Both products are polyurethane foam sprayed wet and expanded in place, and both of them seal air. The difference is what happens inside the foam as it rises: closed-cell cures into millions of tiny sealed bubbles, open-cell cures into bubbles that burst and stay connected. That one structural difference drives the R-value, the moisture behaviour, the rigidity, the weight, the sound performance, and the price.
Here is the short version before the long one. In a Puget Sound house, closed-cell is usually the right call in the crawl space, at the rim joists, and anywhere moisture is part of the problem. Open-cell is usually the right call in generous attic cavities and interior partition walls, and it is a genuinely good product when it is used in the right place. The mistake is not choosing one over the other. The mistake is putting the wrong one in an assembly that needed the other, or paying for one and receiving the other.
The numbers, up front
Closed-cell: about R-6 to R-7 per inch, roughly $1.00 to $2.00 per board foot installed in this market. Open-cell: about R-3.6 to R-4 per inch, roughly $0.50 to $0.70 per board foot. Both air seal. Only closed-cell acts as a vapor retarder, and only at sufficient thickness.
How each one is made, and why the cell structure decides everything else
Both foams arrive on the truck as two liquid components. They meet at the gun, react, and expand on the surface within seconds. What differs is the blowing agent and the chemistry, and therefore what the finished foam looks like under magnification.
Closed-cell: sealed bubbles, packed tight
In closed-cell foam the gas cells stay intact and stay full of the blowing agent, which insulates better than plain air. The cells press against each other with no continuous path between them. The result is a dense, hard, rigid material. You can knock on it. It does not compress under your thumb. Because there is no connected path through the foam, neither air nor liquid water nor, at enough depth, water vapour can move through it easily.
Open-cell: burst bubbles, mostly air
In open-cell foam the cells rupture as the foam rises and the finished material is largely air held in a soft structure of connected cavities. It is light, spongy, and it springs back when you press it. Air does not blow through it, which is why it still seals, but water vapour passes through it freely and liquid water will soak into it and drain out of it like a sponge.
Everything below follows from those two paragraphs. If you understand the cell structure, you can predict every other difference without memorising a table.
The full comparison
| Closed-cell | Open-cell | |
|---|---|---|
| Cell structure | Sealed cells, no connected path | Ruptured, connected cells |
| Feel | Hard, rigid, does not dent | Soft, spongy, springs back |
| R-value per inch | About R-6 to R-7 | About R-3.6 to R-4 |
| Air sealing | Yes | Yes |
| Vapor retarder | Yes, at sufficient thickness | No, it stays vapor open |
| Bulk water | Resists it, sheds it | Absorbs and holds it, then drains |
| Sound absorption | Fair, it is stiff | Better, it is soft and porous |
| Rigidity and racking strength | Adds real stiffness to the assembly | Adds none |
| Weight on the structure | Noticeably heavier | Very light |
| Expansion | Modest, applied in controlled lifts | Large, fills the cavity and overflows |
| Trimming | Rarely, and it is hard work | Routinely shaved flush to the studs |
| Cost per board foot | About $1.00 to $2.00 installed | About $0.50 to $0.70 |
| Best use here | Crawl spaces, rim joists, roof decks, tight cavities | Interior walls, sound control, deep attic cavities |
What each difference actually means in your house
R-value per inch, and why depth matters more than the number
Closed-cell gives you roughly R-6 to R-7 per inch against roughly R-3.6 to R-4 for open-cell, so closed-cell is worth about 1.7 times as much per inch. That only matters when inches are scarce. In a 2×4 exterior wall you have about three and a half inches to work with, and the difference between about R-21 and about R-13 in that cavity is the whole argument. In an attic where we can spray as deep as we like, open-cell just goes on thicker and gets to the same place for less money. Depth is cheap in an attic and expensive in a wall. For what the numbers should actually be in this climate, see our R-value guide for Seattle, climate zone 4C.
Air sealing: both do it, and that is most of the win
This is the part homeowners are most often sold wrong. Both foams stop air movement at the thicknesses we install. In most Seattle houses the air leakage, not the R-value, is what makes the upstairs cold and the bill high, so a properly installed open-cell job and a properly installed closed-cell job feel much more similar than the R-value comparison suggests. If somebody tells you open-cell “does not seal,” they are either confused or selling. The honest framing is that closed-cell adds moisture control and stiffness on top of the air seal, and that is what you are paying the premium for.
Vapor permeability, and what a vapor retarder actually means
A vapor retarder slows the movement of water vapour through an assembly. It is not a raincoat and it is not the same as an air barrier. Air carries far more moisture through a building than diffusion does, which is why the air seal matters first. But in a cold assembly, vapour that diffuses into the cavity and reaches a cold surface can condense, and then you have liquid water inside your wall or roof.
Closed-cell becomes a vapor retarder at enough thickness, commonly cited as somewhere around two inches, though the real figure depends on the product and its published perm rating. Open-cell stays vapor open at any thickness. Vapor open is not automatically bad. An assembly that can dry in one direction is often safer than one that is sealed on both sides. It becomes bad when the assembly has no drying path and the vapour has somewhere cold to land.
Water: what happens when something leaks
Everything leaks eventually. Closed-cell resists bulk water, so a slow leak tends to run across the face of it rather than into it. Open-cell takes the water in, holds it while it is wet, and releases it when conditions allow. On a basement or crawl space wall that is the whole argument for closed-cell. On a roof deck it is the heart of a real and ongoing debate, which is its own section further down this page.
Sound
Open-cell wins here and it is not close. Soft porous material absorbs airborne sound; rigid material transmits it. If you are insulating an interior partition between a bedroom and a media room, or a floor under a teenager, open-cell is the better product and the cheaper one at the same time. That is a rare and pleasant combination, so take it when you can get it.
Rigidity and racking strength
Closed-cell bonds hard to the framing and sheathing and measurably stiffens the assembly. That is a genuine structural side benefit, and it matters most on older, lightly braced framing. Open-cell contributes nothing structurally. Neither product should ever be the reason a structural decision gets made, but if you are already spraying closed-cell in a crawl space on a 1920s Craftsman, the added stiffness is a free bonus.
Weight
Closed-cell is several times denser than open-cell. On a crawl space wall nobody cares. Under a plank subfloor, on an old lath ceiling, or on the underside of a roof deck built with light framing and long spans, the added dead load is something we actually check rather than assume. It is one of the quiet reasons a good installer looks at the framing before quoting the product.
Expansion and trimming
Open-cell expands dramatically. It fills an irregular cavity beautifully, finds every gap around a wire or a pipe, and then overflows past the face of the studs, so the crew shaves it flush before drywall. That shaved surface is a good thing: you can see the whole cavity and confirm it is full.
Closed-cell expands far less and has to be built up in controlled lifts, usually around an inch to two inches at a time, letting each pass cool before the next. Rushing that is one of the classic install failures, because too much foam in one pass traps the heat of its own reaction, which can scorch the core and ruin adhesion. It is also why closed-cell takes longer and costs more in labour, not just in material. Our page on what a quality spray foam install looks like goes through the rest of it.
Cost
Per board foot, closed-cell runs roughly $1.00 to $2.00 installed here and open-cell roughly $0.50 to $0.70. But board feet are not the honest unit for comparison, because you rarely install the same number of inches of both. Compare the two products at the R-value you actually need in that assembly, then look at the price. In an attic, open-cell at twice the depth is usually still cheaper. In a rim joist, closed-cell at two inches is cheaper than any depth of open-cell would be, because open-cell was never going to do the job there. The full breakdown lives on our Seattle spray foam cost page.
Which one belongs where: a Puget Sound decision framework
This is how we actually specify a job, assembly by assembly. It is not a rule, it is a starting point that survives contact with most houses in this region.
| Assembly | Usually | Why |
|---|---|---|
| Crawl space walls | Closed-cell | Ground moisture, standing water risk, and cold outside air all in one place. Open-cell would soak. See crawl space insulation. |
| Rim joists | Closed-cell | Small area, huge air leak, cold surface where vapour would condense. The best dollar per degree in most Seattle houses. |
| Roof deck, conditioned attic | Closed-cell, usually | Moisture control at the sheathing plus more R in less depth between rafters. Open-cell here is possible but conditional, see below. |
| Attic floor, vented attic | Often neither | If the attic stays vented, air seal the ceiling plane and blow in depth. We will tell you when that is the cheaper right answer. See attic insulation. |
| 2×4 exterior walls | Closed-cell | Three and a half inches is all you get. Open-cell in that cavity tops out around R-13 or so, closed-cell roughly R-21. |
| Interior partitions for sound | Open-cell | Better absorber, lower price. There is no moisture story inside a partition wall. |
| Cathedral ceilings | Closed-cell, usually | Shallow rafters, no attic above to vent, and a cold surface overhead. Depth is scarce and the moisture risk is real. |
| Bonus room over a garage | Closed-cell in the floor | That floor sits over unconditioned space. Closed-cell seals the joist bays and handles the cold garage air that makes that room unusable in January. |
If the honest answer for your house is air sealing plus blown-in rather than foam, we will say that, quote that, and be glad to. We would rather be right than expensive.
Which one did I actually get?
This is the question we get asked more than any other by people who already have foam, and it deserves a straight answer. Substitution happens: a job is quoted and paid for as closed-cell, and cheaper open-cell goes in instead. Here is what you can check yourself, in rough order of how reliable it is.
- Press it with your thumb. This is the single best tell. Closed-cell is rigid. Your thumb does not dent it and it does not spring back, because it never moved. Open-cell gives under light pressure and pushes back like a firm sponge.
- Knock on it. Closed-cell sounds and feels like a hard board. Open-cell absorbs the knock and makes almost no sound at all.
- Look at the surface. Open-cell is usually shaved flat and flush with the framing, because it overflowed and had to be trimmed. Closed-cell is normally left as sprayed, with a lumpy, rolling, slightly glossy surface that stops short of the stud face.
- Colour is a hint, not proof. Closed-cell is commonly a tan, cream, or light green, open-cell commonly a pale yellow or near white. Products vary by manufacturer and both fade, so never decide on colour alone.
- Measure the depth against the R-value you were sold. This one is arithmetic and it is hard to argue with. If your contract says R-30 and the foam is four and a half to five inches deep, that is consistent with closed-cell. If it is four and a half inches and it is open-cell, you have roughly R-18, not R-30. Open-cell would have needed about seven and a half to eight inches to get there.
The skin-coat trick
The one you have probably read about: a crew sprays cheap open-cell to most of the depth, then lays a thin closed-cell coat over the face of it. The surface presses hard, looks right, and passes a casual touch test, while nearly all the depth behind it is the cheap product. It is the reason a thumb test alone is not enough on a job you already have doubts about.
To catch it you have to see a cut edge. Cut a small clean square out with a utility knife, pull the plug, and look at the cross section: a thin rigid layer over an obviously soft, lighter, more open material is the answer. If the foam is uniform all the way through, it is one product. Do this somewhere it will not matter, at the edge of a crawl space wall rather than the middle of a finished assembly, and patch it afterwards.
When you need a professional. Be honest with yourself about the limits of a thumb. If real money is in dispute, if the assembly is closed up behind drywall, or if you are heading toward a warranty claim or a contractor complaint, you want a core sample and an independent set of eyes, not a forum diagnosis. Density can be measured from a core, and a measured density settles the argument. We do these assessments in the Seattle area and we will tell you plainly if what you have is fine. Related reading: how to tell if spray foam was installed correctly.
The open-cell roof deck question, answered honestly
You will find people online who say open-cell on a roof deck rots the sheathing, and people who say that is scaremongering. Both camps are arguing from real experience, and the reason they disagree is that the answer depends on the climate and the assembly.
The mechanism is not imaginary. Open-cell is vapor open, so moisture from inside the house can diffuse through it and reach the underside of the sheathing. In winter that sheathing is cold. If enough moisture accumulates there and the assembly cannot dry, the wood stays wet and eventually it suffers. That is why building codes in colder zones require a vapor retarder to be applied over open-cell on a roof deck, and why the failures people photograph and post are mostly from cold climates with high indoor humidity.
Seattle is climate zone 4C, marine. It is milder than the zones where that requirement bites, our winters are wet but not severely cold, and our summer dew points sit well below indoor temperatures, so the moisture picture genuinely differs from both the frozen north and the humid Southeast. That does not make open-cell automatically safe on a roof deck here. It makes it assembly-dependent. Indoor humidity, whether the attic is truly conditioned, ventilation, roof colour, shading, and the age and condition of the sheathing all move the answer.
What we will not do is tell you what the Washington code requires on your specific roof as though it were a settled fact from a web page. Requirements turn on the assembly and on local amendments, and the person whose opinion decides it is your inspector. We confirm it with them before the job, in writing, every time. Our default in this market is closed-cell on roof decks because it removes the question entirely, and we say so knowing it is the more expensive answer. The longer version is on will spray foam rot my roof deck.
How we pick the product for your house
Look at the assembly
Depth available, what is above and below it, whether it is inside or outside the thermal envelope, and where the air is actually moving.
Ask what the foam has to do
Insulate only, or insulate and control moisture, or insulate and absorb sound. The job decides the product, not the price list.
Price both honestly
At the R-value the assembly needs, not per board foot. Sometimes that makes open-cell the obvious buy and we will tell you so.
Put it in writing
Product, manufacturer, and installed thickness on the quote, so what you are paying for is what you can go down and measure afterwards.
Common questions
Is closed-cell always better than open-cell?
No. Closed-cell is better at more things, which is not the same thing as being the right choice. In a deep attic cavity with no moisture concern, open-cell reaches the same R-value for less money, and in an interior partition wall open-cell is both cheaper and better at the job you want done, which is sound. Closed-cell earns its premium where moisture control or scarce depth is part of the problem. In a Puget Sound crawl space, that is nearly always.
How can I tell if I got open-cell instead of the closed-cell I paid for?
Press it. Closed-cell is rigid and will not dent under your thumb; open-cell gives and springs back. Then check the depth against the R-value on your contract, since closed-cell delivers about R-6 to R-7 per inch and open-cell about R-3.6 to R-4. Be aware of the skin-coat trick, where a thin closed-cell layer is sprayed over cheap open-cell so the surface feels right. To rule that out you need to see a cut edge or take a core sample. If money is in dispute, get an independent professional assessment rather than relying on a touch test.
Can you use open-cell on a roof deck in Seattle?
It is done, and whether it is a good idea is assembly-dependent rather than a yes or no. Open-cell is vapor open, so moisture can reach the cold sheathing, which is why colder climate zones require a vapor retarder over open-cell on roof decks. Seattle is zone 4C marine and the moisture picture here differs, but indoor humidity, ventilation, and the condition of the sheathing all change the answer. We confirm the requirement with the inspector for the specific assembly before we quote it, and our default here is closed-cell because it takes the question off the table.
Can you mix the two products in one house?
Yes, and on a lot of houses that is the correct specification. Closed-cell in the crawl space and at the rim joists, open-cell in an interior partition or a deep attic cavity. What you should not accept is the two products mixed inside one assembly without a stated reason, because that is what an unannounced substitution looks like from the outside.
Does open-cell insulation absorb water and stay wet?
It absorbs water, holds it while the source is active, and drains and dries once the source stops. It does not permanently hold water the way wet fiberglass in a crawl space does, but it also tells you nothing while it is wet, because the water is inside the foam and out of sight. That is exactly why we do not use it on a crawl space wall or a basement wall in this region.
Which one off-gasses more?
Both cure and both need you out of the house during the install and for a re-entry period afterwards, which we give you as a real number for your product and your space rather than a generic figure. Odour that is still strong weeks later is not a product difference, it is a sign the foam was mixed off ratio, and that applies to both. Off-ratio foam generally has to be removed rather than aired out.