Fiberglass vs. Spray Foam: A Head-to-Head Comparison
Choosing between fiberglass and spray foam insulation is one of the most common decisions homeowners face. Both materials offer solid thermal performance, but they work differently, cost differently, and suit different situations. This guide breaks down everything you need to know to make the right call.
What Is Fiberglass Insulation?
Fiberglass insulation is made from fine strands of glass woven into a fluffy material. It comes in two main forms: batts and rolls (pre-cut panels) and loose-fill (blown-in). It's the most widely used insulation material in North America, and for good reason — it's affordable, widely available, and effective when properly installed.
- R-value per inch: ~R-2.2 to R-2.7
- Best for: Walls, attics, floors between stories
- Lifespan: 20–30+ years with proper installation
- DIY-friendly: Yes, with proper protective gear
What Is Spray Foam Insulation?
Spray foam is a two-component liquid that expands upon application to fill gaps, cracks, and cavities. It comes in two varieties: open-cell (softer, lower R-value) and closed-cell (denser, higher R-value, moisture-resistant).
- Open-cell R-value per inch: ~R-3.5 to R-3.8
- Closed-cell R-value per inch: ~R-6 to R-7
- Best for: Air sealing, crawl spaces, rim joists, irregular cavities
- DIY-friendly: Small cans only; full applications require a professional
Side-by-Side Comparison
| Feature | Fiberglass | Spray Foam |
|---|---|---|
| R-value per inch | R-2.2 – R-2.7 | R-3.5 – R-7 |
| Air sealing | Poor (requires separate air barrier) | Excellent (seals as it expands) |
| Moisture resistance | Low (can absorb moisture) | High (especially closed-cell) |
| Cost (material + install) | Lower | Higher |
| DIY installation | Yes | Limited |
| Environmental impact | Moderate (recycled glass content) | Higher (chemical blowing agents) |
When to Choose Fiberglass
Fiberglass batts are an excellent choice when you're working with standard stud spacing and have a straightforward wall or attic cavity. It's the go-to option for budget-conscious projects and DIY installations. It performs well as long as it's installed without gaps, compression, or moisture exposure.
When to Choose Spray Foam
Spray foam excels in situations where air sealing is critical — think crawl spaces, rim joists, around pipes and wiring, or in older homes with irregular framing. Closed-cell spray foam also adds structural rigidity and acts as a vapor barrier, making it ideal for below-grade applications.
The Bottom Line
Neither option is universally "better." For large open cavities on a budget, fiberglass wins. For superior air sealing and moisture control in tricky spots, spray foam is worth the extra investment. Many homeowners use both — spray foam in critical areas and fiberglass batts in standard wall cavities.