How does closed-cell spray foam compare to open-cell spray foam for insulating a basement in New Brunswick?

Closed-cell spray foam is the clear winner for New Brunswick basement walls, and most experienced NB basement contractors will not even offer open-cell spray foam for below-grade applications. The reason comes down to moisture — NB's Maritime climate creates conditions that make open-cell spray foam a risky choice against foundation walls.

Closed-cell spray foam has a density of about 2 pounds per cubic foot and delivers R-6 to R-7 per inch. Three inches gives you R-18 to R-21, well above the NB Building Code minimum of R-12.5. Critically, closed-cell foam is a vapour barrier at 2 inches or more, meaning it blocks moisture migration from the cold foundation wall into the wall cavity. It also resists water absorption — if your basement ever has a minor leak or condensation event, closed-cell foam will not absorb the moisture. In New Brunswick, where foundation walls stay near 0°C at soil contact through the winter months and Maritime humidity runs 70-85% in summer, this moisture resistance is not a luxury — it is essential.

Open-cell spray foam has a density of about 0.5 pounds per cubic foot and delivers only R-3.5 to R-4 per inch. You would need nearly 4 inches to meet code minimum, and even then the performance is mediocre for NB's climate. More importantly, open-cell foam absorbs moisture like a sponge. It is vapour-permeable, meaning water vapour passes through it freely. Against a cold, damp NB foundation wall, this vapour permeability allows moisture to condense within or behind the foam. Over time, this trapped moisture leads to mold growth, musty odours, and potential damage to framing members — exactly the problems you are insulating to prevent.

The cost difference is real but narrower than most homeowners expect. Closed-cell spray foam runs $4.00 to $7.00 per square foot installed in the NB market, while open-cell runs $2.00 to $4.00 per square foot. For a typical 800-square-foot basement, you might save $1,500 to $2,500 by choosing open-cell, but you would then need to add a separate 6-mil poly vapour barrier over the foam and accept the lower R-value and moisture risk. When you factor in the cost of a separate vapour barrier and the risk of moisture problems requiring tear-out in a few years, the savings evaporate.

There is one scenario where open-cell spray foam has a place in an NB basement: above-grade rim joists and interior partition walls that are not against the foundation. For partition walls separating basement rooms, open-cell foam provides decent sound dampening and thermal insulation without the moisture concerns that exist at the foundation wall interface. Some contractors use closed-cell on the foundation walls and open-cell on interior partitions as a cost-saving hybrid approach.

For homes in Saint John with heavy clay soils that hold water against foundations, or coastal areas like Shediac and Bathurst with naturally high water tables, closed-cell spray foam is especially important because these conditions produce the highest moisture loads against foundation walls. In Fredericton and Moncton, the same recommendation holds — NB's climate simply does not allow the vapour permeability that open-cell foam introduces against a below-grade wall.

Hire a spray foam contractor who is experienced with basement applications in New Brunswick and carries proper liability insurance. Spray foam installation requires specialized equipment and training — this is not a DIY project. Always confirm the installer will achieve the thickness needed to meet both your target R-value and the vapour barrier threshold of 2 inches for closed-cell.

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

• Thirty Four Renovations
• Gionetterenovations