Two Very Different Approaches to Insulation
Fiberglass batt insulation has been the industry standard for decades — it's widely available, familiar to builders, and relatively inexpensive. Spray foam, while significantly more costly upfront, offers a different set of performance characteristics that have made it increasingly popular in both new construction and retrofits. Understanding the real-world differences helps you decide where each product belongs.
Performance Comparison Table
| Factor | Spray Foam | Fiberglass Batt |
|---|---|---|
| R-Value per Inch | R-3.5 to R-7.0 | R-3.1 to R-3.4 |
| Air Sealing | Yes (complete) | No |
| Vapor Control | Yes (closed-cell) | No (facing only) |
| Installation Skill Required | High (professional) | Low (DIY-friendly) |
| Upfront Cost | Higher | Lower |
| Longevity | Decades (doesn't sag) | Can settle/sag over time |
| Moisture Sensitivity | Low (especially closed-cell) | High (loses R-value when wet) |
| Sound Attenuation | Good (especially open-cell) | Moderate |
Where Fiberglass Still Makes Sense
Despite the performance advantages of spray foam, fiberglass batts remain a reasonable choice in specific scenarios:
- Attic floors with separate air sealing: When an attic floor is already well air-sealed with caulk or rigid foam, deep blown-in fiberglass can achieve required R-values economically.
- Budget-constrained new construction where spray foam is not financially viable across all assemblies.
- DIY projects where the homeowner has the skills and time but not the equipment for spray foam application.
- Interior partition walls where sound control — not thermal performance — is the goal.
The Hidden Cost of Fiberglass: Air Leakage
Fiberglass batts do not air-seal. In a standard stud-framed wall, up to 40% of heat loss can occur through air infiltration around and through the batt — not through the batt itself. This is the performance gap that spray foam addresses so effectively. When comparing costs, always factor in the additional labor and materials required to air-seal separately when using fiberglass.
Real-World Performance Degradation
Fiberglass batts are laboratory-tested under ideal conditions. In real installations:
- Compressed batts lose significant R-value (a batt compressed to half its design thickness can lose up to 50% of its R-value)
- Gaps around obstructions like electrical boxes and pipes create thermal bridges
- Moisture absorption dramatically reduces effective R-value and promotes mold growth
Spray foam, by contrast, conforms to the shape of the cavity and maintains its performance characteristics over time without settling or compressing.
Hybrid Systems: Getting the Best of Both
A cost-effective strategy used by many energy-efficient builders is a hybrid approach:
- Apply closed-cell spray foam to the inner face of exterior walls to create an air/vapor barrier (typically 1–2 inches)
- Fill the remainder of the wall cavity with fiberglass batts
This approach captures the air-sealing and vapor-barrier benefits of closed-cell foam while using lower-cost fiberglass for thermal mass, often hitting required code R-values at a lower total cost than all-foam assemblies.
Which Should You Choose?
The answer depends on your priorities:
- For maximum long-term performance and energy savings: Spray foam wins on every metric.
- For budget-conscious projects with careful installation: Fiberglass with supplemental air sealing remains viable.
- For moisture-prone areas, crawl spaces, and basements: Spray foam is strongly preferred.
- For simple attic top-ups in dry climates: Blown-in fiberglass or cellulose is cost-effective.