Fiberglass vs. Spray Foam: The Full Breakdown
When it comes to insulating your home, fiberglass and spray foam are two of the most popular options on the market. Both have real advantages — and real drawbacks. Choosing the wrong one for your specific situation can cost you money and comfort for years to come. This guide breaks down everything you need to know.
What Is Fiberglass Insulation?
Fiberglass insulation is made from tiny glass fibers and is available in two main forms: batts (pre-cut blankets) and loose-fill (blown-in). It's the pink or yellow fluffy material you've likely seen in walls and attics. It has been the standard residential insulation choice for decades.
- R-value: Approximately R-2.2 to R-2.7 per inch (batts); R-2.2 to R-4.3 per inch (blown-in)
- Cost: One of the most affordable insulation options available
- DIY-friendly: Batts can be installed without special equipment
- Moisture resistance: Low — fiberglass can absorb moisture and lose effectiveness if wet
What Is Spray Foam Insulation?
Spray foam is a two-component liquid that expands and hardens after application, filling gaps and creating an air-tight seal. It comes in two types: open-cell (softer, lower R-value, vapor-permeable) and closed-cell (rigid, higher R-value, moisture-resistant).
- R-value: Open-cell: ~R-3.5 to R-3.7 per inch; Closed-cell: ~R-6.0 to R-7.0 per inch
- Cost: Significantly higher than fiberglass — typically 3–5x more expensive
- DIY-friendly: Generally not recommended without training and proper equipment
- Air sealing: Excellent — spray foam seals cracks and gaps as it expands
Side-by-Side Comparison
| Feature | Fiberglass Batts | Open-Cell Foam | Closed-Cell Foam |
|---|---|---|---|
| R-Value per Inch | ~R-2.2 to R-2.7 | ~R-3.5 to R-3.7 | ~R-6.0 to R-7.0 |
| Air Sealing | Poor | Good | Excellent |
| Moisture Resistance | Low | Low | High |
| Relative Cost | Low | High | Very High |
| DIY Friendly | Yes | No | No |
| Longevity | 20–30 years | Decades | Decades |
When to Choose Fiberglass
Fiberglass is often the right call when:
- You're working with a tight budget and need to insulate a large area
- You want to DIY the installation in an attic or between wall studs
- The area is well-sealed already and air infiltration is not the primary concern
- You're insulating a new construction home with standard framing
When to Choose Spray Foam
Spray foam is usually the better investment when:
- Air sealing is a priority — such as in rim joists, crawl spaces, or leaky attics
- You're insulating an irregularly shaped space with lots of gaps and penetrations
- Moisture resistance is critical (e.g., below-grade applications)
- You're doing a deep energy retrofit and want maximum performance per inch
The Verdict
There's no single "winner" — the best insulation depends on your home's needs, climate, budget, and the specific area being insulated. Many homeowners use both materials strategically: spray foam for air-sealing critical areas and fiberglass batts to fill the bulk of the cavity. Consulting with an insulation contractor for a whole-home energy assessment is always a smart starting point.