Spray Foam vs Cellulose vs Fiberglass for Southern Tier Homes

Fiberglass is the cheapest, cellulose is the best value for depth in an open space, and spray foam is the only one of the three that air seals in the same application. In a heating-dominated climate like Broome County’s, air leakage rather than R-value is often the dominant heat loss path, and that is where the foam premium either earns itself or does not.

This article is written to help you decide what to hire someone to install. It is not a how-to. We are a foam contractor, so read the recommendations accordingly, and note that this piece tells you to buy something other than foam in three of the six situations below.

The three materials

	Fiberglass	Cellulose	Spray foam
R per inch	3.1 – 3.4	3.5 – 3.8 (dense-pack)	3.5 – 3.7 open-cell / 5.0 – 7.0 closed-cell
Installed cost/sq ft	$1.00 – $2.60	comparable to fiberglass	$1.50 – $3.50 open / $3.00 – $5.00 closed
Air seals?	No	Partially, when dense-packed	Yes
vapor behavior	Permeable	Permeable, hygroscopic	Open-cell permeable, closed-cell retarding
Handles getting wet	Poorly; holds water against wood, loses R-value	Absorbs and releases; treated for fire and pests	Closed-cell does not absorb; open-cell does
Settles?	Batts sag over decades	Loose fill settles; dense-pack does not	No
Fills irregular spaces	Badly	Well	Perfectly
Fills a closed cavity	No	Yes, dense-pack	Yes, injection foam

R-value and cost figures are typical ranges. Job-specific foam figures come from the product’s technical data sheet.

The distinction that actually matters: R-value versus air sealing

This is where most insulation decisions go wrong, and it is worth being precise about.

R-value measures resistance to conduction, heat moving through a material. That is what all three do.

Air leakage is heat moving because air physically moves, carrying its heat with it, through gaps, joints, penetrations, and the thousands of small openings in an ordinary house. Insulation with an R-value of 60 does nothing about air moving around it.

In a climate with 83 inches of snow a year and freezing temperatures from November through April, air leakage frequently costs more than conduction does, particularly in older houses. That is why sealing a rim joist, which is a small area, often changes how a house feels more than adding a foot of attic insulation, which is a large one.

Fiberglass does nothing about air movement. Cellulose does something when it is dense-packed into a cavity. Foam does it in the same application as the insulating.

The practical consequence: if your complaint is drafts, cold floors at the edges of rooms, or a house that never feels warm despite the furnace running, more R-value is not the answer and adding fiberglass will disappoint you. If your complaint is a cold room over a garage or an attic with three inches of insulation in it, R-value is exactly the answer and cellulose is the cheap way to get it.

Where each material belongs

Attic floor, open and accessible: cellulose

Blown cellulose is the best value in insulation, full stop, when you have an open space and unlimited depth. It is cheap per unit of R, it fills around obstructions well, it is largely recycled material, and depth is essentially free.

The caveat: it does not air seal the ceiling plane. The optimal assembly, and the one we recommend most often for a simple attic, is foam as an air barrier at the plane with cellulose blown over the top for depth. You pay foam rates for the small job only foam can do and cellulose rates for everything else.

Climate zone 6 wants R-49 in a ceiling under the code’s prescriptive table. Getting there in cellulose alone is inexpensive.

Attic roofline, conditioned attic: closed-cell foam

Not a comparison. Cellulose and fiberglass cannot be applied to the underside of a roof deck in an unvented assembly at all, and the code in climate zone 6 requires air-impermeable insulation there to be a Class II vapor retarder or carry a Class II coating.

If you need a conditioned attic, foam is the material. The question is only which foam. Full comparison here.

Existing closed wall cavities: injection foam or dense-pack cellulose

The two real options for a wall you are not opening.

Dense-pack cellulose is cheaper, well-established, and performs well when it is genuinely dense-packed rather than loosely blown. Its weakness is that density is hard to verify and an under-packed wall will settle and leave a void at the top.

Injection foam cures rigid, does not settle, and seals around wiring and plumbing crossing the cavity. Costs more.

Either way, insist on thermal imaging afterwards. It is the only way anyone can demonstrate a complete fill in work that is behind a wall, and it applies equally to both materials.

Either way, live knob-and-tube wiring is a stop condition. It depends on open air around the conductors to dissipate heat, and burying live circuits in any insulation is a documented fire concern. This is a real constraint in pre-1950 Binghamton and Johnson City housing.

Foundation and crawl space walls: closed-cell foam or rigid board

Not fiberglass, and this is the most consequential “no” on the page.

Fiberglass batts against a cold foundation wall let indoor air through to the concrete, where it condenses, and then hold the water against the studs. It is the assembly behind most of the basement mold stories in this area and it is still being installed.

Closed-cell foam or rigid board applied directly against the masonry keeps the wall face above the dew point. Board is cheaper on a flat, sound wall; foam is required on stone and rubble foundations because board cannot make continuous contact with an irregular surface.

Rim joist: closed-cell foam

The one place where the case is unambiguous. It is a small area, it is the biggest concentrated air leak in most houses, and it needs a continuous air barrier bonded to five surfaces in each bay. Foam does that in one pass.

Fiberglass in a rim joist bay is worse than nothing, because air moves through it, condenses on the cold rim board behind, and the batt holds the water against the wood.

Interior partitions, for sound: open-cell foam or fiberglass

Fiberglass is genuinely fine here and much cheaper. Open-cell foam is better and costs more. Closed-cell is bad at sound because it is rigid.

Where the cheaper material is the right answer

Stated explicitly, because pages like this usually do not.

Buy cellulose instead of foam if: you have an open, accessible attic floor with no ductwork, the ceiling plane below is reasonably intact, and what you want is depth. Foam over the whole area would cost multiples and buy you little that cellulose does not.

Buy fiberglass instead of foam if: you are insulating an interior partition, or you are filling a framed wall that already sits inside a continuous foam layer, or your budget is genuinely fixed and the choice is fiberglass now versus nothing.

Buy dense-pack cellulose instead of injection foam if: the walls are straightforward, the contractor can demonstrate density and verify with thermal imaging, and the price difference matters to you.

Do not buy foam if: your problem is a roof that leaks, a drainage problem in the basement, or a heating system that is undersized. Insulation is not a fix for any of those and it makes two of them harder to diagnose afterwards.

What state incentives cover

NYSERDA’s Comfort Home program is material-agnostic. It pays $2,500 for air sealing plus attic and rim joist insulation and $3,000 for a package adding wall and floor insulation, regardless of whether you use foam, cellulose, or a combination.

That neutrality is worth noticing. The program is paying for the outcome, which means the cheapest way to reach it is the rational choice, and for many houses that is a hybrid rather than foam throughout.

Both require a NYSERDA participating contractor. Amounts change. Comfort Home · EmPower+

Two questions to ask any insulation contractor

1. “Which of these is an air sealing problem and which is an R-value problem?”

If they cannot separate the two for your specific house, they are selling a product rather than solving a problem.

2. “Where in this scope would a cheaper material do the same job?”

An honest answer exists for almost every house. A contractor who says “nowhere” is either working on a very unusual building or is not really answering.

Get a foam insulation quote, and an honest answer about where cellulose would do instead.

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