Heated garages need new approach to insulation

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This article was published 02/12/2017 (2848 days ago), so information in it may no longer be current.

Questions: We currently have a garage in London, Ont., that was totally drywalled, both walls and ceiling, and painted and is in perfect condition. We cut a small hole to see if there is a vapour barrier and insulation, and it appears that there is pink fibreglass insulation but no vapour barrier. We want to convert the garage into a granny suite and were told by one contractor that the existing drywall would have to be pulled off, a vapour barrier and insulation inserted, and then new drywall installed for the walls and ceiling.

Is there any other way to accomplish this without tearing down perfectly good drywall? Perhaps installing strapping, vapour barrier and then new drywall? Any ideas would be greatly appreciated.

Thank you,

— Bill

We have an older Winnipeg home with an attached single garage. The deck above the garage is open, with a torched-on membrane as a walk-on deck, so no insulated space above. I use the garage regularly as a temporary woodworking shop, although the car is always parked there overnight. In spring and fall, I have a 240-volt heater to make the space comfortable. Through the coldest winter months, I seldom heat the garage.

I recently removed some old and crumbling drywall in the garage. There was no insulation or vapour barrier underneath. Walls are on 16” centre and looked dry and in good shape. I redid the wiring in the walls and to the ceiling lights, and would now like to insulate the garage. The wall against the house was redone a few years ago with fresh insulation and vapour barrier, on the house side, and drywall. For my remaining three outside walls and garage ceiling, I was thinking of using Roxul insulation and fire-rated drywall.

Should I use vapour barrier on the inside of these three walls and ceiling? Any reading on this has given me variable advice, with cautions about unhelpfully trapping moisture. I read on one site that you can use a slightly permeable barrier that traps moisture less.

Thanks for any advice,

— Todd

Answer: Both of your questions are strangely similar, despite the different roof designs, so I will address them together. Proper insulation and air/vapour barriers are not critical in garages that are unheated. Since both garages are undergoing modifications for increased use as conditioned spaces, proper insulation and air sealing are critical to preventing condensation, mould, rot and other moisture-related issues.

An unheated garage, whether it has a pitched roof or a flat roof, as in the second respondent’s home, does not require thermal insulation in the exterior walls or attic/roof system. The reason for this is that only limited heat will be present in the garage, from leakage through the house walls and warm vehicles. With no heat, there is little chance of condensation and moisture issues inside the cool attic or roof cavity. Also, there should be minimal sources of moisture in an unconditioned garage, so the small amount of warm air that may penetrate the attic or roof system will also be less prone to condense. That is why the first homeowner’s insulated garage has no current issues, even with a missing air/vapour barrier. Even a slightly heated garage with no insulation, as in the second scenario, may have few current problems, other than damaged drywall, because of limited use in the coldest weather.

The issues change completely with the proposed alterations to the use of both garages. Insulating the flat-roofed Winnipeg building will require insulation and air sealing on both the walls and roof/ceiling cavities, but different approaches should be taken for each. I’m not sure what you read about partially permeable materials, but those can do more harm than good.

Your choice of mineral fibre batts for the walls, covered with a properly caulked and sealed 6MIL poly air/vapour barrier, should be fine. But the unvented ceiling cavity between the joists must be treated differently. Because of the limited space for insulation, and none for adequate ventilation, this space should be completely air sealed, rather than using batts and poly.

To accomplish that, the entire cavity should be filled with blown-in, high-density polyurethane foam, or a combination of that and rigid extruded polystyrene. Both of those types of insulation are quite impervious to air, so poly underneath is not necessary. They both also have high R-values, so a thinner layer will suffice relative to conventional batt insulation. That method will prevent the warm air intrusion that could occur with the rather permeable batts. This will minimize damaging heat loss to the roof surface, while eliminating the chances of condensation and moisture issues in the ceiling cavity.

In the other planned conversion of the garage to conditioned living space, the internal environment will be completely different after the changes. Permanent heating and cooling, along with habitation, will increase the relative humidity and drive warm air upward into the insulated attic. While the drywall will provide a partial air barrier, any holes, joints, or openings will allow a fair amount of moist air to penetrate the insulation. That air will cool as it passes through the insulation, condense and create problems. So, removal of the drywall and installation of a well-sealed 6MIL poly air/vapour barrier, with additional attic vents, is the way to go. Alternatively, cutting a hatch into the attic would allow access for a different air sealing approach. The batts could be temporarily removed and a minimum of five centimetres of foam blown on top of the drywall to air seal it. The batts could then be reinstalled on top to provide adequate thermal resistance. That method would eliminate the need to remove the ceiling drywall, but walls would still need to be stripped to install the poly sheathing.

Changing the use of both garages in question, for increased heating and occupancy, will certainly necessitate installation of a better air/vapour barrier. The most cost-effective way may include use of traditional batts and poly, but will likely require partial air sealing with more costly, but superior, high-density foam insulation in both.

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

trainedeye@iname.com