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Fiberglass is cheap and common, but does it block extreme desert radiant heat as well as spray foam? Review the science on air sealing, R-value degradation, dust penetration, moisture limits, and pricing.
| Feature | Closed-Cell Polyurethane | Standard Fiberglass Batts | Arizona Heat Impact |
|---|---|---|---|
| R-Value Per Inch | ~R-6.2 to R-6.9 | ~R-2.9 to R-3.4 | Foam packs more insulation per inch |
| Air Sealing Performance | 100% Seamless seal | 0% (Vapor permeable) | Foam stops hot wind drafting |
| Convective Draft Loops | Blocks loops entirely | Highly susceptible | Batts let ceiling radiant heat slip |
| Moisture Resistance | Waterproof barrier | Degrades if wet | Foam handles monsoon roof seepage |
| Settling / Sagging | Extremely stable | Sags over decades | Foam conforms permanently |
| Material Cost | Higher upfront | Very low upfront | Payback takes 5-10 years |
Why does spray foam often outperform fiberglass in Arizona? The secret is **air sealing**. Fiberglass batts and loose-fill cellulose resist heat conductive transfer, but do not stop convective air movement.
Gaps around lighting boxes, top plates, and plumbing stacks let hot, dusty attic drafts pass directly into conditioned rooms. Spray foam acts as an air filter and thermal block combined, blocking drafts completely.
Pro: Drastically suppresses attic heat transfer.
Pro: Seals plumbing penetrations permanently.
Pro: Eliminates insulation settling or displacement.
Con: Undergo chemical off-gassing requiring evacuation schedules.
Con: Upfront investment is 3x to 5x higher than fiberglass.
Pro: Extremely low upfront financial investment.
Pro: Easy to blow in or lay down inside simple attics.
Pro: Completely fireproof material composition.
Con: Let convective hot air loop and slip around penetrations.
Con: Subject to settling, reducing R-value thickness over decades.