Open‑Cell Foam on Metal Substrates: Misunderstood, Risky, or Justifiable?

Just Because You Can Doesn’t Mean You Should.

Ask “Can I spray open-cell on metal?” in any contractor group and you’ll trigger a flood of passionate opinions. For some, it’s standard practice. For others, it’s asking for callbacks, corrosion, and regret.

This post doesn’t take a side for the sake of argument — it lays out the science, the risks, the nuance, and the field-tested perspective of spray foam pros who’ve had this debate a hundred times over.

Why the Debate Exists

Metal is fast to heat and cool, which makes it prone to condensation. It also flexes with temperature swings. Open-cell foam, being vapor-permeable and less rigid, doesn’t always play nicely with that.

When applied without a vapor retarder or in the wrong climate zone, open-cell can allow warm, moist air to reach the cold metal and condense behind the foam. That’s where trouble starts — corrosion, microbial growth, or foam delamination.

Still, some contractors say they’ve sprayed open-cell on metal for years without issue. So who’s right?

What the Building Science Says

Closed-cell foam is denser, more rigid, and significantly more resistant to moisture. It forms a vapor barrier and bonds tightly to metal, making it the go-to choice in most metal assemblies — especially in roof decks, unvented spaces, or humid climates.

According to manufacturer guidelines and SPFA recommendations, closed-cell foam is preferred on steel panels, aluminum framing, and other metal substrates where moisture is likely or the substrate may flex under temperature shifts.

One frequently cited concern in these guidelines is dew point formation. Cold metal can easily reach dew point temps — and if vapor from inside the structure is allowed to pass through open-cell foam unchecked, it will condense on the metal. That moisture gets trapped where you can’t see it, and that’s a recipe for callbacks (or worse).

What Contractors Are Saying

In Spray Foam World Wide, this topic comes up often — and while opinions vary, a few themes repeat.

Some contractors argue that open-cell can be sprayed on metal in specific, tightly controlled conditions — such as in dry climates, in fully conditioned buildings, or when a vapor barrier is applied over the foam.

Others say it's not worth the gamble. Many posts include paraphrased sentiments like:

“Metal sweats. Open-cell doesn’t stop that, it just lets it happen behind the foam.”

“If you spray open-cell on a metal roof deck in a humid area, you're basically building a mold sandwich.”

“You might get away with it... but when you don’t, you're the one fixing it.”

There's a shared thread: spraying open-cell on metal is a calculated risk — and not one to take lightly.

Manufacturer Recommendations

Here’s what several manufacturers and industry sources advise:

• SPFA & Metal Construction Association: Recommend closed-cell SPF on metal panels and discourage open-cell in unvented roof assemblies without additional moisture control.

• Carlisle, Gaco, and Johns Manville: Require dry, clean, non-condensing metal surfaces and advise extreme caution with open-cell foam on metal, especially in colder climates or where humidity is present.

While most data sheets don’t outright ban open-cell on metal, they almost universally require a proper vapor management plan — and make it clear that the installer bears the responsibility for moisture failures.

When It Might Be Fine

Some contractors do report success when spraying open-cell on metal, but it typically depends on:

• A fully conditioned, ventilated structure

• No exposure to vapor drive or condensation risk

• Proper coatings or vapor barriers added to the interior

• A dry, consistent climate zone

Even then, it’s not always approved by inspectors — and rarely recommended by foam manufacturers without qualifications.

What Foambid Does to Help You Catch This

Foambid includes a built-in warning system for exactly this situation. If you select Open-Cell while working on a metal substrate, the app immediately triggers a caution message reminding you that open-cell foam sprayed directly against metal can trap moisture and accelerate corrosion. It suggests starting with a closed-cell base coat or considering another insulation strategy entirely.

This feature doesn’t block the workflow — but it puts responsibility back in the estimator’s hands while reminding them of the building science. It’s just one more way Foambid helps contractors avoid expensive callbacks and make better-informed decisions right from the estimate.

Just Because You Can Doesn’t Mean You Should

Yes, open-cell foam can technically be applied to metal. But in most cases, the potential for failure — through trapped moisture, corrosion, or loss of adhesion — makes it a poor fit unless you've controlled for every risk.

If you're doing a small, simple job and understand the vapor flow? Maybe.

If you're trying to save money by using open-cell where closed-cell is clearly safer? You’re gambling with your reputation.

Final Thought: Foam Isn’t the Problem — Conditions Are

The foam doesn’t fail on its own. It fails when it's put in the wrong place under the wrong conditions. That’s the heart of the open-cell-on-metal debate.

So if you’re considering it, be honest about the building, the budget, and the risk. If there’s doubt — or if you’re trying to argue with building science to save a few bucks — step back and rethink it.

Closed-cell may cost more up front, but when you factor in risk, callbacks, and your name on the job, it often turns out to be the better bargain.

Editor’s Note:

Some quotes and positions in this post are paraphrased from ongoing contractor conversations inside Spray Foam World Wide, a private Facebook group. These perspectives reflect field experience and real-world debate, but are not direct attributions unless otherwise cited.

by Gage Jaeger, Owner and Founder of Foambid