Vapour barrier required when insulating garage

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This article was published 17/04/2021 (1602 days ago), so information in it may no longer be current.

Question: I have a non-attached garage. I would like to use it periodically over the winter, maybe a couple of times a week, for a workshop. It will not be permanently heated, only when I use it. A 240 Volt, 5,000 watt electric heater and a small forced-air propane heater will be used. Once it is warm enough, I may turn off the propane unit. I am planning to insulate the roof inside and the walls. Is a vapour barrier really needed in this case and does the fibreglass need to be covered?

Thanks, Rob

 

Answer: Insulating garages for winter heating must always include a polyethylene air/vapour barrier or insulation that is restrictive to air infiltration. The location of the insulation must also be a consideration, include adequate ventilation and should be covered in case of fire.

During pre-purchase inspections I frequently run into detached garages which have been improperly, or inadequately, insulated and are sometimes heated. This combination will always lead to potential health and safety concerns. Most often the problems are seen in the ceiling or attic insulation, which is sometimes covered with black or dark grey sections. These discoloured areas are often due to dirt and soot accumulation and are a result or a missing or poorly sealed air/vapour barrier.

The heated air in the garage will easily penetrate the fibreglass batt insulation in this area and carry with it dirt, dust, and other particulates present. Especially with batts improperly installed in between the rafters, immediately underneath the roof sheathing, this is a major defect. I assume this may be your desired approach, based on the wording of your inquiry. If that is your wish, batt insulation should not be used and the only acceptable methods of insulation are to fill the cavities completely with spray-on high density foam or extruded polystyrene sheets. If using the latter, an extra layer with sealed seams should be installed underneath the rafters to provide a good air seal and enough thermal resistance to prevent damage to the roofing.

While the staining of the insulation may be unsightly and somewhat reduce its effectiveness, the real concern is the moisture that accompanies these particulates into the insulation or attic above. If there is poor attic ventilation this moisture will certainly remain in the attic or insulation and condense. Because the attic is much colder than the garage below, this condensation will freeze in the coldest weather and will likely lead to mould and rot growth in the attic framing and sheathing. Just as concerning is the insulation becoming wet from the melted frost and mould growing on the debris suspended inside the fibres. I frequently see water stains and evidence of small puddles of water on the attic side of the poly sheathing in poorly insulated garages.

As far as covering the insulation with some form of sheathing, that is a strong recommendation. There are two reasons for this requirement and both are related to fire and safety issues. Firstly, installing fire-rated sheathing on the inside of the garage exterior walls and ceiling will help protect the wood structure from easily burning, should a fire start in the garage. With your proposed shop, this is always an increased risk, depending on what machinery and products you are working with. You may also want to rethink the use of a propane heater, as that will also increase the chances of a fire and reduce occupant safety due to unvented combustibles in an enclosed space. With fireguard drywall installed, it may prevent significant damage to the garage structure until a fire can be extinguished. Without any protection, even a small fire can quickly melt the poly sheathing over the insulation. Not only can that lead to a very dangerous inferno, it has an additional safety concern.

Most plastics will release toxic fumes when burnt. If you are inside the sealed and heated garage when this occurs it can lead to serious health concerns, or even death. This is especially important if you decide to insulate the underside of the roof with spray foam or rigid panel insulation. The compounds contained within those hardened materials are highly toxic and will be released when set ablaze. In a small enclosed space, like a garage, the fumes may overwhelm occupants before they have a chance to escape to safety.

For some types of spray foam it may be acceptable to cover the inside with a fire-retardant paint to meet fire safety standards. Otherwise, a layer of 5/8-inch thick fireguard drywall is normally required to achieve the desired fire protection. While the surface paper of the drywall may burn, the gypsum core is an excellent fire-retardant material and can significantly extend the time before a fire reaches the wood framing behind. Even more combustible wall sheathing, like plywood or OSB, may provide reasonable protection if painted with a fire-resistant finish.

The final component required for a properly insulated garage is proper ventilation for the attic above the ceiling insulation. This can be easily accomplished on most detached garages by installing a combination of roof and soffit vents. Ensuring the soffits are not blocked by the new insulation is critical for proper performance and prevention of moisture and mould issues.

It is always necessary to install a proper air/vapour barrier in any heated building, or insulation with an integral superior air/vapour permeability rating. This, along with fire-rated sheathing installed on the warm side, will ensure both safety and moisture damage prevention are adequate for your new garage shop.

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