Renovation & Design

To insulate or not to insulate? That is the (popular) question

Katherine Frey / Washington Post files

Whether to insulate the exterior walls of an attached garage is a common question, both during inspections and from many owners of newly built homes.

Question: My husband and I are in the process of buying our first new home in Ottawa. We were researching regarding insulating the garage walls and stumbled upon your detailed and insightful response regarding the same issue. Some information about our situation: the wall joining the living area of the house, the garage ceiling and garage doors will be insulated as per the builder. The garage will not be heated.

Our question is: should we have the side walls of the garage insulated and sealed, or not? It definitely is expensive — that’s why we want to know whether it’s worth the cost.

We would really appreciate your assistance/input on this issue.

Thanks in advance.

— Rajnoop Kaur

Answer: Whether to insulate the exterior walls of an attached garage is a common question, both during inspections and from many owners of newly built homes. The answer will depend on the location of the home, exposure to wind and other elements and planned use of the garage space.

Insulating the walls in a newly built home may provide some benefit, but is only critical if the garage is to be heated. If you are planning on heating the garage, even temporarily, to warm your vehicles or use as a shop, then insulation and air-sealing is required. This is because you will cause a significant temperature differential between the exterior walls and the interior space of the garage if it is mechanically heated. This will certainly cause condensation and frost to develop the inside of the garage wall sheathing, potentially leading to moisture damage and mould growth. In that situation, insulation will prevent excessive heat loss, while air-sealing the area will prevent warm-air intrusion into the wall cavity, and condensation. It will also make heating the cool space easier, lowering your energy cost.

If you do not plan to heat your garage, then insulation may be more of a hindrance than a help. This may seem contradictory, but leaving the wall cavities open allows for better air circulation and may help evaporate any small amount of condensation that does occur. You may still have some issues with condensation, but these are normally limited to the garage windows and can be easily managed. Leaving vehicle doors open for a few minutes after parking warm vehicles in cold weather may keep the garage windows from sweating and freezing up.

One problem with insulating attached garages is the choice of wall coverings for the insulated walls. Most building codes require a fire-resistant drywall layer to be installed inside the walls. This will certainly be the case for the common wall with the home and ceiling, but may also be required for any perimeter, exterior walls. Drywall is not very moisture-resistant and can easily be damaged or become mouldy if repeatedly wetted. Since attached garages are prone to some snow and debris intrusion, moisture near the bottom of the walls is common. Also, bringing in lawn mowers, yard tools, bikes and other recreational equipment may cause the wall coverings to get wet or damaged.

Since most modern garage walls are sheathed on the outside with OSB or plywood, they can be quite moisture-resistant, even with frequent wetting, as long as they can easily dry afterward. So, another reason to avoid insulating the exterior garage walls is to prevent damage to wall sheathing not well-designed for wet areas. Alternatively, the insulation could be covered with OSB, plywood or cement sheathing, as long as it is allowed by local building officials. If fireguard drywall is required, it should be left a minimum of 12 mm above the garage floor to prevent wicking moisture from the concrete. It could also be covered with a layer of moisture-resistant sheathing after installation to prevent surface damage and moisture-related issues.

To touch upon my earlier point, the location and configuration of your garage may ultimately determine whether you should be insulating all the walls. If your uninsulated garage walls are openly exposed to strong winds, northern or western exposures or have no adjacent homes or buildings nearby, insulation is more likely. If you have lots of vegetation or trees near the garage, this will reduce the effects of wind and cold on the building. Also, many newer subdivisions have multiple homes built in very close proximity, or even semi-attached, which would provide considerably more protection from cold winter weather. If the garage is primarily south-facing, it will get additional solar gain, making it warmer than a garage on the north side of the home and minimizing the need for additional thermal insulation.

If you are planning on using the attached garage in your newly built home for a workshop, or for vehicle maintenance, then insulating it for temporary heating should be done. If you have no plans to use it other than for vehicle and tool storage and access to the home, then insulating the walls is at your discretion. Insulating the walls will make the garage slightly more comfortable, but will not be cost-effective in energy reduction if it is not heated.

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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