What Is Open-Cell Spray Foam Insulation?
Lighter, softer, and more budget-friendly than its closed-cell counterpart. Here is what open-cell spray foam insulation is and where it makes sense in Minnesota.
The lighter of the two spray foam types
Open-cell spray foam insulation comes from the same basic two-component chemical reaction as closed-cell foam, an isocyanate and a polyol resin that mix at the spray gun and expand on contact. The difference is in the formulation and the ratio, which produces a softer, lighter, more flexible foam once it cures.
The name comes from its structure. Instead of sealed, individual cells like closed-cell foam, open-cell foam's cells are broken open and connected to one another, similar in concept to a natural sponge. That open structure is what gives the material its softer feel, its lower density, and its lower cost per board foot.
What the open-cell structure means in practice
Because the cells are connected rather than sealed, open-cell foam is vapor-permeable, meaning water vapor can move through it, unlike the closed-cell foam that acts as a vapor retarder. That is not a flaw, it is simply a different property that fits certain wall and roof assemblies better than a vapor-retarding material would.
Open-cell foam also expands to a much larger volume from the same amount of liquid material, which is part of why it typically costs less per board foot installed than closed-cell foam, even though it has a lower R-value per inch.
R-value: what open-cell foam delivers per inch
Manufacturer data sheets commonly publish open-cell spray foam at roughly R-3.5 to R-3.7 per inch, lower than closed-cell foam's roughly R-6 to R-7 per inch, but still ahead of most batt insulation on an air-sealing basis. In a standard framing cavity with adequate depth, open-cell foam can reach a strong total R-value while air sealing the cavity in the same application.
A deeper 2x6 wall cavity, roughly 5.5 inches, sprayed full with open-cell foam using that per-inch range lands close to the high teens for total R-value, which is why open-cell foam tends to show up more often in newer construction with deeper wall cavities than in older Minnesota homes built with narrower 2x4 framing.
A quieter house, as a side benefit
One property that comes up often with open-cell foam is sound. Its softer, more open structure tends to dampen airborne sound better than the dense, rigid structure of closed-cell foam, which is why open-cell foam sometimes gets specified in interior walls around a home theater, office, or bedroom even when air sealing to the exterior is not the primary concern.
That is a real, structural property of the material, not a marketing claim, though the degree of sound reduction depends on the whole wall assembly, framing, drywall, and any other materials involved, not the insulation alone. It is a detail worth mentioning to your installer if a quieter interior is part of what you are trying to achieve, not just lower bills.
Where open-cell spray foam is typically used
Why vapor permeability is not automatically a downside
It is easy to hear vapor-permeable and assume that is worse than a vapor retarder, but that is not the right way to think about it. Some wall and roof assemblies are actually designed to let a small amount of moisture move through and dry in a particular direction, and a vapor retarder in the wrong spot can trap moisture rather than manage it.
Where open-cell foam is used in a cold climate assembly, it is common to pair it with a separate vapor retarder, like poly sheeting, on the warm-in-winter side, rather than relying on the foam itself to control vapor. Getting that pairing right is a building science decision, which is exactly why it is worth discussing with a spray foam installer who understands how the whole assembly works together, not just the product being sprayed.
This is one of the areas where an experienced spray foam insulation contractor earns their fee. Two homes with similar wall construction can call for different vapor strategies depending on the siding, the sheathing, and the specific roof or wall assembly involved, and getting it wrong in either direction can trap moisture rather than manage it.
Open-cell versus closed-cell: the short version
Open-cell foam costs less per board foot, dampens sound better, and is vapor-permeable. Closed-cell foam costs more, delivers roughly double the R-value per inch, and acts as an air and vapor barrier in one step. Neither one is universally better, the right choice depends on the cavity, the climate exposure, and what the wall or roof assembly is designed to do. Our guide comparing the two goes through the decision in more detail, and our cost guide covers general pricing ranges for each.
Finding the right fit for your project
The best way to know whether open-cell foam, closed-cell foam, or a combination of both is right for your home is an on-site look at the specific cavities involved. Trust Works Company has installed both products across Minnesota for more than 20 years, from our base in Savage to homes statewide. A free estimate is where we look at your specific situation and give you an honest recommendation, not a one-size-fits-all answer.
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