Spray Foam vs. Fiberglass Insulation

R-Value Is Only Half the Story

Fiberglass batts are the standard, affordable insulation choice, rated by R-value per inch of thickness. The problem isn't the fiberglass itself, it's that batts only work well when installed perfectly, and gaps, compression, or air leaks around the batts can cut real-world performance well below the rated R-value. Spray foam expands to fill every gap and crack as it cures, creating an air seal that stops drafts entirely, not just insulating against heat transfer.

Is the Extra Cost of Spray Foam Actually Worth It

For a standard attic floor with good existing air sealing, fiberglass batts remain a perfectly reasonable and far cheaper choice. Spray foam earns its price premium in specific situations: rim joists, irregular framing cavities, and unvented roof assemblies where air sealing genuinely matters as much as R-value. Most cost-effective renovations use a hybrid approach, spray foam in the hard-to-seal spots and fiberglass or blown cellulose everywhere else.

• Open-cell spray foam is cheaper than closed-cell but has a lower R-value per inch and isn't moisture resistant, making it a poor choice for below-grade or exterior-adjacent applications.
• Compressed fiberglass batts lose R-value fast. A batt rated R-19 but squeezed into a cavity too shallow for it can perform closer to R-13 in practice.

Attic Floor vs. Roofline: The Decision Changes by Location

Insulating the attic floor, keeping the attic itself unconditioned and cold, is the traditional approach and works well with either fiberglass batts or blown-in insulation at a much lower cost than foam. Insulating the roofline instead, creating a conditioned attic, is where closed-cell spray foam earns its cost premium, since it seals the irregular rafter cavities and roof deck in a way batts genuinely cannot replicate. Choosing which surface to insulate matters as much as which material, and it's worth discussing both with a contractor rather than assuming attic-floor insulation is always the default.

Cure Time and Ventilation Considerations

Freshly sprayed foam needs proper cure time and ventilation before a space is safely occupied, typically 24 hours minimum, and improperly mixed or applied spray foam can off-gas noticeably longer, which is why hiring an experienced, licensed installer matters more with spray foam than with fiberglass batts. Fiberglass has its own handling consideration, airborne glass fibers during installation, which is why installers wear protective gear, but it doesn't carry the same curing timeline concern once it's in place.