Q: I have an open ceiling in my garage that is insulated with paper-backed R-13 batts with the fiberglass exposed and held in place by metal strips. The ceiling has a lot of pipes and wires running over the ceiling joists.

Q: I have an open ceiling in my garage that is insulated with paper-backed R-13 batts with the fiberglass exposed and held in place by metal strips. The ceiling has a lot of pipes and wires running over the ceiling joists.

I plan to install poly-encapsulated R-19 batts into these spaces. Is it better to remove the old insulation, then install new insulation? Or can I leave the old insulation, remove the metal strips and fit the new insulation into the space? Another option would be to leave the old insulation and use a foil insulation product to enclose the ceiling.

A: To put it bluntly, don’t do it. Don’t install poly-encapsulated R-19 insulation over the existing craft-paper-faced R-13 insulation. "Poly-encapsulated" is fancy term for fiberglass insulation encased in plastic. The plastic cover eliminates glass fibers flying around during installation.

Having suffered through our share of the fiberglass "itchies," we understand why you’d want to go the plastic route. But the comfort comes with an unintended consequence. By adding encapsulated insulation, you’re stacking vapor retarders and creating a sandwich of 4-inch fiberglass between a plastic and a craft-paper vapor retarder. The result is that ambient air in the R-13 fiberglass may give up its moisture and breed mold. The same is true for running a foil-faced insulation over the R-13.

If you insist on installing the poly-encapsulated R-19 insulation, you must remove the existing R-13 insulation. The advantage is no itching during installation (although you might scratch after you remove the R-13). The disadvantage is less insulation. Another alternative is to build up the existing insulation with another layer of unfazed fiberglass insulation to the depth of the floor joist. This should give you somewhere in the neighborhood of 11 inches of insulation, without several vapor retarders. We’d opt for the latter. The plus in our method is greater insulation; the minus is installation that may be a bit uncomfortable.

Fiberglass dust is an irritant to mucous membranes and the skin. It’s important to wear eye protection, a respirator or dust mask, long-sleeved clothing and gloves to protect yourself.

Installing the insulation is a simple matter, more easily accomplished with two people. The first step is to remove the metal strips that hold the existing insulation in place. With a utility knife, cut the new insulation to fit. With assistance from a helper, push the new insulation into place, making sure not to compress it. Finally, reinstall the metal strips as needed to hold the insulation in place. Every 2 to 3 feet should do the trick.

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