Spray foam, blown-in cellulose, and fiberglass batts are three tools that each solve a different problem. There is no single "best" insulation — there's a best pick for each application. Blown-in fills irregular horizontal spaces (attic floors) with no gaps. Batts dimension neatly into standard framing (walls, floors). Spray foam bonds to complex geometries and gives you air sealing built into the insulation layer. The trick is matching material to job, not picking a favorite and forcing it everywhere.
Fiberglass batts
Batts are pre-cut fiberglass rolls sized for standard framing bays — 16-inch on-center for 2×4 or 2×6 walls, 24-inch on-center for some ceilings. They're the cheapest option per R-unit and, when installed cleanly with no compression and no gaps, deliver their rated R-value.
Right pick for: Exterior walls in framing bays. Crawl-space joist bays. New construction where framing is exposed and straightforward.
Wrong pick for: Attic floors with irregular joist heights (gaps everywhere). Retrofit walls (can't stuff a batt behind finished drywall). Any complex geometry.
R-value: R-3.2 to R-4.3 per inch depending on density.
Warranty: Owens Corning batts carry a Limited Lifetime warranty. See our warranty page for details on the products we install.
Blown-in fiberglass
Loose-fill fiberglass blown into an attic with a machine. It fills irregular bays, covers penetrations, and self-levels so there are no gaps. It's the most common attic-floor material in California.
Right pick for: Attic floors, especially retrofits. Homes where you want a straightforward, cost-effective full re-insulation to R-30, R-38, or R-49.
Wrong pick for: Vertical walls (won't stay put without netting). Spaces that need air sealing performance from the insulation itself.
R-value: R-2.2 to R-3.2 per inch installed.
Blown-in cellulose
Recycled newspaper treated with boric acid for fire, pest, and mold resistance. Higher R-per- inch than blown-in fiberglass, denser, and quieter (some soundproofing benefit). Green credentials — largest recycled content of any common insulation.
Right pick for: Attic floors where R-per-inch matters (limited attic depth). Homeowners who prioritize sustainability. Retrofit walls (dense-pack cellulose can be blown into existing wall cavities through small access holes).
Wrong pick for: Damp or moisture-prone locations (cellulose absorbs moisture more than fiberglass).
R-value: R-3.5 to R-3.8 per inch installed.
Open-cell spray foam
A softer, less-dense spray foam that expands aggressively during application. Excellent sound-damping, good air sealing, moderate R-per-inch.
Right pick for: Interior walls where sound matters. Rim joists. Complex framing where a self-conforming material saves labor.
Wrong pick for: Exterior walls in humid climates (can absorb moisture — less of a concern in SoCal). Anywhere you need vapor-barrier performance from the foam.
R-value: R-3.5 to R-3.8 per inch.
Closed-cell spray foam
The highest-performing insulation per inch. Dense, rigid, structural (adds real racking strength to a wall), and a vapor barrier. Also the most expensive.
Right pick for: Unvented "conditioned attic" assemblies. Rim joists in colder climates. Retrofits where limited depth requires maximum R-per-inch. Any spot that needs air sealing + insulation + vapor barrier in one pass.
Wrong pick for: Standard vented attic floors — overkill and overpriced. Homeowners on tight budgets doing a full attic re-insulation.
R-value: R-6 to R-7 per inch.
What each costs, honestly
Order from cheapest to most expensive per installed R-38 attic (roughly, and highly dependent on square footage, prep work, and access):
- Fiberglass batts (in framing bays) — cheapest
- Blown-in fiberglass — close second on attic floors
- Blown-in cellulose — slightly more than fiberglass
- Open-cell spray foam — 2–3× the cost of blown-in
- Closed-cell spray foam — 3–4× the cost of blown-in
The right way to compare is by application, not by material. Ask "what does this material do for THIS attic / THIS wall / THIS crawl space" — not "which one is best overall."
What Pure Eco actually installs
Our default for a full attic re-insulation is blown-in fiberglass or cellulose to R-38 (or R-49 in colder inland zones). We spec batts for exterior walls, Roxul mineral wool where sound or fire matters (media rooms, party walls, near bathroom fans), and closed-cell foam in the specific applications where its performance justifies its cost. If a contractor tells you the same material is right for every application, get a second opinion.
Watch: from our crew
Watch a Pure Eco insulation installation from start to finish:
FAQ
Is spray foam the best insulation, period? +
It's the best-performing per inch and also the most expensive. It's the right pick for tight spaces, complex geometries, and applications where you want air sealing built into the insulation. It's overkill (and overpriced) for a standard attic floor where blown-in cellulose or fiberglass would perform just as well at a fraction of the cost.
Is cellulose insulation flammable? +
Modern blown-in cellulose is treated with boric acid, which makes it Class A fire-resistant — the highest classification. It won't sustain a flame on its own. The concern with untreated cellulose (which no reputable manufacturer sells anymore) is a legacy one.
Can I mix materials in the same attic? +
Yes, and it's common. Batts on the exterior walls, blown-in on the attic floor, and Roxul (mineral wool) around bathroom fans for sound and fire safety is a normal spec on the same job.
Does spray foam off-gas? +
Modern low-VOC formulations off-gas primarily during cure (24–48 hours). After proper curing, a well-installed spray-foam envelope is inert. The key is 'well-installed' — cure time and mix ratios matter. Off-gassing horror stories almost always trace to a bad install.