Skip to content
ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Open Cell vs
Closed Cell
Spray Foam

Which foam goes where: attic roof decks, walls, basements, rim joists, crawl spaces and metal buildings, with the code section behind each call, cost per R, and three tools from the people who make both foams.
Spray Foam Quotes

Not sure which foam your job needs? Get a quote from a certified applicator.

Tell us what you're insulating, where it is and roughly how big. We'll connect you with an ArmorThane-certified spray foam applicator who will quote foam type, thickness and R-value in writing, and can spray open-cell, closed-cell or both.

  • Attics, walls, basements and crawl spaces
  • Rim joists, pole barns and metal buildings
  • Foam type, thickness and R-value on the quote
  • Contractors: ask about ArmorFoam and spray rigs
Rather talk now? Call (417) 831-5090
Step 1 of 3
R-6.5 vs R-3.7
R-value per inch
Closed-cell aged vs typical open-cell
≈2 vs 0.5
Density, lb/ft³
ArmorFoam closed-cell vs ArmorFoam 0.5
II vs III
Vapor retarder class
Closed-cell at about 1–2 in; open-cell never
≈1.5x
Closed-cell cost per R
vs open-cell, 2026 mid prices (2.6x per board foot)

Open-cell and closed-cell spray foam come out of similar guns, stick to the same framing and both stop air. After that they're different materials, and the right one depends almost entirely on where it's going.

Quick answer

Use closed-cell where surfaces are cold, damp or shallow: rim joists, basement and crawl space walls, metal buildings, vans, shallow cavities, and roof decks in climate zones 5–8. Use open-cell where there's depth to fill and no water: 2x6 walls in zones 1–3, roof decks in zones 1–4, and interior walls for sound. Per R-value, closed-cell costs about 1.3 to 1.7 times as much as open-cell at 2026 prices, not the 2–3 times its board-foot price suggests. And on an open, vented attic floor, use neither: blown insulation is cheaper. Jump to the foam picker.

Most comparisons stop at R-value per inch and a list of pros and cons. That's not how the choice gets made on a real job. It gets made spot by spot: how deep the cavity is, whether the surface runs cold or damp, and what your climate zone's code says about vapor. So that's how this page is organized, with the code section behind each call and three tools that do the math for your house.

We make both. ArmorFoam closed-cell and ArmorFoam 0.5 open-cell come out of our plant in Springfield, Missouri, so we don't gain anything by steering you to one. Where the honest answer is neither foam, we'll say that too. For the full specs on each, see our closed-cell spray foam guide and open-cell spray foam guide. This page is the decision between them.

What's in this guide
  1. 1What's the difference?
  2. 2Which foam goes where (foam picker)
  3. 3Attic roof decks (deck builder)
  4. 4Exterior walls and the vapor rule
  5. 5Basements, crawl spaces and rim joists
  6. 6Metal buildings, pole barns and vans
  7. 7Interior walls and sound
  8. 8Cost per board foot and per R (calculator)
  9. 9Moisture, leaks and mold
  10. 10How to tell which foam you have
  11. 11ArmorFoam, and when we'd sell you neither
  12. 12Open vs closed cell FAQ
The basics

What's the difference between open-cell and closed-cell spray foam?

Both are two-part polyurethane foam sprayed from a heated proportioner. In closed-cell foam most of the tiny cells stay sealed, so it cures dense (about 2 lb/ft³), rigid and R-6.5 per inch, and it becomes a vapor retarder at an inch or two. Open-cell's cells break open as it rises. It cures at about 0.5 lb/ft³, soft, around R-3.7 per inch, and it stays vapor-open at any thickness.

PropertyClosed-cellOpen-cellWhy it matters
R-value per inchR-6.5 aged (ArmorFoam, ASTM C518); R-5 to R-7 across brandsR-3.7 typical (one ESR: R-13 at 3.5 in); ArmorFoam 0.5: R-3.9 at 1 inDecides how many inches you need
Core density1.8–2.0 lb/ft³ (ArmorFoam 2.0)0.5 lb/ft³ (ArmorFoam 0.5)Rigid vs soft; material per inch
Air barrierAt 1 in (ArmorFoam, ASTM E2178). DOE uses 1.5 inProduct-specific: 4 in in one ESR. DOE uses 5.5 inStops the air leakage batts can't
Vapor retarder classClass II at about 1–2 in (ArmorFoam: 0.83 perm at 1 in)Class III (vapor-open) at any thicknessDecides cold-climate walls and roof decks
WaterTolerates occasional wetting; not a membraneAbsorbs water; DOE: not below gradeBasements, crawls, flood-prone walls
Feel and strengthHard; stiffens the framing it bonds toSoft and spongy; flexes with framingMetal, vehicles, racking
Sound in the cavityModest absorberBetter absorberInterior walls, media rooms
Blowing agentHFO or water; GWP under 150 since 2025 (EPA)Water (CO₂)Why product names changed in 2025
Installed cost, 2026$1.00–$2.12 per board foot$0.44–$0.73 per board footPer R, the gap is only about 1.5x
Yield per 55-gal set≈5,000 board feet (ArmorFoam, ideal)≈17,000 board feet (ArmorFoam, ideal)Why open-cell is cheaper per inch
Sources: ArmorThane data sheets; HomeGuide (Dec 2025) for brand ranges; IAPMO UES ER-0913 (open-cell ESR); Huntsman design note BES-102 and BSC BSD-149 for vapor and air thresholds; DOE Building America and Energy Saver; 2026 price bands from our spray foam insulation cost guide.

R-value per inch

ArmorFoam closed-cell tests at R-6.5 per inch aged. Across brands, HomeGuide puts closed-cell at R-5 to R-7 and open-cell at R-3.5 to R-3.7. One open-cell evaluation report (IAPMO ER-0913) lists R-3.7 at 1 inch, R-13 at 3.5 inches and R-20 at 5.5; our ArmorFoam 0.5 tests at R-3.9 at 1 inch and R-13.6 at 3.5. Call it 1.76 inches of open-cell for every inch of closed-cell. Whether that matters depends on how many inches you've got. The full thickness chart is in our spray foam R-value guide.

Air and vapor: the difference that decides most jobs

Both foams stop air, which is most of why spray foam outperforms batts. The code calls insulation "air-impermeable" at 0.02 L/s·m² or less at 75 Pa (IRC R202). Closed-cell gets there at about an inch; BSC's unvented-roof digest says the same. Open-cell gets there too, but thicker, and the number is product-specific: one report shows 4 inches. DOE's Building America guidance plays it safe with 5.5 inches for open-cell and 1.5 for closed-cell.

Vapor is where they part ways. The IRC sorts vapor retarders into Class I (0.1 perm or less), Class II (over 0.1 up to 1.0) and Class III (over 1.0 up to 10). Closed-cell becomes Class II at roughly 1 to 2 inches, depending on the product. ArmorFoam measures 0.83 perm at 1 inch. Open-cell stays Class III however thick it is. That one fact decides the foam in most cold-climate walls and roof decks, as you'll see below.

Water, strength and sound

Closed-cell sheds water and shrugs off occasional wetting. Open-cell soaks it up: DOE says it "should not be used below ground level where it could absorb water," and open-cell evaluation reports bar continuous contact with water. Neither one is a roof, a housewrap or a foundation membrane.

Closed-cell is rigid and bonds to what it touches. Industry testing shows it stiffens walls against racking, but no US model code lets you count it as bracing (the numbers are in our closed-cell guide). Open-cell stays soft, moves with the framing and soaks up more sound in the cavity.

Blowing agents

Open-cell foams are blown with water: the reaction makes CO₂. Closed-cell made or imported in the US since January 1, 2025 has to use a blowing agent with a global warming potential under 150 under EPA's HFC rules, which is why most brands moved to HFOs. If your 2026 quote names a different product than a 2023 quote did, that's the reason.

Interactive tool 1 of 3

Open cell or closed cell: which foam goes where?

Closed-cell goes where surfaces are cold, damp or shallow: rim joists, basement and crawl walls, steel, vehicles and cold-climate roof decks. Open-cell goes where there's depth to fill and no water: roof decks in zones 1–4, 2x6 walls in zones 1–3 and interior walls. Here's the house at a glance, then the full chart and a picker that applies your climate zone.

1 2 3 4 5 GRADEBASEMENTLIVING SPACE

Closed-cell   Open-cell   Depends on climate zone and depth

  1. Roof deck, unvented attic. Either foam in zones 1–4; closed-cell or flash-and-fill in zones 5–8.
  2. Exterior walls. Open-cell where the cavity reaches the code R-value; closed-cell or a hybrid where it can't.
  3. Rim joist. Closed-cell, 1.5–3 in.
  4. Basement or crawl wall. Closed-cell or rigid foam board, never open-cell against concrete.
  5. Interior wall. Open-cell (or mineral wool) for sound.

Open cell vs closed cell by location

LocationOur pickWhyCode that drives it
Unvented attic roof deck, zones 1–4Either; open-cell is cheaperDeep rafters to fill and no Class II rule. The attic needs conditioned or dehumidified air.IRC R806.5; Table R806.5 R-5 to R-15 if batts go under the foam
Unvented attic roof deck, zones 5–8Closed-cell or flash-and-fillFoam at the deck must be Class II or coated. Open-cell lets more winter moisture reach the sheathing.R806.5 item 4; Table R806.5 R-20 to R-35
Vented attic floorNeither: blown cellulose or fiberglassUnlimited depth makes loose-fill the cheapest R. Use foam only to air-seal top plates and penetrations.2021 IECC ceiling R-30 to R-60
Cathedral ceiling, shallow raftersClosed-cell or flash-and-battLittle depth; the foam has to carry condensation control. See our cathedral ceiling insulation guide.R806.5
2x6 exterior wall, zones 1–3Open-cellA full 5.5 in is about R-20, which meets the code at the lowest cost.2021 IECC R-13 (zones 1–2), R-20 (zone 3)
2x6 exterior wall, zones 4–8Closed-cell, or open-cell + exterior foamR-30 in the cavity takes 4.7 in of closed-cell. Open-cell tops out near R-20, so it needs R-5 outside.R-30 or 20+5ci; R702.7 vapor rule in 5–8 and Marine 4
2x4 exterior wallOpen-cell in 1–2; closed-cell in 3; add exterior foam in 4–8Full depth is R-13 in open-cell, R-22.8 in closed-cell.R-13 / R-20 / 13+10ci or 20+5ci
Rim joistsClosed-cellCold wood and three leaky joints. See our rim joist insulation guide.IRC R316.5.11: exposed up to 3¼ in
Basement wallsClosed-cell or rigid foam boardConcrete wicks ground moisture. DOE: no open-cell below grade.5ci to 15ci by zone; exempt from R702.7
Crawl space walls (sealed crawl)Closed-cellSame reasons as basements, plus tight access.Same as basement walls
Floor over a vented crawlClosed-cell, or seal the crawl insteadThe underside breathes outdoor humidity all summer.Floor R-13 to R-38 by zone
Metal building or pole barn steelClosed-cell (open-cell only as fill over it)Stops warm, moist air reaching cold steel. Open-cell is vapor-open.Separate metal-building code tables
Vans, trailers, containersClosed-cellThin steel, condensation and vibration.None for vehicles
Interior walls and floorsOpen-cell or mineral woolSoft foam absorbs sound; there's no thermal job to do.No energy-code requirement
Code values: 2021 IECC via DOE/PNNL; IRC R806.5 and R702.7 model text. Thicknesses use R-6.5 per inch for closed-cell and R-3.7 for open-cell. Your local code edition and amendments govern.
Foam picker

Open-cell or closed-cell for this spot?

Pick the location, your IECC climate zone and the framing depth. Most of the South is zone 2–3, the Midwest 4–5, the northern tier 6–7; Marine 4 is the coastal Pacific Northwest.

Studs, rafters or floor joists
Also true here
Either foam; open-cell costs lessZone 4
FoamThickness for R-60Fits the depth?Vapor classAir barrier
Open-cell (R-3.7/in)16.2 inNoClass III (vapor-open)Product-specific; DOE uses 5.5 in
Closed-cell (R-6.5/in)9.2 inYesClass II at about 1–2 inAt 1 in (ArmorFoam, ASTM E2178)

Attic roof deck (unvented attic), 9.25-in cavity. Target: R-60 ceiling (2021 IECC, zone 4).

  • Zone 4 has no Class II rule for the deck, so open-cell is allowed and costs less per R.
  • Filling the 9.25-in rafters gives R-34.2 with open-cell or R-60.1 with closed-cell. The roof-deck builder below prices both and a hybrid.
  • Either way the attic becomes indoor space: give it conditioned or dehumidified air (DOE Building America). Damp open-cell attics are a known problem when that step is skipped.
  • If you add batts under the foam instead, the foam must reach R-15 (Table R806.5).
  • Foam decks rarely reach the prescriptive R-60. Most are permitted through the performance (ERI) path; ask which path your quote assumes.

Read next: cathedral ceiling insulation · get a quote from a certified applicator

How the picker decides: thickness to target = code R-value ÷ R per inch (R-3.7 open-cell, R-6.5 closed-cell, with a 1-inch closed-cell minimum for air sealing); "fits" compares that to the framing depth. Vapor rules follow IRC R702.7 for walls and R806.5 for roof decks. It's a screening tool. Your code official and the product's evaluation report have the final say.

Interactive tool 2 of 3

Open cell vs closed cell spray foam on an attic roof deck

In climate zones 1–4, either foam works on the underside of the roof deck, and open-cell is cheaper. In zones 5–8 the IRC wants the foam against the deck to be a Class II vapor retarder, so the simple routes are closed-cell or flash-and-fill (closed-cell first, open-cell after). All-open-cell in those zones needs a vapor-retarder coating and a conditioned attic.

What the code says: IRC R806.5

Spraying the deck turns the attic into indoor space, and the IRC allows that under R806.5 when the attic sits inside the thermal envelope, there's no Class I vapor retarder (like poly) on the ceiling side, and the insulation meets two moisture rules:

  • Item 4, zones 5–8: "any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation." Closed-cell at its Class II thickness passes on its own. Open-cell doesn't.
  • Table R806.5, hybrids: when air-permeable insulation (batts, loose-fill) goes under the foam, the foam against the deck must reach a minimum R-value for condensation control.
Climate zoneTable R806.5 minimumClosed-cell at R-6.5/inClass II rule (item 4)?
1, 2A, 2B, 3A, 3B, 3CR-50.8 in (spray 1.5 in for a reliable air seal)No
4C (Marine)R-101.5 inNo
4A, 4BR-152.3 inNo
5R-203.1 inYes
6R-253.8 inYes
7R-304.6 inYes
8R-355.4 inYes
IRC R806.5 and Table R806.5 (2015 text; the values are the same in later editions). Zones 2B and 3B with tile roofs need none. The table "contributes to but does not supersede" your energy-code R-value. Closed-cell inches are our math.

Open-cell decks need a dry attic

Open-cell on a roof deck isn't a mistake. It needs its conditions met. Building Science Corporation's field study of eleven in-service foam roofs found sheathing moisture "well within the safe range" at every site; the one failure it reviewed was foam sprayed onto wet OSB. The same report put the trade-off plainly: open-cell "dries more readily" than closed-cell, but "allows more wetting of the sheathing during the winter."

That's why DOE's Building America guidance says to condition the new attic: exhaust with motorized dampers, an HRV or ERV, a dehumidifier, or ducted supply and return from the HVAC system. It also says to fix any roof leak before foaming, and cites BSC's advice to keep winter indoor humidity under 35% in zones 6–8. Skip the attic air and open-cell attics can run damp, which is the source of most horror stories about it.

Flash-and-fill: the cheaper way to use closed-cell

Spray enough closed-cell against the deck to handle vapor and condensation, then fill the rest of the rafter with open-cell. You get closed-cell where it counts and open-cell prices for the bulk of the R-value. Size the closed-cell layer to at least the Table R806.5 value for your zone, the same rule the code applies to foam under batts. In zones 5–8 some inspectors read item 4 as covering the open-cell layer too and will ask for a coating on its underside, so ask before you price it.

Roof-deck builder

All open-cell, all closed-cell, or flash-and-fill?

Compares the three ways to foam an unvented roof deck at the same target R, checks each against R806.5 and prices it at 2026 installed rates.

Sloped deck plus gable ends
Cheapest that reaches R-34: All open-cell$10,915 typical
OptionLayersTotal RTargetCode checkInstalled (low–high)$/sq ft
All open-cell Best value9.25 in open-cellR-34.2MeetsOK in zone 4; condition the attic air$8,140–$13,505$5.46
All closed-cell5.5 in closed-cellR-35.8MeetsClass II at this thickness$11,000–$23,320$8.58
Flash-and-fill2.5 in closed-cell + 5.0 in open-cellR-34.8MeetsClosed-cell layer R-16.3 vs Table R806.5 R-15$9,400–$17,900$6.85

Target R-34.2 (what open-cell gives when it fills the rafters). The 2021 IECC prescriptive ceiling in zone 4 is R-60; most foam decks are permitted through the performance path instead. Prices: 2026 median installed bands, open-cell $0.44–$0.73 and closed-cell $1.00–$2.12 per board foot. Thermal or ignition barrier not included. At totals of R-49 and up, size the closed-cell layer by ratio, not just the table: our cathedral ceiling guide runs that check by city.

Vented attic floor? Neither foam, usually

If the attic stays vented and nothing important lives up there, the insulation belongs on the attic floor, and there you have all the depth you want. Blown cellulose or fiberglass buys R-value far cheaper than either foam; our closed-cell guide says the same. Spend the foam money air-sealing top plates, chases and penetrations first. Shallow rafters in a vaulted room are a different problem, covered in our cathedral ceiling insulation guide.

Walls

Open cell or closed cell in exterior walls?

In zones 1–3, fill 2x6 walls with open-cell: 5.5 inches is about R-20 at the lowest cost. In zones 4–8 the 2021 IECC asks for R-30 in the cavity or R-20 plus R-5 of exterior foam, and only closed-cell reaches R-30 inside a 2x6. In zones 5–8 and Marine 4, open-cell walls also need a Class II vapor retarder on the inside.

Wall and foamThicknessR-valueMeets cavity-only code inVapor in zones 5–8, Marine 4
2x4, open-cell full3.5 inR-13Zones 1–2; zone 3 with R-5 and zones 4–8 with R-10 exterior foamAdd Class II inside
2x4, closed-cell full3.5 inR-22.8Zones 1–3; zones 4–8 with R-5 exterior foamClass II itself
2x4, flash-and-fill (2 + 1.5 in)3.5 inR-18.6Zones 1–2Class II at 2 in closed-cell
2x6, open-cell full5.5 inR-20.4Zones 1–3; zones 4–8 with R-5 exterior foamAdd Class II inside
2x6, closed-cell4.7 inR-30.6All zonesClass II itself
2x6, flash-and-fill (2 + 3.5 in)5.5 inR-26Zones 1–3; zones 4–8 with R-5 exterior foamClass II at 2 in closed-cell
Our math at R-3.7 and R-6.5 per inch against 2021 IECC wood-frame wall values (R-13 in zones 1–2; R-20 or 13+5ci in zone 3; R-30, 20+5ci, 13+10ci or 0+20ci in zones 4–8), via DOE/PNNL. Cavity R-value isn't whole-wall R-value: studs bridge around any cavity fill.

The vapor rule in zones 5–8 and Marine 4 (IRC R702.7)

"Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4." Closed-cell at its Class II thickness satisfies that by itself. Open-cell doesn't, so it needs help: kraft facing, vapor-retarder paint on the drywall, or enough exterior continuous insulation to qualify for a Class III retarder under Table R702.7.1. Huntsman's design note on the subject shows both routes working in a zone 5 2x6 wall. Going the other way, the IBC doesn't allow Class I or II retarders on the inside of frame walls in zones 1–2, which is one more reason open-cell fits the South.

Hybrids need enough closed-cell

Flash-and-fill (or flash-and-batt) in a wall works when the closed-cell layer is thick enough to keep its inner face above the room's dew point and to be Class II. The same design note shows ½ inch of closed-cell under a batt failing on both counts in Buffalo. Plan on about 2 inches, the thickness BSC uses for closed-cell as a vapor retarder. In very cold climates, run the numbers; our rim joist guide has a condensation check that works the same way.

Existing, closed walls

Neither foam here. Open-cell and closed-cell spray foam both go into open framing, so they fit new construction, additions and gut remodels. A finished wall needs a different product (injection foam) or the drywall off.

Below grade

Basements, crawl spaces and rim joists: closed-cell territory

Below grade and at the rim, use closed-cell or rigid foam board. Concrete wicks ground moisture and rims run cold, and open-cell is vapor-open and absorbent: DOE says it shouldn't go below grade at all. Closed-cell air-seals, controls vapor and handles occasional dampness in one pass.

Basement walls

The 2021 IECC asks for R-10 continuous in zone 4 and R-15 in zones 5–8 (R-5 in zone 3). That's about 1.5 inches of closed-cell in zone 4 and 2.3 inches in zones 5–8. BSC's basement information sheet shows 3 inches of closed-cell at R-18 against the concrete and says "no interior vapor barriers should be installed," so the wall can dry inward. Basement walls are also exempt from the IRC R702.7 vapor-retarder rule. To be fair to foam board: sealed rigid foam does this job for less material money if you're doing it yourself. Details, including finished-basement framing, are in our basement wall insulation guide.

Crawl spaces

In a sealed crawl space the walls are the boundary, and closed-cell goes on them at the same R-values as basement walls. In a vented crawl the floor above is the boundary, and its underside breathes outdoor air all summer. That's a poor home for vapor-open, absorbent open-cell. DOE Energy Saver calls an unventilated crawl space "the best option in homes in hot and humid climates" and says the best approach is to "seal and insulate the foundation walls rather than the floor." Our crawl space insulation guide walks through both.

Rim joists

Closed-cell, 1.5 to 3 inches (about R-9.8 to R-19.5). The rim is cold wood with three leaky joints, and closed-cell is the air barrier and the vapor retarder in one pass. The IRC lets foam stay exposed at the rim up to 3¼ inches if it meets density and flame/smoke limits (R316.5.11). DOE's Building America guidance for band joists next to a garage accepts open-cell but prefers closed-cell in zones 1A, 2A, 7 and 8. We'd use closed-cell almost everywhere.

Steel

Metal buildings, pole barns, vans and containers

Put closed-cell against steel. Metal sweats when warm, moist air reaches it; closed-cell bonds to the panel, stops that air and is a vapor retarder at about an inch or two. Open-cell is vapor-open, so on its own it lets moisture through to cold steel in heating climates. It's fine as a cheaper fill layer over closed-cell.

On metal buildings and pole barns, 1 to 2 inches of closed-cell on the panels is the usual starting point, and it also breaks the thermal bridge at purlins and girts. If you want more R-value than that, add open-cell over it or batts inside the framing. Code targets for metal buildings differ from houses; our metal building insulation guide covers them, with a condensation calculator.

Vans, trailers and shipping containers are the same problem with more vibration: thin steel, big temperature swings, and people breathing and cooking inside a small box. Closed-cell bonds, stiffens the skin and keeps moisture off the metal. We'd leave open-cell out of vehicles entirely.

Sound

Interior walls, floors and sound

Open-cell is the better of the two for sound. It's soft, it absorbs airborne noise in the cavity, and it's cheaper. But foam of either kind is only part of a quiet wall: drywall mass, decoupled framing and sealed edges decide most of the sound rating. Mineral wool batts often match open-cell for less.

Where open-cell earns its keep inside a house is the odd-shaped cavity: around ducts, in floor systems full of plumbing, in walls with a lot of penetrations. It fills and seals them in one step, which batts can't. Closed-cell's rigidity works against it here; it isn't a good sound absorber and it's the expensive foam. We cover what spray foam will and won't do for noise in does spray foam insulation reduce noise.

Interactive tool 3 of 3

Open cell vs closed cell cost: per board foot and per R

Installed in 2026, open-cell runs about $0.44–$0.73 per board foot and closed-cell $1.00–$2.12. But closed-cell packs 1.76 times the R-value into each inch, so per unit of R it costs only about 1.3 to 1.7 times as much as open-cell. Open-cell is cheaper wherever the depth exists. Where it doesn't, closed-cell is the only foam that gets there.

2026 priceOpen-cellClosed-cellClosed vs open, per R
Low$0.44/bf = $0.119 per sq ft per R$1.00/bf = $0.154 per sq ft per R1.29x
Typical (midpoint)$0.59/bf = $0.159 per sq ft per R$1.56/bf = $0.240 per sq ft per R1.51x
High$0.73/bf = $0.197 per sq ft per R$2.12/bf = $0.326 per sq ft per R1.65x
Price bands are the median of seven published installed ranges (Angi, HomeGuide, This Old House and others, 2021–2026) from our spray foam insulation cost guide, which also prices whole houses, attics and minimum charges. Per-R math is ours: $/board foot ÷ R per inch (3.7 open-cell, 6.5 closed-cell).
Same-R cost comparison

What does the same R-value cost in each foam?

Enter the area, the R-value you're after and the depth you have. The tool sizes open-cell, closed-cell and a flash-and-fill hybrid to the same R and prices each.

Open-cell
Cheapest foam that fits
$3,189
Open-cell, 5.4 in
$4,800
Closed-cell, 3.1 in
1.51x
Closed vs open, cost per R
OptionThicknessFits?Board feetInstalled (low–high)Per sq ftPer sq ft per R
Open-cell Cheapest that fits5.4 inYes5,405$2,378–$3,946$3.19$0.159
Closed-cell3.1 inYes3,077$3,077–$6,523$4.80$0.240
Flash-and-fill (2 in closed-cell + open-cell)3.9 inYes3,892$2,832–$5,621$4.24$0.212

Same R-20 over 1,000 sq ft at the typical 2026 price. Per board foot closed-cell costs about 2.6x open-cell; per R it’s 1.51x, because each inch does 1.76x the work. Real jobs round up to the next half inch and carry a $1,000–$2,000 minimum. In zones 5–8 and Marine 4, open-cell walls also need a Class II vapor retarder. See spray foam insulation cost for whole-house pricing.

What else changes the bill

  • Minimum job charges. Most contractors carry a $1,000–$2,000 minimum, so small closed-cell jobs cost more per board foot than the band.
  • Vapor-retarder coatings. Open-cell roof decks in zones 5–8 need one. We budget about $0.65–$1.00 per square foot, an estimate based on published vapor-barrier installation prices.
  • Thermal and ignition barriers. Both foams need ½-inch drywall or an equivalent in living space (IRC R316.4; R303.4 in the 2024 IRC), or an ignition barrier or listed coating in attics and crawl spaces entered only for service.
  • Trimming and passes. Open-cell overfills and gets trimmed, so some of what you pay for ends up in bags. Closed-cell goes on in multiple passes, which takes crew time.

Doing a small closed-cell job yourself is realistic with a two-part kit; our DIY spray foam insulation guide compares kits. Whole-cavity open-cell needs a pro rig.

Moisture

Moisture, leaks and mold: the real trade-off

Neither foam rots a house on its own. Water does. Closed-cell keeps indoor vapor off cold surfaces but dries slowly if a leak gets behind it. Open-cell dries faster but lets more winter moisture reach cold sheathing. Fix leaks before any foam goes in, and in cold climates give open-cell a vapor retarder and a conditioned attic.

BSC's roof study modeled how much leakage each assembly can dry out. In Seattle, an open-cell deck handled about 0.6% of rainfall and a closed-cell deck about 1.0%. Miami's open-cell deck handled 1.5%, and Minneapolis's closed-cell deck 0.6%. The lesson isn't that one foam is leak-proof. Neither is, and a roof that leaks needs fixing before it gets foamed.

Mold follows the same logic. EPA's rule is short: "The key to mold control is moisture control." Foam isn't food for mold, but damp dust on it and wet wood behind it are. Keep winter indoor humidity sensible (EPA suggests 30–50%), fix water first, and the foam type stops being the risk.

Claims to take with a grain of salt"Open-cell rots roofs." Not by itself. The risk is cold climates without a vapor retarder or attic conditioning, which the code already addresses.

"Spray foam cuts energy bills 40–50%." ENERGY STAR's modeling puts the average from air sealing plus insulation at 15% of heating and cooling costs (11% of total energy). Leaky houses do better.

"Our closed-cell is R-7.5 per inch." Probably an initial value. The aged value is the one that counts, and for most closed-cell foams it lands near R-6.5.

"Closed-cell makes walls 300% stronger." That traces to a best-case test on a weak baseline wall, and codes give it no bracing credit.

More on what actually goes wrong with spray foam, and what to do about it, in our spray foam insulation problems guide.

Identify it

How to tell open-cell from closed-cell foam

Press it. Open-cell is soft and spongy, dents under a thumb and springs back; it's usually thick and trimmed flush with the studs. Closed-cell is hard, feels like rigid foam board, and usually sits thinner than the framing with a lumpy surface. Color tells you nothing, because it varies by brand.

  • Thumb test. A dent that recovers means open-cell. No dent means closed-cell.
  • Thickness. Foam filling the whole stud or rafter depth and cut flat is usually open-cell. Two or three inches against the sheathing is usually closed-cell.
  • A small plug. Open-cell squeezes like a sponge. Closed-cell is stiff; it crushes rather than springing back.
  • Paperwork. The installer's invoice or the permit file should name the product and thickness. That's the only way to know the R-value and vapor rating for sure.
Applicator in a protective suit spraying soft white spray foam into wall stud cavities, with foam rising past the face of the studs
Soft foam rising past the stud faces, the way open-cell does before it's trimmed flush.
ArmorThane applicators

ArmorFoam open- and closed-cell, and when we'd sell you neither

We make both foams, and a certified ArmorThane applicator can spray either one, or both on the same job. ArmorFoam closed-cell tests at R-6.5 per inch aged (ASTM C518), is an air barrier at 1 inch (ASTM E2178) and measures 0.83 perm at 1 inch. ArmorFoam 0.5 open-cell is a 0.5 lb/ft³, water-blown foam at R-3.9 at 1 inch and R-13.6 at 3.5 inches.

And the cases where we'd point you away from spray foam altogether: an open, vented attic floor (blow it in), a few rim-joist bays under a contractor's minimum (a kit or foam board), the outside of a foundation (rigid board is the norm there), and finished walls you don't want to open.

Get a quote from a certified applicator →
Spray foam applicator in a hooded suit and respirator spraying foam into a framed wall, with finished foam-filled stud bays in the foreground
One rig, one crew, either foam.
R-6.5 / in

ArmorFoam closed-cell

Aged, ASTM C518. About 1.9 lb/ft³, Class II at 1 inch, ≈5,000 board feet per set.

R-3.9 at 1 in

ArmorFoam 0.5 open-cell

0.5 lb/ft³, water blown. R-13.6 at 3.5 inches, ≈17,000 board feet per set.

3.4x

Board feet per set

Open-cell yields about 3.4 times the board feet of closed-cell per set, which is a big part of the price gap.

For contractors

Offer both foams. Quote every spot on this page.

Crews that only spray one foam end up walking past half the house. With ArmorFoam open- and closed-cell on the truck, and polyurea coatings on the same rig, you can bid the attic, the walls, the rims and the crawl in one visit. We supply the foam, the equipment and the training. Not a franchise: no franchise fee, no royalties, startup packages from $25,000.

Become an applicatorStart a spray foam business
Straight answers

Open cell vs closed cell spray foam FAQ

Which is better, open-cell or closed-cell spray foam?

Neither across the board. Closed-cell is better where depth is tight or surfaces are cold or damp: rim joists, basement and crawl walls, metal buildings and roof decks in climate zones 5–8. Open-cell is better, and cheaper per R, where there's room to fill and no water, such as 2x6 walls in zones 1–3 and roof decks in zones 1–4. The foam picker applies your zone.

Should I use open-cell or closed-cell spray foam in my attic?

On the underside of the roof deck, either works in climate zones 1–4, and open-cell usually costs less. In zones 5–8, IRC R806.5 requires the foam against the deck to be a Class II vapor retarder or be coated with one, so closed-cell or flash-and-fill is the simpler route. On a vented attic floor, blown cellulose or fiberglass is cheaper than either foam.

Is open-cell spray foam good for exterior walls?

Yes, in the right climate. A full 2x6 cavity gives about R-20 at the lowest cost, which meets the 2021 IECC in zones 1–3. In zones 5–8 and Marine 4, IRC R702.7 requires a Class I or II vapor retarder on the inside of frame walls, so open-cell walls there need kraft facing or vapor-retarder paint, or you switch to closed-cell.

What are the downsides of closed-cell spray foam?

Price, first: about $1.00–$2.12 per board foot installed in 2026, two to three times open-cell. It hides roof and plumbing leaks and dries slowly if water gets behind it, it makes remodeling and termite inspection harder, and it's unforgiving of poor installation. Like open-cell, it needs a thermal barrier such as drywall in living space.

What are the downsides of open-cell spray foam?

It's vapor-open (Class III), so in cold climates it needs a separate vapor retarder in walls and on roof decks. It absorbs water, so it doesn't belong below grade or against steel. It also needs more depth for the same R-value: about 5.4 inches for R-20 versus 3.1 for closed-cell. Open-cell roof decks need a conditioned or dehumidified attic.

Can mold grow on open-cell spray foam?

Mold needs moisture; the foam itself isn't what feeds it. Where open-cell stays damp from a leak, humid attic air with no conditioning, or vapor condensing on cold sheathing in a cold climate, mold can grow on dust and on the wood behind it. EPA's rule applies: the key to mold control is moisture control.

Is open-cell spray foam waterproof?

No. Open-cell absorbs water, DOE says it shouldn't be used below ground level where it could absorb water, and open-cell evaluation reports bar continuous contact with water. Closed-cell resists water much better, but it isn't a waterproofing membrane either.

How can I tell if I have open-cell or closed-cell spray foam?

Press it. Open-cell is soft and spongy, dents easily and springs back, and is usually trimmed flush with the studs. Closed-cell is hard, feels like rigid foam board and usually sits thinner than the framing. Color varies by brand, so ignore it and check the installer's paperwork for the product name and thickness.

Is open-cell or closed-cell better for soundproofing?

Open-cell. It's soft, absorbs airborne sound in the cavity better than rigid closed-cell, and costs less. But foam is only part of a quiet wall; drywall mass, decoupled framing and sealed edges matter more. See does spray foam insulation reduce noise.

Should I use open-cell or closed-cell in a metal building or pole barn?

Closed-cell against the steel. It bonds to the panels, stops warm, moist air reaching them and is a vapor retarder at about an inch or two, which is what prevents condensation. Open-cell is vapor-open, so use it only as a fill layer over closed-cell. More in our metal building insulation guide.

How much more does closed-cell cost than open-cell?

Per board foot, about 2.3 to 2.9 times: $1.00–$2.12 installed versus $0.44–$0.73 in 2026. Per unit of R-value the gap shrinks to about 1.3 to 1.7 times, because closed-cell gives R-6.5 per inch against open-cell's R-3.7. Where you have the depth, open-cell is the cheaper way to a given R; where you don't, closed-cell is the only foam that gets there.

Can you use open-cell and closed-cell together?

Yes. Flash-and-fill sprays closed-cell against the sheathing first for vapor and condensation control, then fills the rest with open-cell for cheaper R-value. On an unvented roof deck, size the closed-cell layer to at least the IRC Table R806.5 value for your zone, from R-5 in zones 1–3 up to R-35 in zone 8. In zones 5–8, some inspectors also want a vapor-retarder coating on the open-cell.

Do open-cell and closed-cell spray foam both need to be covered?

Yes, in living space. Both are foam plastics and need a thermal barrier equal to ½-inch gypsum board (IRC R316.4; R303.4 in the 2024 IRC). In attics and crawl spaces entered only for service, an ignition barrier or an intumescent coating listed in the product's evaluation report can be used instead.

Can I spray closed-cell or open-cell foam myself?

Small closed-cell jobs, yes, with a two-part kit: rim joists, a few bays, a shed. Kits cost about $1.22–$1.95 per board foot in material alone, close to a pro's installed closed-cell price, and they have thickness limits. Whole-cavity open-cell needs a pro rig. Compare kits in our DIY spray foam insulation guide.

AT
ArmorThane Technical Team · Springfield, Missouri
Written from IRC and IECC code text, evaluation reports, Building Science Corporation and DOE research, and our own ArmorFoam data sheets. Last updated September 26, 2026.

Methodology and sources

R-values use R-6.5 per inch for closed-cell (ArmorFoam, aged) and R-3.7 for open-cell (a typical published value; ArmorFoam 0.5 tests at R-3.9 at 1 inch). Code values are 2021 IECC prescriptive via DOE/PNNL and IRC model text; check the edition and amendments your jurisdiction has adopted. Installed prices are the 2026 median bands from our spray foam cost guide. Thickness, cost-per-R and hybrid sizing are our calculations and are estimates, not quotes.

  1. IRC R806.5, unvented attic and rafter assemblies, with Table R806.5 (2015 text, Huntsman reprint, PDF): huntsmanbuildingsolutions.com/en-US/sites/en_us/files/2019-10/2015-IRC
  2. IRC R702.7, vapor retarders (UpCodes): up.codes/s/vapor-retarders
  3. IRC R202, air-impermeable insulation definition (UpCodes): up.codes/s/air-impermeable-insulation
  4. DOE/PNNL Building America Solution Center, 2009–2021 IECC and IRC minimum insulation: basc.pnnl.gov/information/2009-2021-iecc-and-irc-minimum-insulation-re
  5. DOE/PNNL Building America Solution Center, below-deck spray foam for existing roofs: basc.pnnl.gov/resource-guides/below-deck-spray-foam-insulation-existin
  6. DOE/PNNL Building America Solution Center, garage rim/band joist adjoining conditioned space: basc.pnnl.gov/resource-guides/garage-rimband-joist-adjoining-condition
  7. U.S. DOE Energy Saver, types of insulation: energy.gov/energysaver/types-insulation
  8. U.S. DOE Energy Saver, where to insulate in a home: energy.gov/energysaver/where-insulate-home
  9. Huntsman Building Solutions, design note BES-102: vapor retarders and vapor permeance (Oct. 2022, PDF): huntsmanbuildingsolutions.com/en-US/sites/en_us/files/2022-10/22.00112
  10. IAPMO UES ER-0913, open-cell spray foam evaluation report (rev. May 2026, PDF): forms.iapmo.org/ues_reports/reports/er_0913.pdf
  11. Huntsman Building Solutions, Sealection 500 open-cell data: huntsmanbuildingsolutions.com/en-US/products/open-cell-insulation/seal
  12. Building Science Corporation, BSD-149 unvented roof assemblies for all climates (2007): buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies
  13. Building Science Corporation, BA-1312 spray foam under plywood and OSB roof sheathing (2013): buildingscience.com/documents/bareports/ba-1312-application-of-spray-f
  14. Building Science Corporation, Info-511 basement insulation (2009): buildingscience.com/documents/information-sheets/basement-insulation
  15. GreenBuildingAdvisor, does open-cell spray foam really rot roofs? (Apr. 2014): greenbuildingadvisor.com/article/does-open-cell-spray-foam-really-rot-
  16. Efficiency Vermont Better Buildings by Design, why are open-cell foam attics so damp? (Feb. 2018, PDF): efficiencyvermont.com/Media/Default/bbd/2018/docs/presentations/effici
  17. HomeGuide, spray foam insulation cost (Dec. 2025): homeguide.com/costs/spray-foam-insulation-cost
  18. Angi, spray foam insulation cost (Sep. 2026): angi.com/articles/spray-foam-insulation-cost.htm
  19. ENERGY STAR, seal and insulate savings methodology: energystar.gov/saveathome/seal_insulate/methodology
  20. U.S. EPA, a brief guide to mold, moisture and your home: epa.gov/mold/brief-guide-mold-moisture-and-your-home
  21. U.S. EPA, HFC technology transitions by sector: epa.gov/climate-hfcs-reduction/technology-transitions-hfc-restrictions
  22. ArmorThane, closed-cell spray foam guide and ArmorFoam specifications: armorthane.com/closed-cell-spray-foam/
  23. ArmorThane, ArmorFoam 0.5 open-cell spray foam: armorthane.com/products/armorfoam-products/open-cell-spray-foam/
  24. ArmorThane, ArmorFoam spray foam products: armorthane.com/products/armorfoam-products/

latest news & bLOGS

Learn more

The right foam in each spot, on one quote.

ArmorThane-certified applicators spray ArmorFoam open-cell and closed-cell, so your quote can put each foam where it belongs, with thickness and R-value in writing. Contractors: we make both foams, build the rigs and train your crew, with no franchise fee.

Find a spray foam applicator →Become a spray foam applicator
United States 417-831-5090 · Canada 780-444-6200

CONTACT US

REQUEST A QUOTE OR

BECOME AN APPLICATOR

ArmorThane has built a strong reputation over the past 30 years for producing high-quality, durable protective coatings.

group 115 660555abebfe4
Phone Number

Tel: 417.831.5090

group 115 1 660556c21395c
Email Us

moreinfo@armorthane.com

Step 1 of 3

We want to help you find the best ArmorThane applicator for your protective coating project. Please either fill out the request form below or call us toll free at 417-831-5090, any time Monday-Friday, 8:00 a.m. to 5:00 p.m. Central Standard Time.

RV Find An Applicator

Step 1 of 3

Connect with an authorized ArmorThane professional to get a long-lasting, seamless protective coating for your RV. Submit the form below or call us directly at 417-831-5090 (Monday–Friday, 8:00 AM – 5:00 PM CST).

Facebook Twitter Youtube

Find An Applicator

Applicator Login

Armorthane Logo Outlined
CALL NOW
BECOME APPLICATOR
FIND APPLICATOR

Protective Coating Applications

Products

Applicator Opportunities

Knowledge Center

Galleries