Spray Foam vs Blown-In Insulation
Blown-in and spray foam both fill a cavity, but they behave very differently once the job is done. Here is an honest side-by-side for Minnesota homes.
Two ways to fill a cavity, two different results
Blown-in insulation and spray foam are both applied with specialized equipment rather than hand-fitted like a batt, which leads some homeowners to assume they are more similar than they actually are. In practice, they behave quite differently once the job is finished, and the differences matter most in exactly the areas Minnesota homeowners tend to ask about: attics, and how well a material holds up over years, not just on installation day.
This post compares blown-in insulation, fiberglass or cellulose loose-fill blown into an attic or wall cavity, against spray foam, on R-value per inch, air sealing, long-term performance, and where each one tends to make the most sense.
How blown-in insulation works
Blown-in insulation is loose material, typically fiberglass or cellulose, blown into place with a machine through a hose. In an open attic, it is distributed across the attic floor to a target depth. In an enclosed wall cavity, it is blown in densely enough to fill the space without needing to be removed later.
It is a fast, relatively affordable way to insulate a large, open attic floor, and it is also the standard way to top up existing attic insulation that has settled or fallen below a proper target, since it can be added directly over what is already there without removing it first.
How spray foam differs
Spray foam is applied wet and expands and cures in place, bonding to the framing and any surrounding materials rather than simply resting in the cavity. That is the fundamental difference between the two products: blown-in insulation fills a space, spray foam seals and fills it at the same time.
Because it cures into a fixed, rigid or semi-rigid material, spray foam does not shift, settle, or compress the way loose-fill material can over time.
R-value per inch, and how it holds up over time
Per manufacturer product data sheets, blown fiberglass is generally rated in roughly the R-2.2 to R-2.7 per inch range at typical installed density, and blown cellulose is generally a bit higher, roughly R-3.2 to R-3.8 per inch. Spray foam, again per manufacturer data sheets, runs roughly R-3.5 to R-3.7 per inch for open-cell and roughly R-6 to R-7 per inch for closed-cell, higher than either blown material on a straight per-inch basis.
The other factor worth knowing is that loose-fill insulation can settle over time, which lowers its installed depth, and therefore its effective R-value, below the depth it was originally blown to. That is a normal characteristic of loose-fill material, not a sign of a bad installation, and it is part of why attics topped with blown-in insulation years ago are often found sitting below their original target depth during an inspection. Cured spray foam does not settle in the same way once it is properly installed.
Air sealing is where the two diverge the most
Blown-in insulation, like fiberglass batts, adds thermal resistance but does not stop air movement on its own. Loose material poured or blown over an unsealed attic floor still allows warm, moist house air to bypass it through gaps around can lights, plumbing stacks, the attic hatch, and the top plates of interior walls. According to the U.S. Department of Energy, air leakage of this kind can account for 30 to 40 percent of a home's total heat loss, which is exactly why attic air sealing is recommended as a step before, not instead of, adding blown-in insulation.
Spray foam air-seals and insulates in the same application, which is the main reason it tends to close the gap between rated performance and real-world performance more reliably, particularly in an older attic with a lot of penetrations that were never properly sealed.
Where each one tends to make sense
A general guide, not a rule that applies to every home.
A realistic look at cost
Blown-in insulation is generally less expensive to install per square foot than spray foam, since the material and application process are both simpler and the equipment involved is less specialized. That cost gap is a real factor in the decision for a lot of homeowners, particularly on a large, open attic floor where blown-in insulation can cover a lot of area efficiently.
Spray foam costs more upfront, but it is also solving a different problem in many cases, air sealing plus insulation in one step, rather than insulation alone. Our spray foam cost guide covers general industry pricing ranges if you want a sense of where a project typically falls before requesting a specific estimate.
The bottom line
For a straightforward, well-air-sealed attic floor, blown-in insulation is a proven, cost-effective choice, and it is what we recommend often for that specific application. For rim joists, cathedral ceilings, and any project where air sealing is the priority alongside R-value, spray foam is generally the better-performing choice, even at a higher upfront cost.
The most common approach we recommend for a whole attic project is actually a combination: air seal the ceiling plane first, sometimes with a small amount of spray foam at the trickiest gaps, then bring the rest of the attic floor up to target depth with blown-in insulation. An inspection is where we figure out what combination makes sense for your specific attic.
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