Fiberglass vs. Spray Foam Insulation: Which Saves More Energy & Money?
Fiberglass is spun glass fibers packed in batts; Spray Foam is a two-part liquid that expands into an air-sealing plastic. Both resist heat flow, but use different science.
Homeowners hear “R-value” and assume higher numbers automatically mean bigger savings, so they lump the two materials together—yet one pays for itself in tiny cracks while the other just fills the bay.
Key Differences
Fiberglass costs ~$0.40 per sq ft and slows heat; Spray Foam runs ~$1.50 yet blocks airflow and moisture, trimming HVAC use 20-30 %. Payback is 3-7 years versus 10-15.
Which One Should You Choose?
Dry, accessible attics: fiberglass for quick DIY wins. Rim joists, cathedral ceilings, or humid zones: spray foam’s seal cuts bills and prevents mold, outweighing upfront cost.
Examples and Daily Life
A 1970s ranch in Ohio cut heating 25 % by foaming rim joists; a Phoenix townhouse shaved summer peaks 18 % with fiberglass plus radiant barrier. Climate and budget decide.
Can I mix both materials?
Yes—foam the tricky gaps first, then overlay fiberglass batts to hit target R-value without overspending.
Does spray foam void roof warranties?
Only if the installer skips a thermal barrier; always check shingle manufacturer specs first.