Spray Foam vs Fiberglass Insulation: Cost, R-Value & Rebates
The Core Difference: Insulation vs. Air Sealing
Most homeowners think about insulation in terms of R-value — thermal resistance to heat flow. By that metric alone, closed-cell spray foam wins: R-6 to R-7 per inch versus R-2.2 to R-3.8 per inch for fiberglass. But R-value per inch isn't the whole story.
Heat escapes from homes through two mechanisms: conduction (through walls, attic, and floor assemblies) and convection (air movement through gaps, cracks, and penetrations). Fiberglass insulation addresses conduction well. It does very little for convection — air flows through and around fiberglass batts easily. Spray foam addresses both, because it expands to fill gaps and seals the air barrier while insulating.
In a typical American home, air infiltration accounts for 25–40% of total energy loss for heating and cooling. Adding fiberglass insulation to a leaky attic reduces conductive heat loss but leaves the air infiltration problem largely untouched. Adding spray foam to the same attic addresses both simultaneously.
Types of Spray Foam
Spray polyurethane foam comes in two formulations with very different properties:
| Property | Open-Cell Spray Foam | Closed-Cell Spray Foam |
|---|---|---|
| R-value per inch | R-3.5 to R-4 | R-6 to R-7 |
| Air sealing | Excellent | Excellent |
| Vapor permeability | Permeable (breathes) | Vapor barrier (impermeable) |
| Water resistance | Low | High |
| Cost per sq ft at 3" | $0.44–$0.65 | $1.00–$1.50 |
| Structural benefit | Minimal | Significant (shear strength) |
| Best uses | Interior walls, open attic rim joists | Exterior walls, basement walls, crawlspaces, rooflines |
Open-cell is cheaper and works well in interior applications. Closed-cell is more expensive but provides much higher R-value per inch, vapor control, and water resistance — necessary for below-grade or exterior applications.
Types of Fiberglass Insulation
Fiberglass comes in two forms with different performance characteristics:
Batt Insulation
Pre-cut lengths for fitting between studs or joists. The most common DIY-accessible insulation format. R-value varies by thickness: R-13 for 3.5" (standard stud wall), R-19 for 6.25" (standard 2x6 wall), R-30 for attic applications.
Critical issue with batts: installation quality determines actual performance. Batts compressed into spaces too small, cut improperly, or left with gaps at edges perform significantly below their nominal R-value. A perfect R-30 batt installation provides significantly better performance than a careless installation of the same batt.
Blown-In (Loose-Fill) Fiberglass
Machine-blown loose fiberglass for attic floors and existing wall cavities. More forgiving than batts in terms of installation quality — the blowing process fills irregular cavities better than batts. However, blown-in fiberglass settles over time (10–20% density reduction) and provides no air sealing.
Blown-in fiberglass is the most cost-effective way to increase attic R-value in existing homes. Adding 10–12 inches of blown fiberglass to an attic floor with existing 3-inch fiberglass batts can bring total R-value from R-11 to R-50+ for $1–$1.50/sq ft.
Cost Comparison by Application
| Application | Blown Fiberglass | Open-Cell Spray Foam | Closed-Cell Spray Foam |
|---|---|---|---|
| Attic floor (1,000 sq ft, R-38) | $800–$1,500 | $2,000–$3,500 | $4,000–$6,000 |
| Rim joist sealing | Batts: $200–$400 | $400–$800 | $600–$1,000 |
| Crawlspace walls (500 sq ft) | Not recommended | $1,200–$2,000 | $2,000–$3,500 |
| Basement walls (1,000 sq ft) | Batts: $800–$1,500 | $2,500–$4,000 | $4,000–$7,000 |
Spray foam almost always costs 2–4x more than fiberglass for the same area. Whether that premium is worth it depends on the application and what problem you're solving.
What the Research Actually Shows
Studies on building performance consistently find that air sealing combined with insulation outperforms insulation alone. Lawrence Berkeley National Laboratory research found that homes receiving comprehensive air sealing plus insulation showed 25–35% energy reduction, while insulation-only improvements averaged 10–15% reduction.
This matters for HOMES rebates — the 35% threshold for maximum HOMES rebates ($4,000 standard / $8,000 income-qualified) is much more achievable with combined air sealing and insulation than with insulation alone. If your goal is maximizing rebate eligibility, the air sealing component deserves as much attention as the R-value number.
Rebate Eligibility
HEAR covers insulation and air sealing:
- Insulation: Up to $1,600 (at 80% AMI)
- Air sealing: Included in the insulation/weatherization category up to the $1,600 limit
HEAR doesn't distinguish between spray foam and fiberglass — both qualify as long as they meet minimum efficiency standards. The $1,600 HEAR maximum covers partial attic insulation costs for most projects; it's a rebate supplement, not a full-project payer.
HOMES rebates, based on whole-home energy savings, pay $2,000–$8,000 depending on documented savings percentage. For insulation-heavy projects achieving 35%+ savings, HOMES is where the bigger dollars are. See insulation rebate guide for HEAR and HOMES applications to insulation projects.
Many utilities also offer insulation rebates. These vary by utility and state — California insulation rebates, New York insulation rebates, and other state pages have current utility program amounts.
The Professional Assessment Step
Before committing to an insulation material, schedule an energy audit with a BPI-certified professional or HERS rater. An audit includes a blower door test — a pressure test that measures your home's actual air leakage rate. The results tell you precisely where air is escaping and which interventions will have the most impact.
A blower door test on a leaky home may reveal that your attic insulation is actually at R-30 but your air changes per hour (ACH) rate is 12 — meaning air sealing will reduce your energy use far more than adding more insulation. Without the audit, you might spend $2,000 on fiberglass when $800 of air sealing would do more.
Energy audits are often subsidized by utility programs and are the mandatory first step for HOMES rebate qualification. See the energy audit guide for what to expect and how to find a qualified auditor.
When to Use Each
Use blown-in fiberglass when:
- Adding insulation to an attic floor over existing insulation
- You've already addressed air sealing separately
- Budget is a primary constraint
- The existing air barrier is adequate (newer construction)
Use open-cell spray foam when:
- Sealing interior wall cavities in existing construction
- Rim joist sealing (air sealing + insulation in one step)
- Open attic applications where air sealing is important and vapor permeability is desired
Use closed-cell spray foam when:
- Insulating exterior-facing applications (basement walls, crawlspace walls, exterior sheathing)
- Under-roof deck applications (converting vented to unvented attic)
- Applications requiring vapor control
- High moisture environments
New Construction vs. Retrofit: Different Calculus
The spray foam vs. fiberglass decision looks different in new construction than in retrofit. In new construction, closed-cell spray foam applied to the exterior sheathing (continuous insulation) and to the roof deck (creating an unvented conditioned attic) is the premium approach — it creates an exceptionally tight, high-performance building envelope that exceeds code minimums significantly. The upfront cost premium is substantial but the long-term efficiency advantage is durable.
In retrofit applications, the highest value interventions are usually air sealing of identified leaks (attic bypasses around plumbing and electrical penetrations, rim joists, attic hatch) combined with blown-in insulation to bring R-value to code recommendations. Open-cell spray foam for air sealing specific problem areas and blown fiberglass for bulk R-value is a common and cost-effective retrofit combination.
Insulation and the HOMES Rebate: Thinking About Savings Thresholds
If your goal includes qualifying for HOMES rebates at the 35% savings tier, insulation strategy should be coordinated with an energy auditor who can model the expected savings. Not all insulation paths achieve the same savings percentage — adding R-30 of attic insulation to a well-sealed attic with R-19 existing insulation may produce a 10% energy reduction, while air sealing a leaky attic (20 ACH vs. 3 ACH target) with R-49 insulation and a new heat pump installation might produce a 40%+ reduction.
The auditor's pre-retrofit energy model determines what interventions qualify for which HOMES tier. Commission the audit before selecting insulation materials, not after — the modeling results should guide the insulation strategy, not vice versa. See the energy audit guide for how to get an audit that's useful for HOMES qualification, and the insulation rebate guide for HEAR application specifics.