Fiberglass vs Spray Foam Insulation: Cost, R-Value, and Performance
Quick Answer
- *Fiberglass: $0.50-$1.50/sq ft installed, R-3.1 to R-3.7 per inch, no air sealing, DIY-friendly.
- *Spray foam (closed-cell): $1.50-$3.50/sq ft installed, R-6 to R-7 per inch, excellent air sealing, professional only.
- *Spray foam costs 2-3x more but delivers nearly 2x the R-value per inch and seals air leaks that fiberglass cannot.
- *Best approach for most homes: spray foam in critical air-leak areas, fiberglass everywhere else.
| Feature | Fiberglass | Spray Foam (Closed-Cell) |
|---|---|---|
| Cost installed | $0.50-$1.50/sq ft | $1.50-$3.50/sq ft |
| R-value per inch | R-3.1 to R-3.7 | R-6 to R-7 |
| Air sealing | None | Excellent (fills every gap) |
| Moisture barrier | No (absorbs water) | Yes (vapor barrier) |
| DIY possible? | Yes (batts and blown-in) | No (requires pro equipment) |
| Lifespan | 80-100 years | 80+ years |
| Structural strength | None | Adds racking strength to walls |
What Is Fiberglass Insulation?
Fiberglass insulation is made from extremely fine glass fibers spun into a fluffy mat. It comes in two forms: pre-cut batts (sized for standard wall cavities) and loose-fill (blown in with a machine). It has been the most common residential insulation for over 60 years.
Fiberglass works by trapping air in millions of tiny pockets between glass fibers. Still air is a poor conductor of heat, so the trapped air slows heat transfer. R-value ranges from R-3.1 to R-3.7 per inch depending on density. For a 2x4 wall (3.5 inches), that delivers R-11 to R-13.
What Is Spray Foam Insulation?
Spray foam is a two-component chemical mixture sprayed as a liquid that expands and hardens into a rigid or semi-rigid foam. Closed-cell spray foam (R-6 to R-7/inch) is dense, rigid, and acts as both insulation and air/vapor barrier. Open-cell spray foam (R-3.5 to R-3.8/inch) is softer and cheaper but does not block moisture.
Spray foam’s defining advantage is air sealing. It expands to fill every crack, gap, and irregular surface. Air leakage accounts for 25-40% of heating and cooling loss in typical homes — more than insulation R-value alone. By stopping air movement, spray foam can outperform fiberglass even at similar R-values.
Key Differences
- Cost: Spray foam costs 2-3x more per square foot. For a 1,500 sq ft attic: $750-$2,250 for fiberglass vs $2,250-$5,250 for closed-cell spray foam.
- R-value per inch: Spray foam delivers nearly double the insulation in the same thickness. Critical in spaces where cavity depth is limited.
- Air sealing: The biggest practical difference. Fiberglass does not stop air movement. Spray foam does. In leaky older homes, this alone can cut energy bills 20-30%.
- Moisture: Closed-cell spray foam repels water. Fiberglass absorbs it, losing R-value and potentially growing mold.
- Installation: Fiberglass batts are DIY-friendly. Spray foam requires professional equipment ($300-500/day rental for DIY kits, but pros do it better).
When to Use Fiberglass
- Standard wall cavities in new construction or renovation (most cost-effective).
- Attic floors (blown-in fiberglass is cheap and effective when air-sealed separately).
- Budget-constrained projects where air sealing can be done separately with caulk and foam tape.
- DIY projects where you want to do the work yourself.
When to Use Spray Foam
- Rim joists, band joists, and sill plates (critical air leak areas).
- Crawlspaces and basements where moisture resistance matters.
- Attic rooflines when creating conditioned attic space.
- Older homes with extensive air leakage that cannot be sealed with caulk alone.
- Tight spaces where maximum R-value per inch is needed.
The Bottom Line
Fiberglass is the workhorse of residential insulation — cheap, effective, and proven over decades. Spray foam is the premium option that excels at air sealing and delivers higher R-value per inch. The smartest strategy for most homes: use spray foam where air sealing matters most (rim joists, crawlspace, attic penetrations) and fiberglass everywhere else.
Calculate your insulation needs with our insulation calculator.
Frequently Asked Questions
Is spray foam insulation worth the extra cost?
For most homes, closed-cell spray foam in critical areas (attic, crawlspace, rim joists) combined with fiberglass in walls offers the best value. Spray foaming an entire house costs 3-5x more than fiberglass and the energy savings do not always justify the premium. However, in extreme climates, very tight building envelopes, or problem areas with air leaks, spray foam's air-sealing ability can reduce heating/cooling costs by 30-50%.
What R-value do I need for my attic?
The Department of Energy recommends R-38 to R-60 for attics in most US climate zones. That translates to 10-16 inches of fiberglass batts or 6-10 inches of spray foam. Warmer southern climates need R-30 to R-38. Colder northern climates need R-49 to R-60. Check the DOE's recommended R-values by ZIP code for your specific location.
Does spray foam insulation cause moisture problems?
Closed-cell spray foam is a vapor barrier and actually prevents moisture problems when installed correctly. Open-cell spray foam is vapor-permeable and can trap moisture in some assemblies if not paired with a proper vapor retarder. The key is correct installation by a certified contractor. Poorly installed spray foam (incorrect mixing ratio, wrong temperature) can off-gas, shrink, or fail to adhere — always hire experienced, licensed installers.
Can I install fiberglass insulation myself?
Yes. Fiberglass batt insulation is one of the most DIY-friendly home improvement projects. Unfaced or kraft-faced batts fit between standard 16-inch or 24-inch stud spacing. Wear long sleeves, gloves, safety glasses, and an N95 mask to avoid skin and lung irritation from glass fibers. Blown-in fiberglass requires a machine (available for rent at home improvement stores) but is also DIY-feasible for attics.
How long does each type of insulation last?
Fiberglass insulation lasts 80-100 years if it stays dry and undisturbed. It does not degrade chemically. However, it can settle over time (losing 10-20% of R-value in loose-fill applications) and loses effectiveness if compressed or wet. Spray foam lasts 80+ years and does not settle. Both outlast the buildings they are installed in. The practical lifespan is limited by renovation, pest damage, or moisture issues rather than material degradation.
Calculate your insulation needs
Enter your space dimensions and target R-value to get material quantities and cost estimates.
Insulation Calculator