Spray Foam vs Fiberglass Insulation: An Honest Comparison
Spray foam air-seals and insulates in one step, resists moisture and pests, and lasts the life of the building — but costs more upfront. Fiberglass is cheaper and fine in dry, easy-to-access cavities, but it doesn't stop air leaks, can sag and absorb moisture over time, and delivers a lower real-world R-value. Choose foam where air sealing and moisture matter; fiberglass where budget rules and the cavity is dry.
We install spray foam for a living — but we'll give you the honest version, including when fiberglass is the smarter spend. The single biggest difference isn't R-value on paper. It's air sealing. That's where most of the real-world performance gap comes from.
Air sealing: the deciding factor
Up to 40% of a home's energy loss comes from air leakage, not from heat conducting through the insulation. Spray foam expands to fill every gap and crack, creating a continuous air barrier in one pass. Fiberglass batts are an air filter, not an air seal — air moves right through and around them. You can have a perfectly installed R-13 batt and still feel a draft, because the air finds the gaps the batt can't close.
R-value: paper vs. real world
Per inch, the materials look close to fiberglass: fiberglass batts are roughly R-3 to R-4 per inch, open-cell foam ~R-3.5, closed-cell ~R-6 to R-7. But fiberglass only hits its rated R-value when it's perfectly installed, dry, and with no air moving through it — rare in a real attic. Foam holds its rating because it also stops the air movement that degrades fiberglass performance. (For the foam-on-foam breakdown, see our open-cell vs. closed-cell guide.)
| Factor | Spray foam | Fiberglass |
|---|---|---|
| Air sealing | Yes (air barrier) | No (air passes through) |
| R-value per inch | ~3.5 (open) / 6–7 (closed) | ~3 to 4 |
| Moisture resistance | High (closed-cell) | Absorbs, can stay wet |
| Mold risk | Low | Higher if it gets wet |
| Pests / rodents | Doesn't nest easily | Common nesting material |
| Sagging over time | No (adheres) | Can settle & sag |
| Lifespan | Life of building | ~15–25 yrs, may need replacing |
| Upfront cost | Higher | Lower |
| Install | Professional spray | DIY-friendly |
Moisture & mold
Fiberglass absorbs water and stays wet. Wet fiberglass loses most of its R-value and becomes a substrate for mold. Closed-cell spray foam resists bulk water and acts as a vapor barrier, so it keeps moisture out of the assembly — the reason it's the only sensible choice in crawl spaces, basements, and metal pole barns where condensation forms.
Pests & longevity
Mice, insects, and birds happily nest in fiberglass — it's soft, fluffy, and easy to tunnel. Spray foam is a rigid, bonded mass that pests don't nest in and that doesn't sag, settle, or fall out of the cavity over time. Fiberglass commonly needs attention or replacement at the 15–25 year mark; quality spray foam typically lasts the life of the building.
Cost & ROI
Fiberglass wins on upfront cost — it's cheaper material and often DIY. Spray foam costs more to install but pays back through lower energy bills, smaller HVAC loads, and no replacement cost down the road. In leaky, climate-exposed buildings across our tri-state area, the air-sealing savings are where homeowners see the return. The honest math: foam's premium makes the most sense in attics, rim joists, crawl spaces, and metal buildings — the high-leakage, moisture-prone spots.
The federal 25C tax credit expired December 31, 2025 — we never count it in your numbers. Still-active local programs may help: PA via PPL Electric (up to 75% / $500) and UGI Save Smart; NJ via the Clean Energy Program; NY via utility and state energy programs. Flexible financing is available through Hearth.
When fiberglass is still fine
We won't oversell foam. Fiberglass is a perfectly reasonable choice when:
- The cavity is dry, accessible, and already air-sealed by other means.
- You're on a tight budget and the space isn't moisture- or leak-prone.
- It's an interior partition wall where you only need basic sound or thermal separation.
- You're topping up an attic that's already well sealed and just needs more R-value.
The best results often combine both: spray foam to air-seal and handle the tough spots, fiberglass or blown-in to add depth affordably. We'll tell you which approach fits your building. See our attic insulation page for how we scope a project.
Frequently asked questions
Is spray foam really better than fiberglass?
For air sealing, moisture resistance, and longevity, yes — spray foam seals leaks fiberglass can't and lasts the life of the building. But fiberglass is cheaper and works fine in dry, accessible, already-sealed cavities. The right choice depends on the space and your budget.
Does fiberglass insulation stop air leaks?
No. Fiberglass slows heat but lets air pass through and around it, so it acts as a filter rather than a seal. Since up to 40% of home energy loss comes from air leakage, this is the main reason spray foam outperforms fiberglass in the real world.
Will spray foam pay for itself over fiberglass?
Often, yes. Spray foam costs more upfront but lowers energy bills, reduces HVAC load, and doesn't need replacement like fiberglass can. The payback is strongest in leaky, moisture-prone areas like attics, rim joists, crawl spaces, and metal buildings.
Does fiberglass attract mice and mold?
It can. Rodents nest readily in soft fiberglass, and when it gets wet it loses R-value and can support mold growth. Spray foam is a rigid, bonded mass that pests don't nest in and that resists moisture, especially closed-cell.
Can I use spray foam and fiberglass together?
Yes, and it's often the smartest spend. A common approach is spray foam to air-seal and handle moisture-prone spots, then fiberglass or blown-in insulation to add R-value affordably. We'll recommend the right combination for your building.