What is the difference between closed-cell and open-cell spray foam for a GTA basement?

Closed-cell spray foam is the clear choice for GTA basement foundation walls because it provides higher R-value per inch, acts as its own vapour barrier, and resists moisture — while open-cell spray foam absorbs water, requires a separate vapour barrier, and is generally not recommended for direct application against below-grade concrete walls. Understanding the fundamental differences between these two products will help you avoid a costly mistake in your basement insulation.

The core difference is in the cell structure. Closed-cell spray foam has tiny, completely sealed cells filled with a blowing agent that gives it exceptional insulating properties — R-6.5 per inch. Because the cells are sealed, water and water vapour cannot penetrate through the foam. At 2 inches of thickness, closed-cell spray foam has a vapour permeance low enough to qualify as a vapour barrier under the Ontario Building Code, eliminating the need for separate 6-mil polyethylene sheeting. It is also extremely rigid and adds structural strength to the wall. These properties make it ideal for direct application against cold, damp foundation walls in GTA basements.

Open-cell spray foam has cells that are not fully sealed — they are open and filled with air rather than a blowing agent. This gives it a lower R-value of approximately R-3.7 per inch and makes it permeable to water vapour. Open-cell foam also absorbs water — if moisture migrates through the foundation wall, the foam will become saturated, losing its insulating value and creating conditions for mould growth on the foundation wall behind it. In a GTA basement where the foundation wall is constantly under moisture pressure from clay soils and seasonal groundwater fluctuations, this is a serious concern. Open-cell foam applied to a basement foundation wall requires a separate 6-mil polyethylene vapour barrier on the warm side, adding cost and complexity.

The cost difference is significant. Closed-cell spray foam costs $3.50 to $6.00 per square foot at 2 inches in the GTA, while open-cell foam costs $1.50 to $3.50 per square foot at 3.5 inches (filling a 2x4 stud cavity). At first glance, open-cell appears cheaper, but when you factor in the need for a separate vapour barrier, the lower R-value requiring thicker application, and the moisture risk in a below-grade application, the total system cost difference narrows considerably.

Open-cell spray foam does have legitimate applications in a basement renovation. It is an excellent choice for insulating the basement ceiling for soundproofing purposes — the open-cell structure actually absorbs sound better than closed-cell foam, and moisture is not a concern between heated floors. It is also suitable for above-grade interior walls that are not in contact with the foundation, such as partition walls dividing basement rooms. And for above-grade portions of walkout basement walls, where moisture pressure from the soil side is minimal, open-cell foam can be adequate with a proper vapour barrier.

For the foundation walls themselves — the below-grade concrete walls that define your GTA basement — closed-cell spray foam is the professional recommendation. The higher cost per square foot is justified by the superior moisture resistance, the built-in vapour barrier, the higher R-value per inch (meaning less thickness required, preserving floor space), and the long-term durability in a demanding below-grade environment. An experienced spray foam contractor can insulate the foundation walls and rim joists of a typical 1,000 square foot GTA basement with 2 inches of closed-cell spray foam for $4,000 to $7,000 — an investment that pays dividends in comfort, energy savings, and moisture protection for decades.