Should I install a vapour barrier in my insulated garage in Bathurst NB?

Yes, you absolutely should install a vapour barrier in your insulated garage in Bathurst — skipping this step is one of the most common and damaging mistakes in NB garage construction. In Bathurst's northern Maritime climate, where winter temperatures regularly plunge to -25 to -35 degrees Celsius and the heating season stretches six full months, the temperature difference between the inside of a heated garage and the outside creates intense vapour drive that pushes moisture through your wall and ceiling assemblies. Without a vapour barrier, that moisture condenses inside the insulation, soaking it, destroying its R-value, and creating ideal conditions for mould growth and wood rot in your framing.

The principle is straightforward: warm air holds more moisture than cold air, and when warm, humid garage air contacts a cold surface — such as the backside of exterior sheathing in January — the moisture in that air condenses into liquid water. In a Bathurst garage, this condensation cycle runs from roughly October through April, meaning moisture accumulates in your wall cavities for months. A 6-mil polyethylene vapour barrier installed on the warm side (interior side) of the insulation stops this moisture migration before it reaches the cold zone where condensation occurs. This is not optional in NB's climate — it is a fundamental building science requirement.

Garage environments are actually more demanding on vapour barriers than typical residential rooms because of the unique moisture sources in a garage. When you drive a snow-covered vehicle into a heated garage in Bathurst, the snow and ice on the vehicle melts and evaporates, releasing a significant volume of moisture into the garage air. Road salt accelerates this process. If you use the garage as a workshop, additional moisture comes from concrete curing, cleaning activities, and even your own breathing and perspiration during physical work. All of this moisture is eager to migrate into your wall and ceiling cavities if there is no vapour barrier to stop it.

Installation Details That Matter

The vapour barrier must be continuous and well-sealed. Install 6-mil polyethylene sheeting over the insulation on the interior side of all exterior walls and the ceiling, overlapping seams by at least 6 inches and sealing them with red Tuck Tape (sheathing tape) or acoustical sealant. Seal the poly around all penetrations — electrical boxes, light fixtures, plumbing pipes, exhaust fans — using vapour barrier boots or acoustical sealant. Every gap, tear, or unsealed penetration allows moisture to bypass the barrier and reach the cold side of the assembly, potentially concentrating moisture damage in that area.

Around electrical boxes, use vapour barrier boxes (poly boots) that slip over the electrical box and seal to the surrounding poly sheet. Standard electrical box installations without vapour barrier boots are one of the most common failure points — warm, moist air leaks around the box into the wall cavity and condenses on the cold sheathing directly behind the box.

If you are using closed-cell spray foam insulation (minimum 2 inches), the spray foam itself acts as both insulation and vapour barrier, and no separate poly sheet is required. This is one of the advantages of spray foam in NB garage applications — it eliminates the need for a separate vapour barrier while also providing superior air sealing. However, spray foam costs $3.50 to $7.00 per square foot compared to $1.50 to $3.00 per square foot for batt insulation with a poly vapour barrier.

Do not install a vapour barrier on both sides of the wall assembly. The exterior side of your garage walls should have a weather-resistive barrier (house wrap such as Tyvek) that blocks liquid water and wind but allows vapour to pass through from inside to outside. If you trap moisture between two vapour barriers — poly on the inside and a non-permeable material on the outside — any moisture that does get into the cavity has no way to dry out, guaranteeing rot and mould.

Under the concrete slab, a separate 6-mil poly vapour barrier should be installed before the concrete is poured to prevent ground moisture from wicking up through the slab. This is a different vapour barrier from the one in your walls and ceiling, and it is equally important. A garage slab without a vapour barrier underneath will be perpetually damp, causing floor coatings to peel, stored items to develop mildew, and a general feeling of dampness in the space.

The cost of a vapour barrier for a typical two-car garage in Bathurst is $300 to $600 for materials and $500 to $1,200 for professional installation — a modest investment that protects thousands of dollars' worth of insulation, framing, and interior finishing. Find experienced garage builders and insulation contractors through the New Brunswick Construction Network at newbrunswickconstructionnetwork.com.