Spray Foam vs Fiberglass vs Cellulose in Zone 5

Short answer

Comparing spray foam vs fiberglass vs cellulose, closed-cell foam has the most R-value per inch (about R-7 for current HFO products), and spray foam is the only one of the three that is also an air barrier. Fiberglass and cellulose cost far less per R-value and are often the right call on an open, vented attic floor in climate zone 5. Spray foam earns its price on roof decks, rim joists, basement walls, metal buildings and shallow cavities.

Key points

R-value per inch: HFO closed-cell about 7 (it varies a little by product), open-cell 3.5–3.8, fiberglass batts about 3.7–4.3, blown fiberglass 2.2–2.7, cellulose 3.2–3.8.
Spray foam stops air. Cellulose slows it. Fiberglass doesn't, so it depends on separate air sealing.
Closed-cell sheds bulk water and becomes a Class II vapor retarder by about 2 inches; the exact thickness depends on the product. Open-cell and cellulose absorb water.
Nationally, blown-in insulation to R-49 runs about $1.10–$1.90 per square foot of attic, a fraction of foam's cost for the same R-value.
Pennsylvania's zone 5 targets under the code in effect since January 2026: R-49 ceilings and R-23 wood-frame walls (or R-20 plus R-3.8 continuous, or R-13 plus R-7.5 continuous).

Spray foam vs fiberglass vs cellulose at a glance

Western Pennsylvania is climate zone 5. On the 2021 code map Pennsylvania now uses, the northern-tier counties that used to be zone 6, such as Cameron, Elk, McKean and Potter, are zone 5 too. That is cold enough that air leakage and winter moisture matter as much as R-value.

	Closed-cell foam	Open-cell foam	Fiberglass	Cellulose
R per inch	About 7 (HFO products; check the data sheet)	3.5–3.8	Batts about 3.7–4.3; blown 2.2–2.7	3.2–3.8
Air	Air barrier at about 1 inch	Air barrier at about 3–4 inches	Air passes through	Slows air; not an air barrier
Vapor	Class II retarder by about 2 inches, by product	Vapor-open	Unfaced is vapor-open; facings can retard vapor	Vapor-open
Bulk water	Sheds it	Absorbs it	Fibers don't absorb it; wet batts hold it	Absorbs large amounts
National installed cost	$3.00–$5.00 per sq ft	$1.50–$3.50 per sq ft	Batts $1.00–$2.60; blown $1.00–$2.80 per sq ft	Blown $1.20–$2.80 per sq ft

The batt figure comes from DOE's ratings of R-13 to R-15 for a 3.5-inch 2×4 cavity. The cost figures are national ranges from a consumer cost guide, updated late 2025; the foam ranges don't state a thickness. For FoamGuard's closed-cell, $1.50–$2.25 per board foot works out to $3.00–$4.50 per square foot at 2 inches.

R-value per inch and what it means for depth

Inches needed to reach R-49, Pennsylvania's zone 5 ceiling value, for each material.

To reach R-49 you need about 7 inches of closed-cell (49 ÷ 7, using R-7 per inch as a typical aged value for HFO closed-cell; see R-value per inch), 12.9–14.0 inches of open-cell, 12.9–15.3 inches of cellulose, or 18.1–22.3 inches of blown fiberglass. Call it about 13 to 22 inches for the blown materials.

On an attic floor with room to spare, depth is cheap. In a 2×6 rafter bay, a cathedral ceiling or a basement wall you don't want to frame out, it isn't.

Cellulose settles, by up to 20% over several months according to Fine Homebuilding, so installers blow it to the manufacturer's initial depth and let it settle to the target.

Air leakage: what R-value doesn't measure

R-value is measured on the material, not on the air moving around and through it.

Spray foam bonds to framing and sheathing. Building Science Corporation notes that spray foam systems can act as effective air barriers.
Cellulose is denser than blown fiberglass and better at reducing air leakage, but Building Science Corporation classes damp-spray cellulose as an air retarder, not an air barrier.
Fiberglass lets air through. DOE notes its performance depends heavily on a tight fit with no gaps around framing and electrical boxes, and the ceiling or wall still needs its own air sealing.

Loose fiberglass has one more cold-weather issue. Oak Ridge reported a 30% loss in performance below about 0°F in 1991; a manufacturer says today's products start to lose performance around -20°F. One fix experts suggest is capping it with 2 inches of cellulose.

Vapor and moisture in climate zone 5

In zone 5, the code calls for a Class I or II vapor retarder on the interior side of framed walls, with some exceptions (IRC R702.7; see closed-cell vs open-cell in Climate Zone 5). Closed-cell at about 2 inches is that retarder. Open-cell, unfaced fiberglass and cellulose are vapor-open, so they need a separate retarder or a faced product. DOE notes that batt facings can act as a vapor barrier, an air barrier or both.

Bulk water is a separate question:

Closed-cell rejects it, and FEMA's flood-resistant materials bulletin (Technical Bulletin 2, January 2025) lists closed-cell foam as acceptable below the flood elevation (more on basements).
Open-cell shouldn't be used below ground level, where DOE notes it could absorb water.
Cellulose absorbs large amounts of water, and a roof leak can spread through it unnoticed until the drywall shows it.
Fiberglass fibers don't soak up water, but batts that get soaked hold it against the framing and usually have to come out.

Best uses and where each fails

Material	Best uses in zone 5	Where it fails
Closed-cell foam	Rim joists, basement and crawlspace walls, unvented roof decks, metal buildings and pole barns, shallow cavities	Cost, on open attic floors where depth is free; bad installs (off-ratio, too thick per pass)
Open-cell foam	Interior walls for sound; deep cavities with a vapor retarder on the warm side	Below grade; metal buildings; exterior assemblies without vapor control
Fiberglass	Open attic floors (blown); standard wall cavities, installed carefully	Gaps and compression; skipped air sealing; rim joists, where air reaches the cold rim
Cellulose	Open attic floors; dense-packed retrofit walls	Roof leaks and wet locations; thin coverage if not blown deep enough to allow for settling

The attic: an honest answer

Two ways to insulate an attic: blown insulation on the floor of a vented attic, or closed-cell foam under the roof deck of an unvented one.

If your attic is an open, vented space with nothing stored or running in it, the best value is usually to seal the ceiling's air leaks (top plates, wiring and plumbing holes, the hatch, recessed lights), then blow in cellulose or fiberglass. ENERGY STAR recommends R-60 for an uninsulated zone 5 attic and R-49 when 3 to 4 inches are already there.

Nationally, blowing an attic to R-49 runs about $1,100–$1,900 per 1,000 square feet. Closed-cell to R-49 on 1,000 square feet of roof deck is 7,000 board feet at R-7 per inch, or $10,500–$15,750 at FoamGuard's $1.50–$2.25 range. For a plain vented attic, that's hard to justify, and we'll tell you so.

Foam on the roof deck makes more sense when ducts or equipment are in the attic, when a cathedral ceiling leaves no room for depth, or when the space will be used. A hybrid, closed-cell for the air seal plus batts or blown insulation, can split the cost, as long as there's enough foam to keep the deck from condensing.

Thinking about cost? See what spray foam insulation costs. Want a number for your own project? Price My Project or call (814) 208-3478.

FAQ

Which is best for a basement rim joist?

Closed-cell. It air-seals the rim and handles moisture, while a batt lets humid indoor air reach the cold rim. For a small rim, a DIY kit can work; see DIY spray foam kits vs professional.

Can I put cellulose or fiberglass over existing insulation?

On an open attic floor, generally yes, once the air leaks are sealed. Don't bury recessed lights that aren't rated for insulation contact, and keep soffit vents clear.

Which is best for soundproofing interior walls?

Open-cell foam and fiberglass batts both absorb sound well; closed-cell is only fair. Interior walls don't need air or vapor control, so there's little reason to pay for closed-cell there.