Spray foam insulation costs $1.50 to $5.00 per square foot installed, roughly two to three times what fiberglass batts cost at $1.00 to $2.60 per square foot, according to 2025 data from HomeGuide. The premium buys a higher R-value per inch and, done right, a tighter building envelope. Whether that premium is worth paying depends on where in the house you are insulating and what it does to the rest of the mechanical system, which is the part that gets skipped in most comparisons.
The two materials, side by side
Fiberglass insulates but does not seal air leaks on its own, since it relies on a separate air barrier to do that job. Spray foam does both in one step, expanding to fill gaps and sealing the cavity as it insulates, which is the real source of its performance advantage over fiberglass and also the reason it costs more to install.
The R-value difference is real on its own, and it splits spray foam into two distinct products. Fiberglass batts provide 2.2 to 2.9 per inch. Open-cell spray foam costs $1.50 to $3.50 per square foot and provides 3.5 to 3.7 per inch. Closed-cell spray foam costs $3.00 to $5.00 per square foot and provides 5 to 7 per inch, roughly double fiberglass, according to HomeGuide.
Open-cell versus closed-cell
Open-cell foam is lighter, softer, and expands more during application, which makes it well suited to interior walls, attics in moderate climates, and sound dampening between rooms. Closed-cell foam is denser and more rigid, and it resists water and adds structural rigidity, which is why it is the standard choice for exterior walls, basements, crawl spaces, and any area prone to moisture.
Using open-cell foam where closed-cell is called for, such as a below-grade wall or a humid crawl space, is one of the more common and more expensive mistakes in insulation planning, since open-cell foam does not resist moisture the way closed-cell does.
What foam does to ventilation, and why that matters
A tightly sealed envelope changes what the house needs from its mechanical systems, and this is the part most cost comparisons leave out entirely. ANSI/ASHRAE Standard 62.2 sets minimum requirements for mechanical and natural ventilation specifically because a well-sealed home no longer gets enough fresh air from incidental leakage the way an older, leakier house does. A home insulated with spray foam is exactly the kind of tight envelope this standard exists for, which means the ventilation plan is not optional. It has to be engineered in, typically as a whole-house mechanical ventilation system, rather than assumed to happen on its own.
There is an upside on the HVAC side that gets less attention than it deserves. According to guidance from the U.S. Department of Energy's Building America program, applying spray foam to the underside of the roof deck to create an unvented, conditioned attic brings the HVAC equipment and ductwork inside the home's thermal envelope. That reduces conductive losses through the ducts and means any air that does leak from the ductwork still contributes to conditioning the house rather than being lost to an unconditioned attic, which is a real efficiency gain in homes where the mechanical system lives in the attic.
Where the extra cost tends to pay off, and where it does not
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Roofline and unvented attics: closed-cell foam at the roof deck is one of the more well-documented, high-value uses of spray foam, since it brings ductwork into the conditioned envelope and controls moisture at a complex assembly.
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Exterior walls in humid or high-moisture climates: closed-cell foam's water resistance earns its premium here more reliably than in a dry climate.
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Rim joists and other hard-to-air-seal details: foam's ability to seal odd geometry in one pass often beats the labor cost of trying to air-seal and then batt-insulate the same area separately.
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Simple, rectangular interior wall cavities in a mild climate: this is where fiberglass batts do nearly as well for meaningfully less money, since there is little complex geometry for foam's sealing advantage to earn its keep.
Questions worth asking before you commit to foam
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Is this application open-cell or closed-cell, and does that match the moisture conditions of this specific area?
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If the attic will be spray foamed at the roofline, has the ventilation plan been engineered to meet ASHRAE 62.2, not just assumed?
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Does moving the HVAC system into a conditioned attic change the equipment sizing calculation, and has that been recalculated rather than carried over from a vented-attic assumption?
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What is the cost difference specifically for the areas being considered, not a whole-house blended number?
Insulation strategy is a decision worth making at the design stage, not after framing, since it affects attic ventilation, HVAC placement, and equipment sizing all at once. Book a design consultation and we can help you think through where foam earns its cost on your specific plan.
Frequently asked questions
Is spray foam insulation worth the extra cost?
It depends on where it is used. Spray foam earns its premium most reliably at the roofline in an unvented attic, in humid climates, and in hard-to-seal details like rim joists. In simple wall cavities in a mild climate, fiberglass batts often perform nearly as well for less money.
What is the R-value difference between spray foam and fiberglass?
Fiberglass batts provide R-2.2 to R-2.9 per inch. Open-cell spray foam provides R-3.5 to R-3.7 per inch, and closed-cell spray foam provides R-5 to R-7 per inch, roughly double fiberglass, according to HomeGuide.
Does spray foam insulation require more ventilation?
Yes, in effect. A tightly sealed home no longer gets adequate fresh air from incidental leakage, which is why ANSI/ASHRAE Standard 62.2 requires engineered mechanical ventilation in well-sealed homes rather than leaving air exchange to chance.
What is the difference between open-cell and closed-cell spray foam?
Open-cell foam is lighter and less expensive, suited to interior walls and moderate climates. Closed-cell foam is denser, resists moisture, and adds structural rigidity, making it the standard choice for exterior walls, basements, and crawl spaces.
Does spray foam let you use smaller HVAC equipment?
It can contribute to that, particularly when foam is applied at the roofline to bring ductwork inside the conditioned envelope, which reduces energy losses through the ducts. Actual equipment sizing should still be calculated for the specific home rather than assumed.
