Can I use batt insulation in a crawl space in New Brunswick? | Insulation IQ?

Technically, yes — batt insulation can be installed in a crawl space in New Brunswick. But whether you should depends heavily on the type of crawl space you have, how it is detailed, and whether you understand the serious moisture risks involved. In NB's Climate Zone 6 environment, getting crawl space insulation wrong is one of the most reliable ways to end up with a mould problem.

First, it is important to understand the two fundamentally different approaches to crawl spaces: vented crawl spaces and unvented (conditioned) crawl spaces. These require completely different insulation strategies.

In a vented crawl space, the intent is to allow outdoor air to flow through and carry away any moisture vapour that migrates up from the ground. In this configuration, you would install batt insulation between the floor joists above the crawl space — essentially insulating the floor of the living space, not the walls of the crawl space itself. The crawl space remains outside the thermal envelope. For this to meet the NB Building Code (which aligns with the NBC 2015 as adopted), you need at least R-20 in the floor assembly, though R-24 or R-28 is more appropriate for the cold winters in Fredericton, Moncton, and Saint John. A vapour barrier is still required on the ground (6 mil polyethylene minimum) to reduce ground moisture rising into the space. Batt insulation must fit snugly between joists with no gaps or compression — any air movement through or around batts drastically reduces their thermal performance.

However, vented crawl spaces are increasingly recognised as problematic in cold climates. In New Brunswick winters, cold outdoor air entering the vents chills the crawl space floor, which can freeze pipes and cause significant heat loss through the floor above. Many older NB homes with vented crawl spaces suffer from exactly this issue.

The more energy-efficient and moisture-safe strategy today is an unvented conditioned crawl space, where the vents are sealed and the crawl space walls and rim joists are insulated instead. In this configuration, batt insulation between the floor joists is actually the wrong choice — you'd be insulating the wrong surface. The crawl space walls should be insulated with rigid foam or spray foam to bring the space inside the thermal envelope. If batts are used on the walls, they can absorb moisture and sag over time. Spray foam or a combination of rigid foam plus a vapour-control membrane is strongly preferred for conditioned crawl spaces.

There is another critical concern: ground moisture. New Brunswick has significant seasonal moisture fluctuations, and crawl spaces are exposed to ground vapour year-round. Any batt insulation that gets wet — even from condensation — loses R-value immediately and becomes a substrate for mould and rot. Fibreglass batts are especially susceptible to holding moisture against wood framing. Mineral wool batts are slightly more moisture-tolerant but are not immune to the problem.

For homes that do use batt insulation in a vented crawl space floor, the vapour retarder placement matters. The vapour retarder (typically kraft facing) must face the warm side — toward the living space above — not toward the crawl space. This slows moisture-laden interior air from entering the insulation cavity.

The NB Building Code requires crawl spaces to have a ground cover vapour barrier regardless of the insulation strategy. Under Section 9.25, the ground cover must be 0.10 mm (4 mil) polyethylene as a minimum, though 6 mil is the industry standard. Lapping seams by at least 300 mm and sealing to foundation walls significantly reduces ground moisture intrusion.

In terms of cost, installing R-20 to R-28 batts between floor joists in a crawl space typically runs $1.50 to $3.50 per square foot installed in New Brunswick, depending on access difficulty. Tight, cramped crawl spaces cost more due to labour. Spray foam on crawl space walls for a conditioned approach runs considerably more — $3 to $6 per square foot — but delivers a more durable, moisture-resilient result.

If you are unsure which approach is right for your home, a qualified insulation professional can assess your crawl space's current condition, moisture levels, and framing before recommending a solution. New Brunswick Insulation connects homeowners across the province with experienced contractors who understand the specific demands of NB's climate.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• 3Tone Construction Ltd
• Brunswick insulation & roofing
• moose luxury painting
• Thirty Four Renovations
• Arctic Fox Construction Inc.