Can I insulate garage ceiling?

Hey there, time traveller!
This article was published 21/10/2007 (6653 days ago), so information in it may no longer be current.

QUESTION — We have a bungalow with an open ceiling in the living room and dining room and an attached garage. The garage ceiling is a continuation of the open ceiling in the living room. The house was built in 1970. Ice dams have damaged the ceiling drywall in the garage and needs to be replaced. Should the ceiling in the garage be insulated? It is insulated now with the bats pushed between the rafters. There is no vapour barrier.

Any suggestions would be very much appreciated.

–Ed Hurta

ANSWER — The short answer to your question is no, but that would make for a very brief and uninteresting column. A more reasonable answer would depend on the usage of your garage and whether you plan to heat it. I will elaborate further on this when I explore both possibilities, a heated and unheated garage.

The majority of homeowners with attached garages use them primarily or exclusively for storage of vehicles and other items such as lawn and garden equipment. If that is your situation, then you should remove the damaged drywall and insulation and leave the underside of the roof open. Once the insulation is removed, you should also inspect the underside of the roof sheathing to see if there are any signs of excess rot or moisture damage. If the sheathing is rotten, buckled, or soft, replacement of damaged material when the roofing is changed will be required.

The reason for removal of the insulation is to prevent trapping warm, moist air that may accumulate in this small cavity between the roof sheathing and the drywall below. This warm air may leak out from the house roof system or develop once warm vehicles are parked in the enclosed space. This warm air will rise and leak through openings in the ceiling drywall and accumulate in the fibreglass insulation under the roof sheathing. Because there is little air or vapour impedance installed or allowance for removal of this warm air by passive ventilation, it will remain in the insulation until it cools. This cooling will likely cause condensation within this small cavity, often on the underside of the roof deck. In very cold weather, this condensation will freeze and heavy frost accumulations are possible. This frost may melt on sunny winter days or in the spring and cause moisture damage, rot and mould growth on the roof sheathing and rafters.

The second problem with this insulation method is that the warm air accumulation in the space under the roof sheathing will initially warm the underside of the roof sheathing, which would normally remain fairly cool in the winter. Once warmed up, this will cause snow accumulated on the roof to melt and run down toward the eaves, creating the ice dams you have seen. Removal of the insulation and drywall will not only help prevent moisture damage due to warm air condensation, but will help the roof sheathing remain cool, minimizing ice damming. All in all, there can be no downside to removal of the insulation if the garage is not heated.

Workshop?

The second scenario is that you may be using the garage as a heated workshop in the cold months. In this situation, insulation of the walls and ceilings to prevent heat loss will be a must to prevent quick deterioration of the roofing and other problems. The difficulty with this situation is that the vaulted ceiling is still very difficult to deal with in terms of proper insulation, air sealing and ventilation. You will either have to completely seal the cavity below the roof to prevent air leakage or ensure adequate ventilation to prevent condensation and moisture damage. There are two possible approaches to this dilemma, with one vastly superior to the other, in my opinion.

The first approach is to attempt to reinsulate the vaulted ceiling and allow enough vented air space above the insulation and air-vapour barrier to keep the roof cool. This may be possible by installing a couple of layers of rigid foam insulation on the underside of the rafters to allow a minimum of a few inches of open space for this purpose. In conjunction with the opening in this area, a continuous ridge vent will need to be installed at the peak of the garage roof and soffit venting in the entire area under the roof overhang. The problem with this method is that it is difficult and fairly costly to accomplish and will result in lower head room at the bottom end of the garage ceiling. Depending on the height of the garage walls, this may not allow proper usage or storage of large vehicles in the garage.

The second, more preferred method of insulating the garage roof system to allow heating is to install complete blown-in foam insulation. If the cavity between the rafters is completely filled with the proper type of this retro-fit product, there will be little to no way for air to infiltrate the area. Once installed, the underside of the rafters may be covered with plywood or OSB sheathing, returning the garage to near its original look with a much improved and air -sealed roof system.

As stated at the beginning, the simplest method to prevent the ice damming and other problems you are seeing with your garage is to simply remove the drywall and insulation from between the rafters, which will allow enough air circulation to prevent further issues. This will only work if you plan on leaving the garage as an unconditioned or unheated space. If you want to heat the attached garage and minimize the problems, costly renovations to the entire roof system will be required.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors — Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com