Closed-cell is the premium end of spray foam, and in a damp marine climate like ours it is usually the one worth paying for. It gives you the highest R-value per inch of any foam we install, it stops air movement, and at the right thickness it doubles as a vapor retarder. One product, three jobs.
It is also the product people get cheated on. Closed-cell costs meaningfully more than open-cell to buy and to install, which makes it the thing a corner-cutting crew is most tempted to substitute. That is worth knowing before you sign anything, so we have written the whole page from that angle.
The short version
Closed-cell delivers roughly R-6 to R-7 per inch, seals air, and resists moisture. Open-cell delivers roughly R-3.6 to R-4 per inch, seals air, and does not resist moisture. In Seattle crawl spaces, rim joists, and roof decks, that moisture difference is usually the deciding factor, not the R-value.
Where closed-cell earns its price in a Seattle home
We do not recommend closed-cell everywhere. It is the right call when the assembly has to handle moisture as well as heat loss, which in the Puget Sound is more often than most people expect.
- Crawl space walls and rim joists. This is the single most common closed-cell job we do here. It handles ground moisture and stops the cold air that makes your floors miserable in January.
- Roof decks on a conditioned attic. When you want the attic inside the thermal envelope, closed-cell on the underside of the sheathing does it in a single step.
- Shallow cavities that need every inch. A 2×4 wall or a tight cathedral ceiling cannot fit enough open-cell to hit a useful R-value. Closed-cell can.
- Anywhere water is a genuine possibility. Closed-cell does not absorb and hold water the way open-cell and fiberglass do.
And where we will tell you not to bother
Interior partition walls for sound. Attic floors in a house where you are keeping a vented attic and blown-in would do the same job for far less. A garage ceiling with no living space above it. If the honest answer is that a few hundred dollars of air sealing gets you most of the benefit, we will say so, and we will give you that number instead.
Closed-cell versus open-cell, plainly
| Closed-cell | Open-cell | |
|---|---|---|
| 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 |
| Handles bulk water | Resists it | Absorbs and holds it |
| Adds rigidity | Yes, noticeably | No, it stays soft |
| Relative cost | Higher per inch | Lower per inch |
| Typical use here | Crawl spaces, rim joists, roof decks, tight cavities | Interior walls, sound control, generous attic cavities |
If you want this in more depth, including how to work out which one is already in your house, read open-cell vs closed-cell spray foam.
What a proper closed-cell install actually involves
This is the part that decides whether you are happy in ten years, and it is the part that is invisible once the drywall goes back on. Four variables matter more than anything else.
1. Substrate temperature
Closed-cell has a temperature range it will bond and cure in. Spray it on a cold Seattle February morning onto cold sheathing without preparing the space and it can fail to adhere, fail to reach its rated R-value, or pull away from the substrate later. We check the surface, not the weather app.
2. Mix ratio
The two chemical components have to meet in the right proportion, at the right temperature, at the right pressure. Off-ratio foam is the source of nearly every horror story you have read. It may never fully cure, it can keep off-gassing for months, and it generally cannot be fixed in place. It has to come out.
3. Pass thickness
Closed-cell is applied in controlled lifts, not one thick coat. Too much in a single pass traps heat as the foam reacts, which can scorch the core, weaken adhesion, and in the worst cases create a genuine fire risk. Building the depth in passes takes longer. We take longer.
4. Cure and re-entry
You need to be out of the house during the install and for a re-entry period afterwards. We will tell you the real number for your job before the day, based on the product and the ventilation we can get into that space. Anyone who tells you that you can sit in the next room is not doing you a favour.
If you already have foam in your house and you are worried it was done badly, that is a common and reasonable worry. Here is how to tell whether your spray foam was installed correctly, including what you can check yourself.
What closed-cell spray foam costs in the Seattle area
Closed-cell is normally priced per board foot, which is one square foot at one inch thick. In this market it commonly lands somewhere around $1.00 to $2.00 per board foot installed, and open-cell sits lower at roughly $0.50 to $0.70. As a rough whole-project figure, a 1,000 square foot area at three inches often lands somewhere around $2,200 to $5,400. The spread is wide for real reasons rather than sales reasons:
- How many inches the assembly needs. Two inches in a rim joist and five inches on a roof deck are very different jobs on the same house.
- Access. An 18 inch crawl space with a small hatch costs more to work in than an open basement wall, because it is slower and harder on the crew.
- Removal first. If wet batts or rodent damaged blown-in has to come out before we can spray, that is its own line item. See insulation removal.
- Prep and protection. Masking, containment, and ventilation are part of doing it properly.
Anyone who gives you a firm price for your house over the phone, without looking at it, is guessing. We are not going to do that. For the full breakdown including typical project ranges, see spray foam insulation cost in Seattle.
How we handle your job
Look at the actual house
We get into the crawl space or attic, find where the air is moving, and check what the existing insulation is really doing.
Specify honestly
Closed-cell where it earns its price, something cheaper where that is genuinely the better buy. In writing, with the reasoning.
Install it properly
Correct substrate temperature, correct ratio, correct depth in controlled passes, with the space contained and ventilated.
Check it and stand behind it
We walk it with you, confirm coverage and depth, and tell you exactly when it is safe to be back in the space.
Common questions about closed-cell spray foam
How thick does closed-cell need to be to act as a vapor retarder?
Generally you need a meaningful depth, commonly cited as somewhere around two inches, before closed-cell performs as a vapor retarder rather than just insulation. The exact figure depends on the specific product, so we go by the manufacturer data for what we are actually spraying rather than a rule of thumb. If a contractor quotes you one inch and calls it a vapor barrier, ask them which product and what its published perm rating is at that thickness.
Will closed-cell spray foam rot my roof deck?
The concern is real but it is worth being precise about. Foam bonded to the underside of sheathing means a roof leak no longer shows up as a stain on your ceiling, so water can sit against the wood unnoticed. Closed-cell is less risky here than open-cell because it does not absorb and hold that water, but the honest answer is the same either way: deal with the roof before you insulate under it, and understand that you are making the deck harder to inspect. We will walk through that with you before you spend anything.
Does closed-cell spray foam off-gas?
During installation and for a cure period afterwards, yes, which is why you need to be out of the house and why the space has to be ventilated properly. Correctly mixed and correctly installed closed-cell stops having a noticeable odour after it cures. Foam that is still strongly odorous weeks or months later is a sign it was mixed off ratio, and that foam usually has to be removed rather than aired out.
Can closed-cell spray foam be removed if something goes wrong?
It can, but it is unpleasant and expensive. Closed-cell bonds hard to the substrate, so removal is mechanical: scraping and cutting it off, often in a confined space, sometimes with damage to the surface underneath. This is the whole reason we are so particular about ratio, temperature, and pass thickness. The cheapest version of this job is the one done correctly the first time.
Is closed-cell worth the extra cost over open-cell?
In a crawl space, at a rim joist, or anywhere moisture is part of the problem, usually yes. In a generous attic cavity where you have plenty of depth to work with and no moisture concern, often not. We would rather put you in the cheaper product and be right than sell you the expensive one and be wrong.