Spray Foam vs Fiberglass Insulation: R-Value, Cost & Payback
Spray foam insulates and air-seals simultaneously — a major advantage in energy performance. Fiberglass costs far less but does nothing to stop air infiltration, which accounts for 30–40% of heat loss in most homes.
Head-to-Head Comparison
| Aspect | Spray Foam | Fiberglass Batts |
|---|---|---|
| R-value per inch | 3.7 (open) / 6.5 (closed) | 2.2–2.7 |
| Air sealing | Excellent — monolithic | None — requires separate barrier |
| Moisture barrier | Closed-cell: yes | No |
| Material cost (per sq ft) | $1–3 (open) / $2–5 (closed) | $0.30–0.60 |
| Installed cost (per sq ft) | $1.50–4 | $0.50–1.50 |
| Lifespan | Lifetime of building | 20–30 years (can settle) |
| DIY friendly | Limited (kit for small areas) | Yes |
| Fire rating | Requires thermal barrier | Passes as-is |
Pros & Cons
Spray Foam
- ✓Air seals and insulates in one step
- ✓Higher R-value per inch — fits more R in thin cavities
- ✓Closed-cell acts as a vapor barrier
- ✓Doesn't sag or settle over time
- ✓Strengthens wall/roof structure slightly
- ✕5–10× more expensive than fiberglass
- ✕Requires professional installation (health hazards uncured)
- ✕Off-gassing during installation
- ✕Difficult to remove for electrical/plumbing work
- ✕Some formulations not environmentally friendly (HFC blowing agents)
Fiberglass Batts
- ✓Lowest cost insulation option
- ✓DIY-friendly — no special equipment
- ✓Easy to install around wiring and pipes
- ✓Widely available, no lead time
- ✓Easy to remove for repairs
- ✕Does not air seal — requires separate vapor/air barrier
- ✕Can sag or settle over time
- ✕Loses effective R-value when air moves through it
- ✕Moisture can accumulate if vapor barrier is missing
- ✕Lower R-value per inch vs foam
Cost Breakdown
Cost for a 2,000 sq ft home
Fiberglass batts in walls and attic: $2,500–5,000 materials + $500–1,500 labor = $3,000–6,500 total. Spray foam (open-cell walls + closed-cell rim joists): $8,000–15,000 professionally installed. The premium is $5,000–9,000. At 35% energy savings on a $2,400/year bill, that's $840/year saved — payback in 6–11 years.
Hybrid approach (recommended)
Most energy-conscious builders use a hybrid: spray foam on the rim joists, band joist, and any penetrations (where air sealing matters most) + blown-in cellulose or fiberglass in the attic floor. This delivers 80–90% of spray foam's performance at 50–60% of the cost.
Bottom Line
Open-cell spray foam in walls and closed-cell in rim joists and crawl spaces is the gold standard for energy performance — expect 30–50% lower heating/cooling costs vs fiberglass alone. But at 5–10× the cost, payback takes 5–12 years. For new construction where air sealing is part of the scope, spray foam is usually worth it. For retrofits, blown-in cellulose often provides the best cost-per-R-value improvement.
Related Calculators
Frequently Asked Questions
Is spray foam worth the extra cost?+
In new construction where it's bid as part of the initial scope, usually yes. The energy savings over a 30-year mortgage typically exceed the cost premium significantly, especially in climates with extreme summers or winters. In retrofits, the ROI is narrower and depends heavily on your current energy bills.
What R-value do I need?+
DOE recommendations: Attic — R-38 to R-60 depending on climate zone. Walls — R-13 to R-21. Floors over crawl spaces — R-19 to R-30. Colder climates (zones 5–7) need the higher end. Your local energy code specifies minimums.
Can I spray foam over fiberglass?+
Not practically — spray foam needs to adhere to the substrate (framing, sheathing). The standard approach for upgrade is to remove existing batts, spray foam the cavities, or add a continuous rigid foam layer on the exterior (retrofitting from outside). Blown-in cellulose can be added on top of existing batts in attics.
Does spray foam cause moisture problems?+
Closed-cell spray foam can cause moisture issues if improperly specified — it's a vapor retarder, so moisture in the wall can't dry to the inside. The key is to ensure vapor drive is understood: in cold climates, closed-cell goes on the warm side of the insulation. Open-cell spray foam is vapor-permeable and more forgiving.