Renovation & Design

Insulating a garage a worthy project but proper steps required for health, safety

Controlling fire, containing gases critical considerations

Question: I have an attached, unheated garage. I am considering spray-foaming the walls and ceiling. The roof vents will still be exposed and the shared wall to the house has drywall. My question is, do I need to drywall over the exposed spray foam on the exterior walls as well? I do not intend to heat the garage permanently, but will use a space heater if I need heat to work in the winter. Thanks for your time, Sean.

Answer: Using high density polyurethane spray foam to insulate an attached garage may be a very good idea, but leaving it exposed is not. This product not only provides excellent thermal protection for the thickness, it is also water resistant and an excellent air barrier. Because of these properties, concerns with moisture and condensation issues, common in garages, should not be an issue. The only downside of using spray foam is the cost, which may be substantially more than other types of traditional insulation. For the health of your family and for fire protection, spray foam must be covered with a fire-rated sheathing.

It is unclear from your question whether your attached garage already has a drywall ceiling, or if you are planning on spraying the underside of the roof sheathing with the foam insulation. If you currently have a drywall ceiling, and have not made the common error of referring to the underside of the roof as a ceiling, then there should be no concern. The ceiling should already be constructed from 5/8-inch fireguard drywall, which should be secured to the bottom of the trusses or ceiling joists. The foam could be blown in between the bottom of the trusses, or ceiling joists, above this proper sheathing. The gypsum sheathing will then provide all the covering you require, both for aesthetics and fire safety.

If you are planning on blowing the foam onto the underside of the roof sheathing, to give a more spacious feeling to the insulated space, there are other things to consider. First, the current roof vents will have to be removed, and the roofing patched, as they will no longer be required. Adding this type of insulation to the underside of the roof, in sufficient quantity, will provide a sealed roof system, rather than a traditional vented attic space. Putting the air and moisture resistant insulation in this location will stop heat and air from transferring through to the roofing and the exterior, so venting the top of the garage would be counterproductive. If this is indeed your proposal, the insulation should certainly be covered with fire-rated sheathing to prevent burning of the foam during a fire.

Any exposed foam insulation, whether it is spray applied or in rigid sheets, should be covered with fire-resistant sheathing inside the living space, or garage. The main reason for this is to prevent easy combustion of the plastic insulation in the event of a fire. When this type of insulation is burned, it may give off toxic fumes that can be fatal to anyone occupying the same enclosed space. The entry door to the home from the garage is required to be a solid-core, fire rated door, with a self-closer and weatherstripping, for the same reason. Neither exhaust from the vehicles, or fumes from a garage fire, should be allowed to easily enter the living space, which could compromise the health of those inside the home.

Adding a layer of fireguard drywall, or other fire-rated sheathing, will extend the time it takes for a raging fire to penetrate to the highly combustible foam insulation. This may be a matter of several minutes, which can be critical in allowing safe escape of the home occupants. Especially with a fire in an attached garage, which may go unnoticed until it is quite severe, preventing deadly smoke and toxic fume intrusion until the home can be vacated may save lives. If your home is newer, it may also have a heat sensor mounted on the ceiling, which should be connected to the home’s hard-wired smoke detectors. This will alert anyone in the living space to the danger while it is still safe to evacuate through the exterior doors.

Another alternative, which will likely be less costly, is to install the spray foam on the walls only and insulate the attic with traditional insulation. Blowing in cellulose fibre insulation above the garage ceiling, after ensuring a proper 6MIL poly air-vapour barrier is in place, may be a better idea. Especially since you will have enough space to add much more insulation than with the foam, a higher level of thermal resistance can be reached, with less expense. You will also be able to leave the roof vents in place, since they will be required to vent the newly insulated attic. In that situation you will still need to cover the wall foam with sheathing. Also, ensuring vented soffits are left open for proper attic airflow will provide a better insulated garage for less cost.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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