How the two differ in kind
These are not two versions of the same thing. A batt is a fibrous blanket cut to a module and pushed into a bay, and it does one job: it slows heat conducting across the cavity. Air passes through glass fibre almost unimpeded — a batt is a filter, not a barrier — so in a batt-insulated wall the air-control layer has to exist somewhere else entirely, in taped sheathing, a membrane, or sealed penetrations detailed by whoever is doing that work. Spray foam arrives as two liquids mixed at the gun, expanding and curing against every surface they touch. It insulates, and in the same pass it becomes the air barrier at the plane of the framing; closed-cell at code thickness becomes the vapour retarder too. One material is a component of an assembly, the other is three layers arriving as one operation.
That is why comparing the resistance printed on each product answers the wrong question. In a real house, air leakage carries a large share of what an older wall loses, and no quantity of fibre in the bay touches it — but a batt-filled wall behind a properly detailed, taped, blower-door-verified air barrier gets to the same place, because the air control was simply bought in a different operation. So the honest first question is not which material performs better. It is whether the air barrier is actually going to be built and proven by someone else's careful work. New build with taped sheathing and a test at the end: batts fill the cavity and foam is buying something you already own. A gut retrofit on old board sheathing, or balloon framing, or a job that gets one crew visit and no test at the end: foam is you buying air control by construction rather than by supervision, which is worth a great deal when supervision is the thing you cannot supply.
Two constraints then settle most of the remainder. The first is depth, and it is the only place product resistance per inch genuinely decides anything: in a shallow wall you cannot deepen, closed-cell reaches numbers fibre cannot, and where the cavity is generous or exterior continuous insulation is carrying the performance, that advantage is one you are paying for and not using. The second is permanence. A foamed bay is closed for good — cured foam is bonded to studs and sheathing, wiring is entombed in it, and a plumber's later route means cutting out, chasing and re-spraying a patch by hand. A batt lifts out with one hand and goes back. Note also what neither one does: both live between the studs, so both leave the framing bridging the wall. Recovering that is a job for a continuous layer outboard or inboard of the studs, and no choice made in this comparison substitutes for it.
The factors that actually differ
Show
| Spray foam | Fiberglass batts | |
|---|---|---|
| What the material is actually for | Insulation and air barrier in a single pass — cured foam seals to studs, sheathing and every penetration it reaches, and closed-cell at code thickness is the vapour retarder as well. | Thermal resistance, and only that. Air moves through glass fibre nearly freely, so the air-control layer must be built somewhere else in the assembly or it does not exist. |
| How much rides on the installer | Chemistry rather than craft: substrate temperature, ambient conditions, lift thickness and the ratio of the two components. None of that is judged by eye, and a bad pass is largely invisible once the face is trimmed — the mild version underperforms silently, the severe version smells, and its remedy is removal. | Nearly everything. The label figure assumes full contact on every face, no compression, no void at the top of the bay and every cable split rather than stuffed behind — which is exactly why rating schemes grade batt installations rather than assume them. |
| Bays that are not rectangles | Conforms and adheres. Pipes, blocking, drilled plates, the narrow sliver beside a stud pack, a retrofit bay that was never on module — the foam does not know the difference. | Made to a module. On-module bays take a batt straight from the pack; every odd width, wire, box and pipe is a hand-cut and a split, which the crew either performs properly or quietly does not. |
| When cavity depth is fixed | Closed-cell carries the highest resistance per inch of anything sprayed into a bay — near enough double glass fibre — and that is the entire argument in a shallow wall nobody is going to deepen. Open-cell sits close to fibre and earns its keep on air sealing instead. | Resistance is bought with thickness, and thickness is cheap wherever you have it. A deeper cavity, or a rigid layer outboard of the studs, closes the same gap without buying resistance by the inch. |
| Water, drying and finding a leak | Closed-cell stops vapour and keeps indoor moisture off the sheathing — but it also stops the wall drying inward, so a leak from outside is trapped behind a layer that conceals it. Open-cell is vapour-open and lets the stain appear. | Open in both directions; the wall dries whichever way it can. Wet fibre gives up most of its value while wet and recovers as it dries, and a soaked batt is a visible, cheap, replaceable object. |
| Sound between rooms | Open-cell absorbs about as well as fibre does. Closed-cell is dense and rigid, stiffening the cavity and coupling the two faces of the wall — worse for airborne sound than either open-cell foam or a batt, which surprises people who bought it for everything else. | The default acoustic fill, and the reason most tested partition assemblies contain a limp fibrous blanket rather than anything stiffer. |
| Fire and the code overlay | It is a foam plastic. Code requires a thermal barrier over it — normally the gypsum you were fitting anyway — an ignition barrier at minimum in concealed spaces, and in certain exterior wall constructions a specifically tested assembly rather than an assembled one. | The glass itself does not burn. The kraft facing does, which is why faced batts have to be covered and must never be left exposed in an attic, a basement or a garage. |
| Changing the wall later | One shot. The foam is bonded to the framing, the wiring is inside it, and a later service run means cutting it out, chasing, and hand-patching a cavity that was sprayed as a continuous layer. | Reversible by design: pull the batt, do the work, put it back. It is why a building still being altered keeps fibre in its walls whatever the energy model would prefer. |
| Where the cost lands and how it scales | A crew, a rig and a chemical set — mobilisation plus material, priced by wall area, and heavier on every axis. The air seal comes inside that same area price whether the wall was leaky or already tight. | A stocked commodity you can carry home and fit yourself, priced by area — with the air sealing it still needs priced separately, and THAT cost follows joints, edges and penetrations rather than area. It scales with the wall's perimeter and its holes, which is why it lands hardest on a cut-up, opening-heavy elevation and lightest on a long blank one. |
Which one, and when
Choose spray foam when…
- Cavity depth is fixed and the target is not reachable inside it — a shallow retrofit wall, a rim joist, a bay you cannot make deeper without moving the finishes.
- Nobody else is going to build the air barrier. Old sheathing, balloon framing, one crew visit and no blower-door test at the end: this is the case foam was made for, and the case where its premium buys something real.
- The bays are not rectangles — irregular framing, blocking everywhere, services already in place, spacing that was never on module.
- Closed-cell is being specified deliberately as the condensation-control layer against the sheathing, at the thickness the climate-zone table sets rather than at whatever happens to fit.
- You want insulation, air and vapour control from one trade in one operation, and you accept in exchange that the wall becomes permanent.
Choose fiberglass batts when…
- An air barrier is being built and proven anyway — taped sheathing or a membrane, sealed penetrations, a blower-door number at the end. Then the cavity's only remaining job is resistance, and that is the job batts do most cheaply.
- The wall is not finished changing: services still to run, a window that may yet move, a plan being drawn while the building goes up.
- You are doing the work yourself, in stages, in an occupied house. Batts pause at any bay, need no ventilation period, and never require anyone to leave the building.
- The cavity is deep enough, or continuous exterior insulation is doing the heavy lifting. Depth is the cheap way to buy resistance and product is the expensive way, so the order of operations matters.
- You want a wall with no chemical failure mode. A bad batt job is a visible defect corrected for nothing on the afternoon before the drywall goes on.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which actually performs better in a finished wall?
- It depends, and it depends on one thing more than all the others: whether an air barrier is going to be built and tested by some other part of the job. Where it is, the two walls converge, because the batt wall has already bought foam's headline advantage in a separate and cheaper operation — and the remaining difference is resistance per inch, which only matters if the cavity is too shallow to reach the target with fibre. Where no air barrier is coming, the walls diverge sharply, and not in fibre's favour: a perfectly filled batt bay behind leaky sheathing still lets air move through and around the insulation, and the loss does not appear on any label. So do not ask which material is better. Ask who is sealing this wall, in which operation, and how anyone will know it happened.
- Which is cheaper?
- Batts, comfortably, on material — and this site will not attach a figure to that, though the shape of the difference is more useful than a number. Foam is a crew, a rig and a chemical set: mobilisation plus material, priced by area, with a specialist market rather than a competitive commodity one. Batts are stocked everywhere and can be fitted with labour you supply yourself. The honest comparison is not material to material, though, because the two are not buying the same scope. Foam's price includes an air barrier; the batt wall still needs one, and that cost behaves differently — it follows joints, edges, top and bottom plates and every penetration, so it scales with the wall's perimeter and its holes rather than with its area. On a long, simple, opening-light wall that is a small addition. On a cut-up elevation full of windows and service runs it is not. Then price the cost of being wrong, which is wildly asymmetric: a poor batt fill is corrected for nothing before the drywall, and a foam problem is a demolition job. Run the quantities on both calculators first — packages from your product's printed coverage on one side, board feet or volume from area and thickness on the other — then take those quantities to local quotes.
- Open-cell or closed-cell — does that change the comparison?
- More than the headline choice does, and treating spray foam as one product is the most common mistake made on this decision. Open-cell is light and vapour-open: it fills a bay completely, air-seals it, absorbs sound about as well as fibre, lets the wall dry in both directions and shows a roof or cladding leak as a stain on the ceiling below. Its resistance per inch sits near glass fibre, so against batts it is buying air sealing, not depth. Closed-cell is dense and rigid: the highest resistance per inch available in a cavity, a vapour retarder in its own right at code thickness, a real contributor to racking strength, and the one that must be specified thick enough for its climate zone if it is being relied on for condensation control. Its costs are the mirror image — it is the dearest of the three, the worst for sound, and it hides a leak instead of revealing one. Choosing between open-cell and closed-cell is genuinely a different decision from choosing between foam and batts, and it should be made second, on its own merits.
- Is flash-and-batt a compromise or the right answer?
- Usually the right answer, and it is what a lot of experienced crews build when nobody is watching the marketing. A thin flash of closed-cell against the sheathing supplies the air seal, the vapour control and the condensation control at the surface where those matter, and batts fill the rest of the depth with cheap resistance. The rule that governs it is not a preference: the adopted code sets a minimum for the foam layer by climate zone — how much of the assembly's resistance has to sit outboard of the fibre — and going thinner than that table moves the condensing surface into the sheathing — the exact failure the flash was meant to prevent. Look the figure up for the jurisdiction on the permit rather than eyeballing the lift. Done properly, the hybrid also keeps most of the cavity reversible, since the batts still lift out for a later cable even though the flash never will.
- What actually goes wrong with each, and what does the fix cost?
- Batts fail by installation: voids at the top of a bay, compression behind wiring, a batt stuffed around a cable instead of split, a bay left short at a corner. Every one of those defects is visible while the wall is open, and every one is corrected in minutes with the material already on site. Foam fails by chemistry: components sprayed off-ratio, a substrate colder than the manufacturer's window so the first lift never bonds, lifts laid too thick so the exotherm cooks the middle. Those defects are largely invisible once the face is trimmed. The mild version underperforms quietly. The severe version is foam that never fully cures and keeps releasing an odour, and its only reliable remedy is removal — scraping cured plastic off studs and sheathing, sometimes taking the sheathing with it. This asymmetry is worth more weight than most comparisons give it: one product's worst case is an afternoon and the other's is a gut. It argues for a foam contractor with references you can telephone, a documented substrate temperature, and a job not sprayed on the coldest morning of the year.
- Will foaming the walls make the house too tight?
- A house does not become too tight; it becomes tight enough that ventilation stops being optional. Air that used to arrive through the walls by accident was never controlled, never filtered and never balanced, and it carried its heating or cooling cost with it — losing it is the point of the exercise. What has to replace it is mechanical: continuous extract from the wet rooms at minimum, and in a genuinely airtight house a supply path too, ideally with heat recovery so the ventilation is not simply the leakage you just paid to eliminate. Two things deserve checking before the sprayers arrive rather than after. Any atmospherically vented combustion appliance — an open-flue boiler, a gas water heater drawing its air from the room — needs its combustion air and its draught reconsidered in a tightened building, because backdrafting is a safety problem, not a comfort one. And a wall that no longer dries by air movement has to be able to dry by diffusion in at least one direction, which is the decision made when you choose between open-cell and closed-cell.