Spray Foam Myth-Busting: Isn’t That the Stuff That Melts Your Roof?

Every one of these myths comes from a kernel of truth…

Spray foam has been used in buildings for over 40 years. It’s code-approved, performance-proven, and trusted in everything from high-end custom homes to refrigerated warehouses. And yet, every time we walk a job, we still hear the same wild claims:

“I heard it makes your house too tight.”

“It off-gasses forever, right?”

“One inch is plenty — foam’s magic!”

Nope. Nope. And... definitely nope.

So here’s a little myth-busting. Not for the engineers or code nerds (like me). For the real-world folks who’ve heard one too many YouTube-based objections to the product they spray every day.

Myth #1: “Spray Foam Is Waterproof, So It’ll Fix My Basement Leak”

I feel like I personally get this one a lot. I don't know if it's just a central-Nebraska thing or what, but spray foam, especially closed-cell, is water-resistant, not waterproof. It can repel moisture. It can even survive floods in some applications. But it’s not a substitute for a sump pump, drainage tile, or common sense.

If water is entering your home through cracks in the slab, it’s not going to stop and say, “Oh — I guess I’ll turn around. Foam’s here.”

Myth #2: “You Only Need One Inch — Foam’s That Good”

Foam is that good… but physics still applies. One inch doesn’t magically hit your code-required R-values. It might seal the structure, but it won’t insulate it properly. One inch of closed-cell might get you R-6 to R-7 — but that’s not going to cut it on an attic deck in Michigan.

Also: good luck explaining to an inspector why your “air seal pass” was supposed to be the final pass.

Myth #3: “Spray Foam Will Rot the Roof From the Inside”

If you spray open-cell foam on the underside of a roof deck in the wrong climate without accounting for vapor drive, yes — you can create moisture issues. But the problem isn’t the foam. It’s the assembly design.

Closed-cell foam is a Class II vapor retarder. It won’t let moisture in. Open-cell foam is vapor-permeable — it will. The trick is knowing when and where each is appropriate.

If you’re seeing roof rot, it’s probably not the foam’s fault. It’s the lack of understanding around what was being sprayed, where, and why.

Myth #4: “Spray Foam Makes the House Too Tight — It Needs to Breathe”

If your house is “breathing” through the walls, what you really have is air leakage — not ventilation. That’s not healthy. That’s not efficient. And it’s not something to brag about.

Spray foam tightens the building envelope. That’s a good thing. If you want fresh air, install an ERV or HRV system. Let the machine breathe for you — not the gaps in your framing.

Myth #5: “I Can Just DIY This With a Kit and Save Thousands”

Sure — and you can cut your own hair, too.

Spray foam application isn’t just squeezing a trigger. It’s about temperature control, ratio balancing, yield management, substrate prep, and safety. A DIY kit might work in a crawlspace corner. But if you try to DIY an attic or garage and end up spraying off-ratio foam into fiberglass or onto subfloor? That “savings” turns into a nightmare.

We’ve seen it. We’ve re-sprayed it. It’s not worth it.

Myth #6: “It Off-Gasses Forever and Makes the House Toxic”

Yes, spray foam has an odor while curing. That’s chemistry. But when it’s applied correctly and cured, it becomes inert — meaning no ongoing off-gassing.

Most manufacturers recommend a 24- to 48-hour re-entry window after application, depending on ventilation. If your home still smells like chemical weeks later? That’s either off-ratio foam or poor ventilation — and you should call the installer back.

Also worth noting: the same folks panicking about foam often have MDF furniture, vinyl floors, and plug-in air fresheners. Just saying.

Myth #7: “Foam Is Just Fancy, Expensive Insulation”

It’s not “just insulation.”

Foam is your air barrier, your vapor control, your R-value, and in some cases, your structural reinforcement. Try calculating the labor, material, and time needed to install separate products for each of those — and suddenly, foam doesn’t look so expensive.

Foam isn’t about being cheap per inch. It’s about being smart per benefit.

Myth #8: “Isn’t spray foam super flammable?”

Not when it’s installed correctly. Cured spray foam (especially closed-cell) is tested to meet ASTM E84 Class I fire ratings, which means it won’t support a flame once the ignition source is removed. Is it fireproof? No. But neither is your drywall, your framing, or your vinyl floor.

That’s why code requires a thermal barrier—usually drywall—over any exposed foam. Follow the code, follow the install instructions, and you’re good. Spray foam isn’t the accelerant some people make it out to be.

Myth #9: “Foam causes termite infestations”

Spray foam doesn’t attract termites, and it’s not a food source. But it can cover their tracks. If you have an active infestation and then spray foam over it, you’re hiding the evidence. That’s the risk—not the foam itself.

In many southern states, codes even require termite inspection gaps or treated barriers below foam installs for this exact reason. Handle the bugs before you spray, and this isn’t a problem.

Myth #10: “You’ll never be able to change anything once foam is in”

This one has some truth behind it—but it’s also a design problem, not a foam problem. Yes, closed-cell foam is hard to remove. And yes, if you didn’t plan for future wiring, plumbing, or HVAC work, digging through foam will be a pain.

But that’s why good contractors talk about service zones, access panels, and foam-free chases before the gun comes out. Plan your penetrations. Coordinate with trades. Foam where it matters—and leave the rest accessible. The “permanent” part of foam is one of its biggest strengths, not a drawback, when it’s used with intention.

Final Word: Foam Doesn’t Fail — Bad Installs Do

Every one of these myths comes from a kernel of truth… that got stretched, twisted, and re-shared like a game of telephone.

Foam works. Period. But only when it’s designed and applied correctly. The problem isn’t the chemistry — it’s the shortcuts, the assumptions, and the bad installs that give it a bad name.

So the next time someone says “Isn’t that the stuff that melts your roof?” — send them this post.

by Gage Jaeger, Owner and Founder of Foambid