Spray foam wins on performance and fiberglass wins on price. Foam delivers R-3.6 to R-7 per inch depending on type and seals air completely; fiberglass delivers about R-3.2 per inch and does not stop air at all. For most Utah County retrofits the deciding factor is air leakage, which is where fiberglass simply does not compete. Utah Valley Foam is an independent insulation referral service covering Utah County, and the honest version of this comparison includes the cases where fiberglass is the right call.
Head to Head
| Closed cell foam | Open cell foam | Fiberglass batts | |
|---|---|---|---|
| R-value per inch | R-6 to R-7 | R-3.6 to R-3.9 | about R-3.2 |
| Depth to reach R-49 | 7 to 8 inches | 13 to 14 inches | 15 to 16 inches |
| Stops air leakage | Yes, completely | Yes, completely | No |
| Vapor barrier | Built in at 2 inches | No | Separate install |
| Fills irregular cavities | Expands to fill | Expands to fill | Leaves gaps |
| Sags or settles over time | No | No | Yes |
| Safe against concrete | Yes | No | No |
| DIY friendly | No | No | Yes |
| Installed cost | $1.00 to $1.70 / board foot | $0.45 to $0.75 / board foot | Lowest |
Why Air Sealing Decides It in Utah
R-value measures how well a material resists heat passing through it, and bigger is better. What it does not measure is air moving around the material. Fiberglass is fibrous and air passes straight through, which is why a fiberglass attic can measure R-38 on paper and still bleed heat through every wire hole, top plate, and can light.
In a climate zone 5 winter that gap between the paper number and the real performance is the whole ball game. It is also why ice dams keep forming on roofs that technically have “enough” insulation. Foam seals and insulates in one pass, so the installed performance matches the specification.
When Fiberglass Is Genuinely the Better Choice
Said plainly, because pretending otherwise would be useless:
- When the real alternative is doing nothing. A budget that stretches to fiberglass but not foam should buy fiberglass. An under insulated attic is a worse outcome than a cheaply insulated one.
- Open, accessible attic floors with simple geometry. No complex framing, no knee walls, few penetrations, and the air sealing handled separately. Fiberglass performs close to its rating in that situation and costs far less.
- When you want the assembly reversible. Batts come out. Foam is permanent. If you expect to run wiring, move ducts, or renovate soon, permanence is a drawback.
- Topping up an attic that is already sealed and already close. If the ceiling plane, the flat surface between your top floor and the attic, was properly sealed and the attic sits at R-30, blowing more loose fill to reach R-49 is the cheapest R-value you will ever buy.
- Genuine DIY. Batts are homeowner installable. Spray foam is not, and should not be.
Which to Use Where
| Situation | Our recommendation |
|---|---|
| Rim joists | Closed cell. The rim joist is the band of wood where your floor framing sits on top of the foundation, and it needs air sealing and vapor control in one step. |
| Basement walls, below grade | Closed cell. Fiberglass against concrete traps moisture. |
| Crawl space walls | Closed cell. |
| Vented attic floor, complex framing | Foam, or fiberglass over a proper foam air seal. |
| Vented attic floor, simple and already sealed | Blown fiberglass to R-49. Cheapest route. |
| Unvented roof deck, HVAC in the attic | Open cell, or closed cell where depth is tight. |
| Interior walls for sound | Open cell, or fiberglass if budget is the constraint. |
Spray Foam vs Fiberglass FAQ
Is spray foam worth the extra cost over fiberglass?
In a leaky older Utah County home, usually yes, because you are buying the air seal rather than just the R-value. In a home that is already well sealed and only needs more depth on the attic floor, fiberglass gives more R-value per dollar and foam is hard to justify.
How much more does spray foam cost than fiberglass?
Open cell runs $0.45 to $0.75 per board foot and closed cell $1.00 to $1.70. Fiberglass is cheaper per unit of R-value in every case. The comparison only becomes favourable to foam once you price the air sealing that fiberglass cannot do.
Can you put spray foam over existing fiberglass?
Not usefully in an attic. Foam needs to bond to the ceiling plane to seal it, and a layer of fiberglass in between defeats that. Clean dry fiberglass can sometimes stay if we are adding depth above a separate air seal, but where sealing is the goal the old material comes out. See insulation removal.
Does fiberglass meet Utah code?
Yes, if installed to the required R-value. Climate zone 5 asks for R-49 in the ceiling, which is roughly 15 to 16 inches of blown fiberglass, and installers typically blow deeper to allow for settling. Full breakdown on our Utah insulation code requirements page.
Which lasts longer?
Foam. It does not sag, settle, or absorb moisture, and it stays put for the life of the building. Fiberglass compresses and settles over time, and loses most of its value if it ever gets wet.
If you want a straight opinion on which fits your house, describe the project on the phone. Call (801) 691-7136, or compare closed cell and open cell directly.