What is the moisture permeability difference between open-cell and closed-cell spray foam in a Moncton basement application?

Open-cell spray foam is vapour-permeable (allowing moisture to pass through), while closed-cell spray foam acts as a vapour barrier — a critical difference for Moncton basement applications where moisture control is essential.

In Moncton's Maritime climate, this permeability difference fundamentally changes how each foam type performs on basement walls. Closed-cell spray foam has a perm rating of less than 1.0 (typically 0.8-1.2 perms at 2 inches thickness), making it a Class I vapour retarder that blocks moisture movement from the soil through the foundation wall into your basement. Open-cell spray foam has a perm rating of 15-50+ perms, making it highly permeable to water vapour — moisture passes right through it.

For Moncton basements, this creates a significant application difference. When you apply 2-3 inches of closed-cell foam directly to your foundation wall, you get insulation (R-12 to R-18), air sealing, and vapour control in one application. The foam prevents soil moisture from migrating through the concrete into your basement air, reducing humidity and preventing condensation on cold surfaces during Moncton's humid summers.

Open-cell foam on basement walls requires a separate vapour management strategy. Since moisture can pass through the foam, you need either a vapour barrier on the interior side (6-mil polyethylene over the foam) or a vapour-permeable approach that allows the wall to dry inward. However, this creates complexity — if moisture gets behind the foam (through foundation cracks or bulk water infiltration), it can accumulate against the concrete with no escape path.

Moncton's specific moisture challenges make closed-cell foam the preferred choice for most basement applications. The city's proximity to the Bay of Fundy creates year-round humidity, and many Moncton homes sit in clay soils that retain moisture against foundation walls. Closed-cell foam's impermeability prevents this soil moisture from becoming indoor humidity that would otherwise require constant dehumidification.

The cost difference reflects this performance gap — closed-cell foam runs $4.50-$7.00 per square foot installed in Moncton, while open-cell costs $2.00-$3.50 per square foot. However, closed-cell's vapour barrier properties eliminate the need for separate poly installation and provide better long-term moisture protection in below-grade applications.

When open-cell might work in Moncton basements: interior stud walls built against already-insulated foundation walls, where the primary foundation insulation already handles moisture control, or in basement ceiling applications for soundproofing between floors.

For direct foundation wall application in Moncton's moisture-prone environment, closed-cell spray foam's vapour impermeability makes it the superior choice for long-term performance and moisture management.