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Q.Any suggestions on the best way to insulate the ceiling of a garage? The garage will have an in-law suite above, with living, sleeping, and bath areas. The floor construction will be 2x10 joists, or possibly 11 7/16-inch wood I-joists. I'm especially concerned about plumbing freezing if the overhead garage door is inadvertently left open on a cold winter day (it can drop to 13°F below in our area in winter). I also want to ensure a comfortable warm floor in the winter months.

A.Michael Uniacke, owner of Advanced Insulation in Prescott, Ariz., replies: When it comes to insulation, quality control is at least as important as the choice of materials and methods. If all the work is done carefully, you can effectively insulate a floor over a cold space using fiberglass batts. But the more plumbing, wiring, central vacuum pipes, and the like that run through the floor, the harder it is to achieve a careful fit and the more I'm inclined to use a blown-in or sprayed-in product. I'd rather rely less on hand labor and more on a mechanical process that has some measure of inherent quality control.

A critical detail like this also needs good coordination between the builder, the insulator, and the other subcontractors (especially the plumber). The big worry is the potential for pipes to freeze. If plumbing runs within the floor system, it's vital to have the plumber hold the piping as close to the subfloor as possible, away from the cold, lower side of the assembly. This is an instance where the effectiveness of insulation can work against you: The farther the plumbing is from the conditioned space, the colder it will get.

In extremely cold climates, the plumbing should be attached to the subfloor and not the framing members, because of the thermal bridging caused by wood's lower insulating value. Framing has an R-value of about 1 per inch; 3 1/2 inches up, a 2x8 might provide a thermal resistance of R-3 to R-4. If copper piping is attached halfway up the floor joist with a typical copper fitting, you're risking a freeze on some bitter cold night. So locating the plumbing correctly makes a big difference.

If you use fiberglass batts, use full 16-inch batts between wood I-joists, or 15-inch batts for conventional framing. You want a batt that fits snugly into the cavity right out of the bag. Also, you want the cavity completely filled from subfloor to ceiling drywall. So for 2x8 floor framing, an 8-inch (R-25) batt will work best. The batts should even be a little compressed -- if you can't fill the cavity completely, use another system.

For peace of mind, you might want to upgrade to either a dense-blown cellulose system such as par/PAC (877/937-3257, www.parpac.com), or a low-density spray foam such as Icynene. We like par/PAC, which relies on a reinforced poly that gets attached to the framing members with a 1-inch crown roof staple. We attach the poly drum tight and then blow the cavity with cellulose at a density of 3 to 3.5 pounds per cubic foot.

Icynene and similar products also can do an excellent job, as long as the cavity is completely filled and any plumbing is attached close to the warm side. Icynene can be cost competitive with par/PAC because less site labor is involved, but finding a spray foam contractor is a challenge in many areas.

No material or process is perfect. Batts can be installed haphazardly. Cellulose and spray foam provide more assurance than fiberglass, but only if they're done right. Cellulose can be installed at densities so low that the product settles, creating cold spots. And if spray foam is not installed flush with the bottom of the floor joist, thin spots here and there can put plumbing at risk. So no matter what system you choose, be sure you inspect it before the work is covered up.