Resource Guide
Both materials insulate. Only one of them also air-seals. Here is how spray foam and fiberglass batts actually compare for a Las Vegas home.
Fiberglass batts insulate by trapping air within glass fibers, but they do not stop air from moving around or through them. Any gap at a corner, outlet box, or compressed section becomes a path for conditioned or unconditioned air to bypass the insulation entirely.
Spray foam expands to fill the cavity and bonds to framing, which means it insulates and air-seals in a single step. That distinction matters more in a climate like ours, where a huge share of summer heat gain comes from air infiltration, not just conduction through material.
Fiberglass batts typically offer around R-3 to R-4 per inch. Open-cell spray foam offers roughly R-3.5 to R-4 per inch, similar to fiberglass, while closed-cell spray foam offers significantly more, typically R-6 to R-7 per inch, which matters where cavity depth is limited.
Fiberglass batts are the lower-cost material up front, both in materials and labor. Spray foam costs more per square foot installed, but the total-cost comparison should also account for the air-sealing benefit, which reduces work needed elsewhere.
Fiberglass can be a reasonable, budget-conscious choice for simple, easily accessible wall cavities with straightforward framing. Spray foam tends to make the most sense in rooflines, rim joists, crawl spaces, garages, and any irregular cavity where perfect batt installation is difficult in practice.
FAQ
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