Core Comparison: Cost, R-Value, and Air Sealing
| Type | R-Value / Inch | Cost / Sq Ft | Air Sealing | Best For |
|---|---|---|---|---|
| Fiberglass batt | R-3.1 – 3.4 | $0.50 – $1.20 | None | New framing, accessible walls |
| Blown-in fiberglass | R-2.2 – 2.7 | $0.80 – $1.50 | None | Attic floors, existing walls (dense-pack) |
| Blown-in cellulose | R-3.5 | $0.75 – $1.40 | Partial | Attic floors, dense-pack walls |
| Open-cell spray foam | R-3.5 | $0.50 – $1.50 | Excellent | Sound control, crawl spaces, rim joists |
| Closed-cell spray foam | R-6.5 | $1.00 – $3.00 | Excellent + vapor barrier | High-performance zones, below-grade, moisture areas |
| Rigid foam board (EPS) | R-3.8 | $0.75 – $1.50 | Good (taped seams) | Exterior continuous, basement walls |
| Rigid foam board (XPS) | R-5.0 | $1.00 – $2.00 | Good (taped seams) | Below-grade, foundation perimeter |
The Air Sealing Difference
This is the most important performance distinction between spray foam and batt/blown-in insulation. Batt and blown-in products provide thermal resistance only — they don't stop air movement. In a leaky home, air bypasses the insulation through gaps, cracks, and penetrations, dramatically reducing the effective thermal performance of the insulation layer.
Closed-cell spray foam simultaneously insulates and air-seals, acting as both a thermal barrier and an air/vapor barrier in a single application. This is why spray foam often delivers greater energy savings than its R-value advantage alone would predict — especially in older, leaky homes.
Practical implication: In an attic floor application, you can often get equivalent real-world performance by combining blown-in insulation (at lower cost) with targeted air sealing using caulk, foam sealant, and weatherstripping at penetrations. A blower door test ($300–$500) can quantify your home's leakage and identify where air sealing will do the most good.
Where Each Type Performs Best
Batt Fiberglass or Mineral Wool
- Best for: New construction with open stud bays, accessible open attic cavities, remodels where drywall is removed
- Limitations: Must fit tightly in cavities without gaps; compressed batts lose significant R-value; provides no air sealing; can absorb moisture if vapor management is wrong
- Cost advantage: Lowest installed cost for accessible applications; DIY-friendly
Blown-In (Fiberglass or Cellulose)
- Best for: Attic floors (deep blown-in is the industry standard for attic insulation), existing closed wall cavities (dense-pack), hard-to-reach spaces
- Cellulose advantage: Recycled content; better sound attenuation; fire-retardant treated; slightly better R-value than fiberglass blown-in
- Fiberglass advantage: Does not settle over time; inorganic (no mold concern even if damp)
Closed-Cell Spray Foam
- Best for: Rim joists (structural perimeter of floor system — the most air-leaky area in most homes), crawl space walls, cathedral/vaulted ceiling cavities, basement walls, exterior sheathing overlay, shipping container and metal building applications
- Key advantage: R-6.5/inch allows meeting code R-values in thin cavities (2x4 walls can achieve R-20+ with 3 inches); vapor barrier eliminates need for separate vapor retarder in most applications
- Cost trade-off: Worth it in moisture-critical or space-constrained applications; overkill for open attic floors where blown-in performs comparably at 1/3 the cost
Open-Cell Spray Foam
- Best for: Sound control (better sound attenuation than closed-cell), interior wall applications where vapor permeability is desired, large volumes where cost matters
- Limitations: Not a vapor barrier; not appropriate for below-grade or moisture-prone applications
Climate Zone Guidance
| Climate Zone | Recommended Strategy | Key Consideration |
|---|---|---|
| Hot-Humid (Gulf Coast, SE) | Spray foam on roof deck (unvented attic) or blown-in + air sealing; closed-cell in crawl space | Moisture management critical — wrong vapor placement causes mold; consult energy auditor |
| Mixed-Humid (Atlanta, DC) | Blown-in attic + dense-pack walls + closed-cell rim joists; air sealing throughout | Double-check vapor barrier placement — interior in colder months, exterior in hotter months |
| Cold (Minneapolis, Boston) | Blown-in to R-49–60 in attic; closed-cell spray foam on basement walls and rim joists | Ice dam prevention requires good attic air sealing AND insulation; blown-in alone is insufficient if attic floor has significant penetrations |
| Desert (Phoenix, Las Vegas) | Blown-in attic + radiant barrier; batt in walls standard; spray foam optional for rim joists | Vapor drive is outward in cooling season — vapor barrier positioning differs from cold climates |
Frequently Asked Questions
Is spray foam worth the extra cost over batt?
It depends on application. For attic floors with accessible open cavities, blown-in cellulose or fiberglass is excellent value at $0.75–$1.50/sq ft. For rim joists, crawl spaces, and cathedral ceilings where you need both insulation and air sealing in a confined space, closed-cell spray foam is typically worth the $1.00–$3.00/sq ft premium over alternatives.
What R-value do I need in my attic?
DOE recommendations: Southern states (Zone 2–3): R-38. Midwest and Mountain (Zone 4–5): R-49. Northern states (Zone 6–7): R-49–60. Your local energy code may require equal or higher values. Your contractor will confirm code requirements. A blower door test can identify whether air sealing is a bigger priority than adding more R-value — in very leaky homes, air sealing delivers more savings per dollar than additional insulation depth.
Does the IRA cover insulation upgrades?
Yes — the IRA 25C Energy Efficiency Tax Credit provides 30% of cost (max $1,200/year) for qualifying insulation materials meeting IECC 2021 standards. Both spray foam and blown-in insulation can qualify. Keep all invoices and manufacturer certification statements for Form 5695 on your tax return.