What it is
Open-cell polyurethane foam expands to roughly a hundred times its liquid volume and cures into a soft, low-density material - typically around half a pound per cubic foot, which is why it is sold as half pound spray foam or 0.5 lb foam - in which the cells are interconnected.
That open structure is the whole story. It is what makes the foam light, cheap per unit of volume, absorbent, acoustically effective, and vapour-permeable.
Where it performs
Air sealing. This is its strongest suit and the one most often overlooked. Open-cell is an excellent air barrier at typical installed thicknesses, and air leakage is the larger part of the problem in most buildings.
Sound. For airborne noise, open-cell substantially outperforms closed-cell. Soft porous materials absorb sound energy; rigid dense ones transmit it. This is the one application where the cheaper product is the better one. See soundproofing.
Filling deep cavities economically. Where the depth is available and the assembly is dry, open-cell fills it at a fraction of the cost per unit volume.
Expansion into awkward geometry. Its high expansion ratio fills irregular spaces that closed-cell would need more passes to cover.
Where it does not belong
Anywhere it can get wet. Open-cell absorbs and holds water. Below grade, in contact with concrete, in a crawl space, in a livestock building or on a steel deck exposed to condensation, it is the wrong product and it will cause the problem it was meant to prevent.
Where depth is limited. At roughly R-3.5 to R-3.8 per inch against closed-cell's R-6, it needs nearly twice the thickness for the same R. In a rafter bay or a van wall, that decides it.
As a vapour retarder. It is vapour-permeable by design. An open-cell assembly in Alberta needs a separate vapour retarder detailed for the climate zone, and getting that detail wrong is how open-cell assemblies fail here.
The Alberta vapour question
This is the point that matters most in our climate, and it is where a lot of bad advice circulates.
In a cold climate, an assembly insulated with open-cell needs its vapour control designed rather than assumed. Where the vapour retarder goes, what its permeance should be, and whether the assembly can dry - these are climate-zone questions with real answers, and getting them wrong puts moisture inside the wall.
Closed-cell sidesteps it by being its own vapour retarder. Open-cell does not, which means it needs a designer's attention rather than a default.
The honest comparison
Open-cell is cheaper per board foot and cheaper per unit of volume filled. It is not cheaper per unit of R-value, and in a depth-limited assembly it may not reach the target at all.
The right way to choose is per assembly, not per building. Many Alberta jobs correctly use both - closed-cell on the exterior envelope, roof and anything below grade, open-cell on interior partitions and floor systems where sound is the goal.
See closed-cell for the other side of the comparison.
Open Cell Spray Foam Insulation: Common Questions
Is open-cell safe to use in Alberta at all? Yes, in the right assemblies with the vapour control designed for it. It is not a product to default to without that design.
Will it soak up a roof leak? It will absorb water, which some regard as a diagnostic advantage - the leak shows itself rather than tracking somewhere else. Either way, it must be able to dry, and a wet assembly needs the leak fixed.
Is it better for sound? Yes, materially. This is the application where it wins outright.
Which should I use in my walls? It depends on cavity depth, the rest of the assembly, the budget and whether a vapour retarder is being installed anyway. See wall insulation - all the legitimate options are laid out there.
Get advice for your assembly
Get a quote and describe the assembly - we will tell you which product belongs in it, including when it is the cheaper one.