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An attic floor filled with gray blown cellulose between wooden joists, gable vent at the far end

Loose fill against foam

Cellulose vs spray foam

Foam air seals and cellulose does not — but cellulose is dense enough to resist air movement far better than batts, at a fraction of foam's price.

Side by side

Close-up of loose cellulose insulation — gray shredded recycled paperCellulose

Blown cellulose

R per inch
R-3.5
Depth for R-30
8.5"
Installed cost
$1.05–$2.10
DIY
Yes — hired blower
  • Dense enough to resist air movement well
  • Fills around obstructions completely
  • Largely recycled content
  • Can be removed and inspected later

Choose this when: Attic floors, dense-packed finished walls, most retrofits.

More on cellulose
Cream-colored spray foam expanded to fill the cavities between wall studsSpray foam

Open-cell spray foam

R per inch
R-3.6
Depth for R-30
8.5"
Installed cost
$3.90–$6.60
DIY
No — rig and supplied air
  • Air seals as it insulates, in one pass
  • Expands into gaps nothing else reaches
  • Stays put permanently — no settling
  • Works in cavities with no floor to hold fill

Choose this when: Rim joists, cathedral ceilings, unvented roof assemblies.

More on spray foam

The cost gap

Installed, cellulose runs $1.05–$2.10 per square foot at R-30 against $3.90–$6.60 for open-cell foam. Over a whole attic that is a large difference, and for a vented attic floor it buys you very little: depth up there is nearly free, so the cheapest R-value per dollar wins and that is loose fill.

The economics flip where depth is constrained or the cavity cannot hold loose fill — a sloped ceiling, a rim joist, an unvented roof deck. There foam is not a premium version of cellulose; it is the only thing that works.

Which one can dry

This is the difference people underweight. Cellulose is hygroscopic — it takes on moisture and gives it back, and a small roof leak above it will show as a stain you can see. Open-cell foam is vapour-open too, which is why it is specified in assemblies that need to dry inward.

Closed-cell foam is the opposite: above about two inches it is its own vapour barrier, which is a feature in a crawl space and a hazard in a roof deck that also has an exterior barrier. A roof sealed on both faces cannot dry in either direction, and a leak in it stays hidden until the sheathing fails.

Cellulose also carries a borate treatment, which makes it fire-resistant and unattractive to insects and rodents — a genuine advantage in an attic over a long time.

Which to choose

  • Vented attic floor: cellulose. Foam here is money with nothing to show for it.
  • Finished wall retrofit: dense-pack cellulose — it is the main way an existing wall gets insulated at all.
  • Rim joists: closed-cell foam. Cellulose cannot seal that geometry.
  • Cathedral or unvented roof: foam, because the assembly has to be air-sealed to be safe.
  • Budget is the constraint: cellulose, every time. Spend the difference on air sealing first.

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