Does Closed-Cell Spray Foam Really Make a Building Stronger? Yes—But There’s an Asterisk

Closed-cell foam can make a building stronger. That doesn’t make the foam the structural plan...

Spend enough time around closed-cell spray foam and eventually somebody will tell you that it makes a building stronger.

Sometimes the claim is pretty reasonable: closed-cell stiffens a wall, ties metal panels together, or helps bond roof sheathing to the framing. Other times it gets a little more ambitious. You’ll hear that closed-cell makes a building “300% stronger,” that it basically glues a pole barn together, or that once the cavities are filled with two-pound foam you don’t really need to worry about the bracing anymore.

That last version is where structural engineers tend to start looking uncomfortable.

The funny part is that the basic claim is actually true. Closed-cell spray polyurethane foam can add substantial stiffness and racking resistance to framed assemblies, and there is legitimate laboratory testing—and some pretty remarkable real-world storm damage investigations—to back that up. Modern high-wind retrofit guidance even recognizes specific closed-cell polyurethane foam applications for strengthening roof-deck connections.

The problem starts when we take “this tested wall became much stronger after closed-cell foam was added” and turn it into “spray foam makes your house 300% stronger.”

Those are two very different statements.

Yes, Closed-Cell Foam Can Add Real Strength

Closed-cell SPF is rigid, dense, and adhesive. Unlike fiberglass or loose-fill insulation, it doesn’t just occupy the space between structural members. When properly installed against compatible substrates, it bonds to the framing, sheathing, or metal skin around it. Once it cures, those materials can behave more like a connected assembly.

That matters most when the building is trying to rack.

Picture a rectangular framed wall. Push hard enough sideways against the top and that rectangle wants to become a parallelogram. Wind loads can do exactly that to a building. Structural sheathing, fasteners, diagonal bracing, hold-downs, and the rest of the lateral-load system are there to keep the wall from deforming.

Closed-cell foam can help.

A series of wall-panel tests conducted for the spray foam industry in the 1990s and 2000s found substantial increases in maximum racking load after closed-cell SPF was added. In one 1992 NAHB Research Center test, a vinyl-clad wood wall without foam reached an average maximum racking load of about 913 pounds. A comparable wall containing three inches of closed-cell SPF reached about 2,800 pounds. In that particular assembly, the foam-filled wall carried a little over three times the maximum racking load of the unfoamed version.

That is not a rounding error.

But before we turn that into the next sales slogan, keep reading.

About That “300% Stronger” Claim...

This is one of those cases where the number comes from something real and then gets beaten nearly to death by marketing.

If a wall goes from 913 pounds to 2,800 pounds, the second wall has roughly 307% of the original wall’s maximum racking capacity. Another way to say that is that capacity increased by about 207%.

Those are not mathematically the same as saying it became “300% stronger.”

And more importantly, that impressive result came from one particular wall construction.

When the same 1992 testing used plywood sheathing, the baseline wall was already much stronger. The 16-inch-on-center plywood wall increased from approximately 2,890 pounds to 5,300 pounds with closed-cell foam. That is still a huge gain—but now we’re talking about roughly an 83% increase, not tripling the capacity.

The same pattern showed up in later testing. A 1996 metal-stud wall clad with drywall increased from about 2,400 pounds with fiberglass to 5,380 pounds with closed-cell SPF. An OSB-clad assembly, however, went from roughly 4,800 pounds to 6,000 pounds—a much more modest 25% increase. A later test using relatively weak polyiso sheathing roughly doubled its racking load after closed-cell foam was added.

There’s a pretty obvious lesson buried in those numbers:

The weaker the assembly was to begin with, the more dramatic the percentage improvement could look.

So the next time somebody says closed-cell makes a building “300% stronger,” there’s a perfectly reasonable follow-up question:

Stronger than what?

A lightly clad wall?

A conventional OSB wall?

A properly engineered shear wall?

Those are not interchangeable comparisons.

Then the Lab Results Met a Hurricane

Laboratory testing is one thing. What makes this subject really interesting are the buildings where closed-cell foam appears to have made a very visible difference during actual high-wind events.

These case studies are not controlled laboratory experiments, and we shouldn’t pretend they are. Hurricanes don’t conveniently apply identical wind pressures to two structures while researchers hold every other variable constant.

Still, some of the comparisons are hard to ignore.

White’s Lumber: The Old Roof Stayed, the New Roof Left

One of the best examples comes from White’s Lumber in Port Isabel, Texas.

Part of the post-frame building had corrugated 29-gauge metal attached to wood framing with a roughly two-pound closed-cell foam applied in a “picture frame” pattern behind the panels. Years later, another section of the roof was replaced with new metal, but the foam reinforcement was not recreated.

When Hurricane Dolly hit the area, major portions of the newer, unfoamed metal roof blew off, while the much older metal backed by closed-cell foam remained in place. According to the case report, both sections used similar metal and fastening, and the newer section actually had the closer fastening pattern. Investigators found the failed metal had pulled over the fasteners; the foamed section had the additional bonded support between the skin and framing.

That doesn’t prove foam is a substitute for properly engineered metal-panel attachment.

It does make “closed-cell adds structural reinforcement” feel a little less theoretical.

Pascagoula: Two Connected Roof Sections, Two Very Different Outcomes

Hurricane Katrina produced another striking comparison at the Pascagoula Shrimp and Ice Company in Mississippi.

A RICOWI hurricane investigation documented severe damage where internal pressurization destroyed a tongue-and-groove wood roof deck on part of the facility. Connected to that same structure was a lightweight metal-building section insulated w

ith closed-cell SPF.

According to the published case study, the closed-cell-insulated section survived without damage, and the investigators reported no damage in the areas of the building insulated with ccSPF.

Again, I would not turn that into “spray foam saved the building” as though we ran the storm twice without foam for comparison.

But if one connected section fails dramatically while a lighter, foam-reinforced section remains intact, it is certainly relevant field evidence.

Port Isabel RV Park: Two Hurricanes, One Foam-Covered Building

The recreation building at Port Isabel RV Park may be the longest-running example.

After repeated exterior leakage problems in the late 1970s, roughly 1½ inches of closed-cell SPF was applied over the outside of the wood structure. The building later went through Hurricane Allen in 1980. The case report says nearby winds exceeded 125 mph and that the foam-covered structure experienced no reported damage or leakage while other buildings nearby suffered significant wind and water damage.

Nearly three decades later, Hurricane Dolly hit the same area. The same building reportedly came through with only a small crack and no water intrusion.

Thirty years and two major hurricanes is at least worth paying attention to.

Sometimes the Foam Really Is Holding Things Together—and That’s Not Always Good News

There’s another real-world example that demonstrates closed-cell’s strength in a completely different—and much less flattering—way.

In 2023, the American Society of Home Inspectors published a case involving a 1957 Long Island home with a chronically damp crawlspace. Closed-cell spray foam had been installed after Hurricane Sandy. Years later, an engineering inspection found that the floor joists, beams, and sill plates had deteriorated so badly that the wood had essentially lost its structural integrity.

The engineer reported that the closed-cell foam was the only thing keeping portions of the floor system together. Some joists could be penetrated effortlessly with a screwdriver, and pieces of the framing broke away when touched. Much of the damage had been obscured by the foam. The house ultimately had to be lifted so the entire failed floor system could be replaced.

If you wanted proof that cured closed-cell foam can have some serious rigidity and adhesion, that’s a pretty dramatic example.

It is also a fantastic example of why that strength should never be confused with a structural repair.

The foam helped the failed framing hold together well enough that the floor was still there. It did not restore the rotten wood, recreate the original load path, or make the structure safe.

In fact, the foam made the true condition harder to see.

That’s worth remembering when somebody wants to foam an old building because “it’ll tighten everything up.”

It probably will.

That doesn’t mean whatever is underneath it is suddenly healthy.

Modern Wind Standards Took the Idea Seriously

The hurricane stories are interesting, but closed-cell’s structural contribution is not limited to old industry anecdotes.

Current Building America guidance describes strengthening an existing roof from inside the attic by applying two-part closed-cell polyurethane foam adhesive at the roof sheathing seams and framing connections. The foam helps seal those joints against water intrusion while improving the connection between the roof deck and framing. FEMA wind-retrofit guidance and IBHS FORTIFIED details recognize this kind of below-deck polyurethane reinforcement for existing roofs where the exterior covering is not being removed.

That distinction matters.

The recognized retrofit isn’t:

“Spray six inches of whatever insulation foam is on the truck and congratulations—your roof is hurricane-proof.”

It is a specific installation detail intended to strengthen particular connections.

That’s engineering.

And it actually supports the broader argument better than the hype does: when we understand where polyurethane foam contributes strength, test the assembly, and install it accordingly, that strength can be useful enough to become part of a formal resilience strategy.

So What Is the Foam Actually Doing?

Think about a sheet-metal wall on a post-frame building.

Without foam, the panel is attached to the framing at individual fasteners. Push on the middle of a bay and the metal flexes. Wind loading pulls and pushes against the sheet, and those forces eventually have to find their way through screws or other discrete connections back into the framing.

Now bond a rigid closed-cell material across much of the back of the panel and onto the framing members.

The panel still has screws. The framing still carries the structural loads. But the panel and framing are now connected over a much larger area. Loads can be distributed differently, and local movement can be reduced.

Anybody who has sprayed a lightly framed metal building with closed-cell has probably felt this firsthand. A wall that popped and rattled when you leaned against it in the morning can feel noticeably more solid after the foam cures.

That is not your imagination.

It also does not give you permission to start cutting out the X-braces.

Pole Barns and Metal Buildings Are Where the Claim Gets Tempting

This is probably where spray foam contractors are most likely to oversell the structural benefit because the difference can be so obvious.

Thin metal panels, long framing spans, and lightly clad walls can have a fair amount of movement before insulation. Bond closed-cell foam directly to the skin and framing and the finished building can feel dramatically tighter and more rigid.

That’s a legitimate benefit to mention.

Where the contractor needs to pump the brakes is when the conversation changes from:

“Closed-cell will add rigidity to this assembly.”

to:

“You don’t need that structural bracing anymore.”

Those are completely different claims.

If the building was engineered with X-bracing, shear panels, girts, purlins, diaphragm screws, specific fastener spacing, or any other structural component, the insulation contractor should not decide it’s optional because the foam feels stiff.

If somebody wants the foam counted toward the building’s designed lateral resistance, let the structural engineer do that calculation using a product and assembly with data that actually support it.

Foam applicators are allowed to appreciate engineering.

We don’t become structural engineers because the wall quit rattling after lunch.

What About Vertical Loads?

This is another place where the word strength gets us into trouble.

The most impressive closed-cell testing deals primarily with racking, uplift, panel attachment, and resistance to deformation. Those are not the same thing as the building’s primary vertical load path.

Your studs still carry the floors and roof.

Your headers still span openings.

Your beams still carry loads.

Your columns still need to be sized correctly.

Your footing still needs to support everything above it.

Spraying closed-cell between undersized studs does not suddenly mean you can add another story. Filling the bays around a cracked header does not repair the header. And spraying around a rotten sill plate doesn’t put the missing wood back.

Closed-cell can help components work together and resist movement. It should not be confused with magically increasing every type of structural capacity in the building.

The Foam Product Matters Too

Another problem with repeating old structural-test numbers is that closed-cell SPF is not one universal product.

Different formulations have different densities, tensile strengths, compressive strengths, adhesive properties, and cured physical characteristics. Substrate condition matters. Foam thickness matters. Framing spacing matters. Sheathing matters. Fasteners matter. Installation quality matters.

The NAHB and ATI testing proves that closed-cell SPF can contribute substantial racking resistance.

It does not prove that every modern two-pound foam sprayed into every wall will reproduce those exact numbers.

Even the 2010 IIBEC paper that documented the impressive testing and hurricane case studies concluded that additional research, specifications, installation guidelines, quality-assurance procedures, and code approvals would be needed before ccSPF could broadly be relied upon as a primary structural-enhancement material.

That caveat has aged pretty well.

Why Open-Cell Doesn’t Get the Same Structural Reputation

Open-cell foam still adheres to its substrate, fills cavities, and can create an excellent air-control layer. But mechanically it is a very different material.

It is lower density, much softer, and far more flexible than two-pound closed-cell. Push on cured open-cell and it gives. Push on properly cured closed-cell and you immediately understand why one gets discussed as structural reinforcement and the other generally doesn’t.

That does not make open-cell inferior insulation. The two products are simply doing different things.

If structural rigidity is genuinely one of the secondary benefits a customer wants from the insulation system, closed-cell is the foam with the history and testing behind that conversation.

How I’d Sell the Benefit Without Overselling It

I think contractors should talk about this.

Closed-cell foam costs substantially more than many conventional insulation options. Customers deserve to know what they’re buying, and increased rigidity is one of the genuine secondary benefits the product can provide.

There’s nothing wrong with telling the owner of a metal shop that closed-cell will bond to the panels and framing and will likely make the building feel noticeably more rigid. There’s nothing wrong with explaining that controlled wall testing has shown major increases in racking capacity in certain assemblies. And there’s nothing wrong with pointing out that polyurethane foam is used in recognized high-wind roof retrofit details.

What I would stay away from is the magic percentage.

“Closed-cell makes your building 300% stronger.”

It sounds great. It is also vague enough to be almost meaningless.

Three hundred percent stronger in what way? Racking? Uplift? Compression? Compared with what wall? At what foam thickness? With what product? What sheathing? What framing spacing?

A better—and defensible—way to put it is:

Closed-cell spray foam has been shown to substantially increase the racking strength and rigidity of certain framed assemblies, although the amount of improvement depends on the construction and should not be treated as a substitute for required structural bracing or engineering.

It isn’t quite as sexy.

It also won’t make an engineer spit out his coffee.

Final Thoughts

So, does closed-cell spray foam really make a building stronger?

Yes.

Not “technically yes if you squint at a marketing brochure.” Actually yes.

Closed-cell SPF has more than doubled the racking capacity of some tested wall assemblies. In other tested walls, the increase was more modest but still measurable. Roof-deck testing has shown large gains in wind-uplift resistance. Field investigations after hurricanes have documented foam-reinforced portions of buildings surviving while nearby or connected unreinforced sections failed. And modern wind-retrofit guidance recognizes specific two-component closed-cell polyurethane applications for improving roof-deck attachment.

That's a pretty strong case that the benefit is real.

But the foam is adding strength to an assembly that already has a structural system. It is not giving the insulation contractor permission to redesign that system from the spray rig.

The hurricane straps still matter. The shear walls still matter. The X-bracing still matters. The rotten joist is still rotten. And if the engineer wants a certain number of nails around the edge of an OSB panel, “don’t worry, we’re foaming it” probably isn’t an acceptable substitution.

Sell the benefit. It’s a good one.

Just sell the benefit that actually exists.

Closed-cell foam can make a building stronger. That doesn’t make the foam the structural plan.

by Gage Jaeger, Owner and Founder of Foambid