesn’t. The company isn’t making that claim now—or at least I didn’t see it on the label—but buyer beware! Don’t put this insulation over existing insulation and assume that it will breathe. It absolutely doesn’t.
I bought a roll, tore off some of the plastic wrapping (holes and all) and then I covered a steaming pot of water with the perforated wrap. Sure, some of the water vapor probably did escape through the holes but a lot of it condensed on the underside of the wrapping. Experiment with this yourself: Take some kitchen plastic wrap and punch holes in it. Then cover a glass that you just filled halfway with steaming water and watch what happens. If you use this wrapped insulation, you invite the same potential condensation in your attic.
In addition, you really shouldn’t install wrapped fiberglass insulation without a mask, eye protection, gloves, long-sleeve shirt and pants, etc. At least I wouldn’t. Once you cut the roll, you’ve exposed the fiberglass. Buy the regular unfaced batts instead. They’ll let water vapor through and they’re cheaper.
To me, this stuff is a rip-off. I have had clients remove this type of insulation when I’ve found it in their attics over the top of older insulation. The most recent one was a widow, and she didn’t need this kind of headache. Perhaps her late husband installed it, thinking he was taking good care of their house. Despite paying for the materials and the time to have it installed, sorry to say but it now really needs to come out. Fortunately, one of my recommended insulation contractors was willing to unwrap it for her and re-install the old fiberglass.
Simply put, wrapped fiberglass insulation is not good for the durability of a home. It will allow water vapor to condense and any water in a home where you don’t want or expect it can be a nasty thing. Think mold or rot. Besides, there are so many other good alternatives, like unfaced batts, blown fiberglass, blown cellulose … .
David Rabenau, LEED AP, is a certified HERS rater and BPI-certified building professional (building analyst & envelope). This blog first appeared on blog.smhes.com.