Spray Foam vs Fiberglass Insulation: Cost, Performance and Where Each Wins

Fiberglass has been the default insulation in American homes for decades. It is cheap, easy to install and widely available. Spray foam costs more, but it does things fiberglass simply cannot. Choosing between them comes down to what part of the house you are insulating and what problem you are trying to solve.

How they work

Fiberglass traps air in a fluffy mat of glass fibers. Air pockets slow heat flow. It does not stop air from moving through it.

Spray foam expands into place and cures into a solid foam that fills gaps and bonds to framing. It insulates and air seals at the same time.

Side by side

Where fiberglass wins

• Budget projects where first cost matters most
• Interior walls for sound dampening
• Vented attic floors, where blown fiberglass or cellulose is cheap to pile deep
• DIY jobs where you can take your time and cut batts carefully around obstacles

Where spray foam wins

• Rim joists, the classic leak point in basements
• Cathedral ceilings and roof decks, where space is tight and air sealing is critical
• Crawl spaces and basement walls, where moisture is a concern
• Metal buildings, where condensation is the main problem
• Older homes with lots of irregular cavities and air leaks

The hybrid approach

Many builders use a “flash and batt” method: an inch or two of closed-cell foam for air sealing and vapor control, then fiberglass to fill the rest of the cavity. It captures much of foam’s benefit for less money. Ask your contractor whether it makes sense in your climate zone.

Energy bills and payback

Spray foam’s air sealing can reduce heating and cooling costs, especially in leaky homes. How fast it pays back depends on your climate, energy prices and how leaky the house was before. A blower door test before and after is the best way to know what you are actually getting.

Health and installation

Fiberglass irritates skin and lungs during installation but is inert once in place. Spray foam requires occupants to stay out during spraying and for the manufacturer’s re-occupancy period, and must be installed by trained crews. See our post on the hidden perils of foam insulation for an honest look at what can go wrong.

For a wider look at insulation materials, ArmorThane’s guide to types of insulation covers cellulose, mineral wool and rigid boards too. And for builder perspectives on mixing materials, Built Smart Foam has practical case studies.