Spray Foam vs Fiberglass Insulation
Verdict: Choose spray foam where air sealing, tight spaces or moisture control matter and you can pay more. Choose fiberglass batts for open, easy cavities and a lower price, but air seal first.
Spray polyurethane foam expands to fill gaps. A Department of Energy Building America guide gives open-cell foam about R-3.6 per inch and closed-cell about R-6, and says both are excellent at air sealing. Fiberglass batts are about R-3.2 per inch, so a 3.5 inch batt is near R-11.
Air sealing is the real gap. The guide says spray foam blocks air flow far better than fiberglass or cellulose. DOE also warns that fiberglass loses R-value when compressed and that you should air seal before adding insulation. Foam costs more, and closed-cell is nearly three times open-cell per board foot in that guide.
Spray Foam vs Fiberglass Insulation side by side
| Trait | Spray Foam | Fiberglass Insulation |
|---|---|---|
| R-value per inch | Open-cell about 3.6; closed-cell about 6 | About 3.2 for batts |
| Air sealing | Excellent; fills gaps and voids | Does not stop air flow on its own |
| Cost (DOE guide, per board foot) | Open-cell $0.35 to $0.40; closed-cell $1.10 to $1.25 | Typically lower per R-value |
| Water vapor | Open-cell 5 to 10 perms; closed-cell under 1 perm at 2 inches | Not covered in the sources used here |
| Liquid water | Open-cell absorbs it; closed-cell is hydrophobic | Not covered in the sources used here |
| Space efficiency | Closed-cell gives more R in thin spaces | Needs more thickness for the same R |
| Compression | Not applicable | Compressed batt loses rated R-value |
| Placement | Fills gaps and voids as it expands | Batts must fit without gaps or compression |
| Thermal bridging | Heat still flows through studs and joists | Heat still flows through studs; metal framing loses more |
Last verified October 2026. Typical values; individual animals, products and local prices vary.
Which one fits you
Spray Foam
- Air leaks and drafts are your main problem, such as in an attic or rim joist
- You have thin walls or tight spaces where R-value per inch matters
- You want a moisture barrier from closed-cell foam
- Cavities are irregular, with gaps, voids or odd shapes
Fiberglass Insulation
- You have open attic floors or stud bays and want low cost
- You will seal gaps first with caulk and foam
- Your budget is tight and you want the lowest cost per R-value
- Stud bays or attic floors are regular and open, so batts fit without compression
How this works
The quiz compares air leakage, space, budget and who installs the insulation against the DOE and ENERGY STAR data. It does not size R-values for your climate zone, which depends on location. Quiz answers are weighed equally, and "Not sure" answers are skipped.
Prices in the DOE guide are older and vary widely, so use them for relative cost only. A home energy assessor can recommend where each type fits. Prices in the DOE guide are older and vary widely, so use them for relative cost only. A home energy assessor can recommend where each type fits in your house.
Where these numbers come from
The figures and defaults on this page are based on:
- U.S. DOE Building America: Which Spray Foam Is Right For You?, Gives open and closed-cell R-values, cost, air sealing and vapor data. (checked October 2026)
- ENERGY STAR: Insulation Fact Sheet, Gives fiberglass R-value per inch, compression loss and air sealing advice. (checked October 2026)
Planning guide. R-value needs and prices depend on climate zone and installer, so confirm locally.
Frequently asked questions
Is spray foam better than fiberglass?
For air sealing and R-value per inch, yes. A DOE guide says spray foam blocks air flow far better than fiberglass. It also costs more, so fiberglass can be the better value in open, regular cavities if you air seal first.
What is the R-value of spray foam vs fiberglass?
Open-cell spray foam is about R-3.6 per inch and closed-cell about R-6 per inch, per a DOE guide. Fiberglass batts run about R-3.2 per inch, so a 3.5 inch batt is about R-11.
How much does spray foam cost?
The DOE guide listed open-cell at about $0.35 to $0.40 per board foot and closed-cell at about $1.10 to $1.25, which is older pricing. Today's installed prices vary by region and job size, so get quotes.
Should I air seal before insulating?
Yes. DOE says to air seal before adding insulation, because insulation can hide leaks and make them harder to reach later. Fiberglass also loses R-value when compressed.
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