Where Spray Foam Wins
- Rim joists and band joists. The highest-leverage spot in a typical basement. 3 inches of closed-cell gives you R-21 and seals every gap simultaneously.
- Basement and foundation walls. Moisture-resistant, doesn't support mold growth, and bonds directly to concrete.
- Crawl space encapsulation. Part of a complete system with vapor barrier and sealed vents.
- Cathedral ceilings and vaulted rafters. Limited depth + no soffit airflow = closed-cell foam is often the only code-compliant option.
Open-cell foam has a place on interior partition walls for sound dampening, and in cavities where moisture isn't a concern.
Where Cellulose Wins
- Attic floors. Loose-fill cellulose blown to R-49 or R-60 is significantly cheaper than the equivalent in foam, and an attic floor doesn't need foam's vapor barrier property.
- Existing walls (dense-pack retrofit). Can be installed through small access holes, fills every stud bay, and works around wiring and plumbing without voids.
- Between-floor assemblies. For sound dampening between stories.
Cellulose is also the more environmentally friendly option — 85% recycled paper fiber, low embodied carbon, fully recyclable.
Where They Fail
Cellulose in the wrong spot: cellulose in a wet cavity will get soggy, lose R-value, and potentially mold. Cellulose under recessed lights that aren't IC-airtight rated is a fire risk. Cellulose in a cathedral ceiling without proper venting can trap moisture.
The lesson: contractors who spray foam everything, or blow cellulose everywhere, are taking shortcuts. Every assembly needs a scope call.
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Still have questions?
This guide was written by Carlos Rivera. If your situation has a wrinkle we did not cover, call us direct. Most questions we answer by phone take five minutes.