Spray Foam vs Fiberglass Insulation

Spray foam vs fiberglass insulation is the choice most people reach once they have decided to insulate and a contractor has quoted both. The two materials sit at opposite ends of the insulation market. Fiberglass is the cheap, familiar batt that has filled wall cavities for seventy years. Spray foam is the premium product that seals and insulates in a single pass. This guide puts them head to head on the six things that actually decide which one belongs in your wall: R-value per inch, air sealing, moisture behaviour, cost, installation, and how long each lasts. Then it sets out a plain rule for when to pick each.

The performance figures below come from US Department of Energy material data, ICC-ES evaluation reports, and manufacturer technical sheets, cross-checked in 2026 against current installed-price surveys. They are planning ranges, not single numbers, because insulation performance and price both move with product grade, climate, and who does the work. Where you need the exact R-value or the exact spend for your own wall, the calculators linked through this guide do the arithmetic.

Side by Side: Spray Foam vs Fiberglass

The fastest way to read the difference is in one table. Closed-cell spray foam leads on nearly every technical measure; fiberglass leads on price and simplicity. Open-cell spray foam sits in between, sharing foam's air seal but landing at an R-value close to fiberglass.

Property	Fiberglass batt	Open-cell spray foam	Closed-cell spray foam
R-value per inch	R-2.9 to R-3.8 (about R-3.2)	R-3.5 to R-3.7	R-6 to R-7 (about R-6.5)
Air sealing	None on its own	Air barrier	Air barrier
Moisture / vapour	Vapour-open; loses R when wet	Vapour-open; absorbs water	Vapour barrier at 2 in or more
Structural strength	None	None	Adds racking strength
Installed cost (per sq ft)	$0.65 to $2.00	$1.50 to $3.00	$3.00 to $7.00
DIY-friendly	Yes	No	No
Lifespan (kept dry)	80+ years	80+ years	80+ years

The headline is simple. If the wall has deep, accessible cavities and your budget is tight, fiberglass does the job for a fraction of the price. If the cavity is shallow, the wall meets damp ground, or air leakage is the real problem, spray foam earns its premium. The rest of this guide explains why each row of that table falls the way it does.

R-Value per Inch and the Space You Have

R-value per inch decides how much thermal resistance you can pack into a fixed cavity depth. Fiberglass batts deliver about R-3.2 per inch, open-cell foam about R-3.7, and closed-cell foam about R-6.5, roughly double the fiberglass figure. That gap only matters when space is limited, which in a stud wall it almost always is.

Take a standard 2x4 wall with 3.5 inches of cavity. A fiberglass batt fills it to about R-13, open-cell foam reaches a similar R-13, and closed-cell foam hits R-22.75 in the same depth, clearing the R-20 wall requirement that fiberglass cannot meet without a deeper wall. Where the cavity is generous, fiberglass closes most of the gap simply by being thicker, which is why the R-per-inch advantage rarely justifies foam on its own. For how layered assemblies and continuous insulation add up across every material, the way R-values stack from material layers covers the rules that apply to both products.

Air Sealing: The Difference That Decides Most Jobs

R-value measures resistance to heat conducting through solid material. It says nothing about air leaking through gaps, and in most homes that air leakage moves more heat than conduction does. This is the dimension where the two materials genuinely diverge. Fiberglass is air-permeable: it slows conducted heat but air drifts straight through it, so a fiberglass wall still needs a separate air barrier of taped sheathing or housewrap to perform.

Spray foam, both open and closed cell, expands to fill every void and cures into an air barrier in the same step. The US Department of Energy notes that this air-sealing action can remove the separate caulking, housewrap, and joint-taping tasks a fiberglass wall depends on. That is foam's real selling point, more than the R-value bump. A leaky wall packed with high-R fiberglass underperforms a sealed wall with modest foam, because the draft path defeats the insulation. Before you compare materials, it pays to size the R-value your climate zone actually requires, since the target sets how much of either material you need.

Moisture, Vapour, and What Happens When Walls Get Wet

Water behaviour separates these materials as sharply as air sealing does. Fiberglass is vapour-open and has no moisture control of its own; worse, when it gets wet the fibres clump and the batt loses much of its rated R-value until it dries. Open-cell foam is also vapour-permeable, which lets a wall dry in both directions in mixed climates, but it will absorb liquid water if a wall leaks. Closed-cell foam is the outlier: at two inches or more it becomes a vapour barrier in its own right, controlling moisture movement without a separate poly sheet.

That is why closed-cell foam is the standard pick for basements, crawl spaces, and any wall meeting damp soil, where fiberglass facing traps moisture against cold masonry and invites mould. If your wall needs a dedicated vapour layer instead, the poly sheeting estimate for crawl spaces and cavities sizes it, and for block walls the composite R-value of a CMU assembly shows how little the block itself contributes before any insulation goes on.

What You Pay, and Where the Money Goes

Price is where fiberglass wins outright. Installed, fiberglass runs roughly $0.65 to $2.00 per square foot, open-cell foam $1.50 to $3.00, and closed-cell foam about $3.00 to $7.00 on typical wall work. Across the board, spray foam costs two to three times what fiberglass does for the same area. The foam premium buys the air seal, the higher R-per-inch, and in the closed-cell case the vapour control, not a small upgrade to an otherwise identical product.

Those bands are planning figures; the real number turns on thickness, access, and region. A full closed-cell fill of a deep cavity or a cathedral roofline climbs well past the wall range, while open-cell on an attic floor stays cheap by volume. For a project-specific total, run your area and target thickness through the board-foot cost estimate for open and closed cell, and to weigh insulation against other envelope work the payback estimate across insulation, windows, and air sealing puts the spend in context.

Before you commit to either material, it helps to fix the target first. The tool below checks the minimum R-value your climate zone demands for walls, attics, and floors, which is the number both materials have to hit.

Installation, Lifespan, and Health

Fiberglass is a genuine do-it-yourself material. Batts cut with a knife, press into cavities, and need no special kit, so a careful homeowner can insulate a room over a weekend for the cost of the batts alone. Spray foam is not a DIY job at any real scale. It needs heated, pressurised equipment that holds a tight chemical ratio, and off-ratio foam fails to cure, emits isocyanate vapour, and triggers a re-entry delay after spraying. That work belongs with a certified installer. One install upside for open-cell foam is sound: it damps airborne noise better than fiberglass at the same thickness, so the partition design for quieter interior walls often leans on it.

On lifespan, both materials last the life of the building when kept dry, but they age differently. Fiberglass holds its R-value for 80 years or more if it stays dry and uncompressed, yet it can sag or settle in wall cavities and is ruined by repeated wetting. Spray foam adheres permanently and does not slump, though closed-cell foam sheds a small amount of R-value in its first year or two as the blowing agent stabilises, after which it holds steady.

How to Choose Between Them

Strip away the marketing and the decision comes down to the surface in front of you and what problem it has. The four cases below cover almost every residential wall, ceiling, and floor.

• Choose fiberglass when the budget leads, the cavity is deep and accessible, the job is a DIY one, and the wall already has a sound air barrier in the sheathing. This is most new-build interior framing.
• Choose closed-cell spray foam when the cavity is too shallow to hit your R-value target with batts, the wall meets damp ground, or air leakage is the real fault you are fixing. Basements, rim joists, and cathedral rooflines are its home ground.
• Choose open-cell spray foam when you want foam's air seal in a generous interior cavity, value the vapour-open drying, or are chasing sound control on a partition rather than maximum R-value.
• Use flash-and-batt when you want the air seal without paying foam prices for the full depth: an inch or two of closed-cell against the sheathing, then fiberglass to fill the rest of the cavity.

Notice that three of the four cases are not all-or-nothing. Flash-and-batt in particular gets you foam's air-tightness at the sheathing and fiberglass economics for the bulk of the wall, which is why it has become the default on many gut renovations.

Matching the Material to the Job

When we opened the walls during our Northumberland renovation, the old fiberglass had slumped in the cavities and gone damp where the gable wall had been letting water in for years, doing almost nothing. We sprayed closed-cell foam on that cold, damp gable to seal and insulate it in one go, and used cheap fiberglass batts in the dry interior partitions where air sealing was already handled and budget mattered more. The same house took both materials, each on the surface that suited it. That is the honest way to read spray foam against fiberglass: not which is best in the abstract, but which is right for this wall, this moisture risk, and this budget, with flash-and-batt waiting whenever the answer is a bit of both.