Should You Use Open-Cell Foam on Roof Decks? Here’s What Building Science Says
- Feb 10
- 7 min read
Ask five contractors if open-cell foam is okay to use on a roof deck, and you’ll get five different answers. Some won’t touch it. Others use it almost exclusively. Most will tell you their decision is based on cost, inspector approval, or the way they’ve always done it.
But this isn’t a question of preference. It’s a question of building science — and increasingly, liability. The open-cell vs. closed-cell debate on roof decks has nothing to do with which product you like better. It’s about what happens when vapor meets temperature. And it’s about whether your assembly is capable of drying — or not.
If you get it right, you can deliver a high-performance roof that lasts. Get it wrong, and you’re putting the client, the structure, and your name at risk.
Let’s take a proper look at when open-cell is safe, when it’s not, and why so many failures begin before the rig even pulls onto the jobsite.
Roofs Are Not Walls
A lot of contractors think if open-cell foam works great in walls, it should work great in roofs too. The trouble is, roofs behave differently — not just in construction, but in physics.
Wall assemblies often have two directions for drying: inward and outward. Even if moisture gets into a wall cavity, it can typically release in either direction. Roofs, on the other hand, are usually a one-way street.
Most roof decks are topped with asphalt shingles, synthetic felt, standing seam metal, or tile. All of these materials create a vapor-closed exterior surface. Once you foam the underside of the deck, that roof is officially sealed on both sides. If moisture builds up inside that assembly — whether from construction wetting, vapor diffusion, or interior humidity — the only place it can go is back into the conditioned space below.
And if the foam you’ve installed allows vapor to move freely, and there’s no dedicated drying path, that moisture’s going to find a home in the coldest part of the roof. Usually right inside the sheathing.
Closed-Cell vs. Open-Cell: What Really Separates Them on Roof Decks
Property | Closed-Cell Spray Foam (ccSPF) | Open-Cell Spray Foam (ocSPF) |
R-Value per Inch | Typically between 6.0 and 7.0. High insulating power in thin layers — often used to meet R-value requirements with less thickness. | Ranges from 3.5 to 3.9. Requires more depth to achieve the same R-value, which can be limiting in shallow roof assemblies. |
Vapor Permeance | Less than 1 perm at 1" thickness. Qualifies as a Class II vapor retarder. At greater thicknesses, approaches Class I (vapor barrier). | Often 10 perms or more — even higher at full depth. Fully vapor-open, which means it allows moisture diffusion through the foam. |
Air Barrier | Yes — acts as a complete air seal at 1" and above. Prevents convective vapor movement (vapor carried by air leaks). | Yes — open-cell stops air leakage, but not vapor diffusion. It seals drafts but still allows moisture vapor to pass through. |
Water Absorption | Extremely low. Closed-cell foam repels water and will not retain bulk moisture if exposed. Also adds structural rigidity. | Higher absorption. Open-cell foam can take on and hold water if exposed to leaks or prolonged condensation. |
Material Density | Roughly 2.0 lb/ft³. Dense, rigid, and adds racking strength to framing and sheathing. | Roughly 0.5 lb/ft³. Softer and more flexible, offers no added structural support. |
Sound Dampening | Minimal. Its dense structure is not effective at absorbing sound. | Excellent sound attenuation due to open-cell structure and softness. Often used in interior walls for acoustic benefits. |
Drying Potential | Not intended to allow drying. It blocks vapor — which is a benefit in assemblies that are otherwise closed. | Can allow assemblies to dry inward, but only works when a drying path exists and exterior layers are vapor-open (rare in roof decks). |
Cost per Board Foot | Roughly 2× the cost of open-cell. More expensive, but adds structural value and moisture control. | Cheaper per inch and fills cavities quickly — but can create hidden risks if used improperly. |
Best Use Cases | Roof decks, vapor-sensitive assemblies, cold climates, small cavities, hybrid roofs, below-grade applications. | Vented attics, interior walls, warm climates with proper vapor control, acoustic separation zones. |
The Dew Point Is Not Optional
Let’s say you’re insulating the underside of a roof deck with open-cell foam. Everything looks good — solid coverage, proper adhesion, no gaps. But what you can’t see is what’s happening inside the assembly over the next six months.
In winter, the warm, humid air inside the building wants to move toward the colder outside. That moisture travels through the foam. If it reaches a cold surface — like the roof sheathing — and the temperature is below the dew point, condensation occurs.
And it doesn’t take much. Even small amounts of moisture, when repeatedly absorbed into OSB or plywood, will eventually lead to mold, rot, and structural decay. Unlike a vented attic where air circulation might carry some of that vapor away, an unvented roof with sprayed foam is a closed environment. You’ve sealed it tight. That means any mistake in your assembly design stays locked inside — for years.
Code Approval Doesn’t Mean Durability
The International Residential Code (IRC) allows multiple paths to building an unvented roof assembly. One of those paths includes using open-cell spray foam applied directly to the underside of the deck. But that path also comes with conditions — vapor retarder coatings, climate zone limitations, and controls on interior humidity.
In Climate Zones 1 through 3 — think Texas, Florida, and most of the Gulf Coast — open-cell foam can be used on the underside of the roof deck if a Class III vapor retarder is installed on the interior side. Usually this is a specific vapor-retarding paint, and it still requires proper thickness and continuous application.
In Zones 4 and above, which covers most of the U.S., inspectors tend to get more cautious. Open-cell assemblies often need additional strategies like dehumidification, rigid foam above the roof deck, or a fully hybrid roof design to keep the dew point in check. Even then, many officials prefer closed-cell — and with good reason.
The takeaway? Code compliance is not a guarantee of durability. It tells you what’s allowed — not what will actually work.
The Physics of a Failing Roof
Failures involving open-cell foam on roof decks are not hypothetical. They’ve happened — in houses, in multifamily builds, even in commercial construction.
In one case documented by Building Science Corporation, open-cell foam was used in a cold climate cathedral ceiling assembly without a vapor retarder. After just four winters, the OSB sheathing was visibly decayed. Vapor had been migrating through the foam each heating season, condensing against the cold deck, and soaking into the wood. The assembly never had a chance to dry.
In another case, a builder used open-cell under a complex hip-and-valley roof with no venting and synthetic felt. The foam looked perfect. But when the roof was opened up years later due to unrelated repairs, the sheathing was dark, soft, and saturated — even though there was no visible leak.
These aren’t edge cases. These are everyday assemblies: complex roofs, winter climates, non-breathable underlayments, and high interior humidity.
Vapor Retarder vs Vapor Open — The Real Line in the Sand
The difference between these two foams isn’t just price or density — it’s how they manage vapor.
Closed-cell foam is naturally vapor restrictive. At 1" thick, it qualifies as a Class II vapor retarder. At full depth, it can become a full vapor barrier. That’s why it’s ideal for assemblies where you want to stop moisture before it reaches the sheathing.
Open-cell, on the other hand, has a permeance often 10 perms or higher. That means it allows vapor to move through it — and into the sheathing, if conditions allow.
If a roof assembly is a sandwich, then open-cell foam is the peanut butter that never hardens. Moisture enters, gets absorbed, and has nowhere to go. Eventually, you’ve got moldy toast under your shingles.
It’s Not Always a No — But It’s Rarely a Yes by Default
There are situations where open-cell foam can be used safely in a roof:
Warm, humid climate with minimal heating load
Fully conditioned attic with consistent HVAC and low RH
Venting details above the deck or breathable exterior layers
Proper Class III vapor retarder installed on the foam
Hybrid assemblies with rigid foam above the roof
Actively dehumidified attic zones
But these are the exception, not the rule. And they require planning, coordination, and clear documentation — not guesswork.
Know When to Walk Away
If you're being asked to install open-cell foam against a cold roof deck, in a climate that sees frost, with no vapor control, no ventilation, and no drying path — the answer should be simple:
No.
Not “maybe with a vapor paint.”Not “we’ve done it before and it was fine.”Not “if we do it thinner it won’t trap as much.”
Just no.
Because when that assembly fails, and someone starts cutting out soggy OSB, your name’s the one they’ll remember — not the inspector’s, not the builder’s.
Bid With the Assembly in Mind
Every roof deck bid isn’t just about coverage and yield — it’s about accountability. And when you spray foam against the underside of a roof, you’re taking responsibility for how that space behaves across the seasons.
You don’t have to swear off open-cell forever. But you do need to know when it doesn’t belong. And when it doesn’t? That’s your cue to up-design the assembly — or walk.
Because in spray foam, the jobs you walk away from define your reputation just as much as the ones you spray.
Final Thoughts
Open-cell foam isn’t the problem. Misusing it is.
It’s a reliable, consistent, and cost-effective material when used in the right assemblies. But that’s exactly what makes it so risky when it’s used in the wrong ones. It’s easy to install. Easy to bid. Easy to justify — until it ends up trapping vapor inside a roof system that can’t breathe.
Roof decks demand respect. They are vapor-closed by nature. Once you seal the inside face with foam, you’ve created a box. That box needs to perform — in January, in July, and ten years down the line.
So stop and ask the right questions. What’s the climate? What’s the roof material? Where’s the dew point? Can it dry? Has it failed before?
Because you’re not just spraying foam. You’re building an assembly.
If you’re not confident in how that assembly will perform: Bid it with closed-cell. Or don’t bid it at all.
by Gage Jaeger, Owner and Founder of Foambid