The short version
Cellulose blown-in on the attic floor is the right answer for most San Diego attics. About a third the cost of spray foam, performs to R-49 reliably, and resists air movement better than fiberglass.
Spray foam at the roof deck is the right answer when you have a cathedral ceiling you cannot vent properly, when you are conditioning the attic, or when air-sealing matters more than R per dollar.
Cost per R-value (installed)
| Product | Cost per sq ft of R-49 | Best assembly |
|---|---|---|
| Blown-in cellulose | $1.40 – $2.80 | Vented attic floor |
| Open-cell spray foam | $5.50 – $9.50 (at roof deck depth) | Cathedral ceiling, sound |
| Closed-cell spray foam | $7.50 – $13.00 (at roof deck depth) | Rim joists, unvented decks |
Cellulose is roughly 4 to 5 times cheaper per unit of insulation. That math doesn't lie.
Where spray foam earns its premium
- Cathedral or vaulted ceilings you cannot vent, open-cell at the roof deck handles the assembly in one product.
- Conditioned attic conversions, closed-cell at the roof deck moves the HVAC inside the envelope.
- Rim joists in crawlspaces, two inches of closed-cell foam stops the biggest air leak in the house.
- Sound assemblies, open-cell foam in interior walls quiets a room better than batts.
Where cellulose wins
- Attic top-ups on existing R-13 or R-19, just blow on top.
- Empty walls in a 1960s tract home, drill-and-fill dense-pack from interior or exterior.
- Open framing in additions and new construction, loose-fill in attic, batts in walls.
- Air-movement reduction, dense-pack at proper density resists migration.
The decision framework
If your attic ceiling is flat and your HVAC is up there, blown-in cellulose plus air-sealing wins. Period.
If you have any vault, cathedral, conditioned attic, or rim joist in the scope, spray foam comes into the mix.
If a contractor is selling you spray foam everywhere as the only answer, get a second opinion. Most San Diego attics don't need the most expensive product, they need the right product for the assembly.