Should You Spray Foam Over Old Insulation? When It Works, When It Backfires, and How To Price the Fix

Thou shalt not foam over batts?

It always sounds easy. “Just spray right over the old stuff.”

A homeowner says it. A builder shrugs it out loud. Sometimes even a rookie crew member asks why you’re bothering with the messy removal.

And if you’re being honest, there’s a part of you that wants to believe them. Because spray foam over existing insulation sounds like a shortcut—like a cheat code that might just work if the house is dry enough, if the client signs the waiver, if the stars align.

But here’s the truth: in most cases, spraying foam over old fiberglass or cellulose isn’t just risky—it’s dumb. Sometimes it’s code-legal but performance-stupid. Other times, it opens you up to lawsuits, odor complaints, trapped moisture, yield loss, or all of the above.

This post walks through the science, the codes, the real-world liabilities, and how to price the work like a pro—not just a guy with a gun.

Code vs. Common Sense

Let’s get one thing out of the way: building code doesn’t outright ban spraying foam over existing insulation.

The IRC Section R316 lays out thermal barrier rules for spray foam, and R806 covers attic ventilation. Neither of them say “thou shalt not foam over batts.” But just because it passes inspection doesn’t mean it makes sense.

Closed-cell foam is a vapor retarder. Fiberglass and cellulose are not. When you sandwich them together—permeable insulation sealed behind an impermeable layer—you block off that cavity’s ability to dry. And if there’s moisture present, or air leaks, or inconsistent coverage? That’s a recipe for rot.

Even manufacturers like Carlisle and BASF specify in their tech sheets that substrates must be “clean, dry, and free of contaminants”. There’s a reason you don’t see “spray-over-fiberglass” diagrams in any reputable install manual.

The Moisture Trap That Keeps On Giving

Joe Lstiburek, founder of Building Science Corporation, says it best:

“Don’t bury your sins with foam.”

Covering up mold, moisture, or debris with a high-performance air sealant doesn’t fix the problem—it hides it until it festers.

In cold climates, that leads to condensation and freeze-thaw damage behind the foam. In hot, humid zones, it breeds mold and moisture cycling.

The cavity can’t dry to the inside because you sealed it. It can’t dry to the outside because there’s no venting. Even open-cell foam, when sprayed over aged batt insulation, can choke off airflow and cause problems.

You didn’t insulate—you trapped something.

The Smell That Won’t Go Away

Even if you dodge the moisture bullet, there's another one waiting: odor.

Spray foam is exothermic. The chemical reaction inside the foam generates core temperatures between 160°F and 200°F. That’s hot enough to volatilize whatever is underneath it.

So if you spray over dirty fiberglass—especially insulation that’s absorbed cooking odors, rodent urine, mildew, or years of attic funk—you might lock in a smell that gets reactivated every time the attic heats up.

We’ve seen cases where the foam job looked perfect, passed inspection, and still had to be ripped out because of unbearable odors.

You can blame the substrate. You can cite the MSDS. But at the end of the day, you sprayed it, and now the house stinks.

So When Is It Allowed?

There are some narrow cases where spraying over existing insulation might be defensible.

• Vented crawlspaces, where old fiberglass is intact and the foam is part of a dry, ventilated assembly

• Engineer-approved designs, where dense-pack cellulose and closed-cell foam are layered under specific dew-point control strategies

• Non-critical areas, like garage ceilings or outbuildings where thermal performance isn’t vital

But even then, the substrate must be:

• Bone-dry (check it with a moisture meter)

• Visibly clean

• Free of rodent contamination, dust, or mold

And if you can’t see it? You can’t spray it.

The Insurance You Think You Have

Let’s talk liability.

Your foam product might carry a limited warranty, but only if you install it according to manufacturer instructions. That usually means applying it over clean, dry substrates—not mystery batts from 1985.

Most insurance policies exclude coverage for damage related to:

• Improper surface prep

• Workmanship defects

• Mold or vapor barrier failures

If you cause a mold issue by spraying over damp insulation, you own the fix. If the client sues and the carrier denies the claim, you pay out of pocket.

Some contractors assume they’re covered because they’re “following code.” But legal protection doesn’t come from code—it comes from documentation and due diligence.

The Cleanup Isn’t Just Labor (And Might Be Regulated Material)

Insulation removal is the most undervalued part of the job. It’s messy, exhausting, and usually underestimated. But it can also cross into HAZMAT territory fast.

Vermiculite = Stop Work

If the house was built before 1990 and you see granular insulation that looks like kitty litter? That’s vermiculite—often contaminated with asbestos, especially if it came from the notorious Libby, Montana mines.

You can’t eyeball asbestos. And the EPA guidance is blunt:

“If vermiculite insulation is present, assume it may contain asbestos and treat it accordingly.”

Here’s the right process:

1. Pause work immediately

2. Notify the homeowner in writing

3. Get it tested by a licensed asbestos inspector

4. Wait for lab results before disturbing anything

If it comes back positive? It’s not your job. You need to refer them to a certified abatement contractor.

Disturbing asbestos without a license is a fast track to legal trouble, lost insurance, and possibly criminal charges in some jurisdictions.

Even When It’s Not Hazmat, It’s Still Work

Fiberglass batt removal might seem simple, but attic tear-outs often involve:

• Rodent contamination (mice, raccoons, bats)

• Feces and urine in the insulation

• Decades of dust and roof debris

• Nails, wires, or unsafe framing to work around

You’ll need:

• Tyvek suits, gloves, and respirators

• Shop vacs and containment bags

• A plan for dump fees or landfill restrictions

If it’s bagged insulation, you’ll typically need one 42-gallon contractor bag per 25–30 square feet. A 1,200 sqft attic? That’s 40+ bags, plus hauling.

Don’t forget the soft costs:

• Break time due to heat exhaustion or crawl fatigue

• Setup and tear-down hours

• The risk of someone putting a boot through the ceiling

  
  

How To Price It

Foambid allows you to quote insulation removal as a separate line item. You should.

We see contractors charge:

• $0.75–$1.50/sqft for basic removal

• $2.00–$3.50/sqft for contaminated or nested areas

• Hourly + disposal fees for crawlspaces and small jobs

You can also add a precondition clause:

“If vermiculite or hazardous materials are discovered, all work will stop and a licensed abatement firm must clear the site before spray foam installation continues.”

This protects you, your crew, and the homeowner.

What To Put In Writing

Don’t leave room for misunderstandings. Your quote should include:

• Assumptions: “All existing insulation to be removed by others unless otherwise stated.”

• Exclusions: “Bid does not include removal unless listed. No warranty applies if substrate remains.”

• Preconditions: “Substrate must be clean, dry, and free of contaminants before foam is applied.”

And if the customer declines removal? Have them sign a waiver or acknowledgment. Check out this post for more information on solid SPF contracts.

Final Thoughts: The Jobs You Walk From Build Your Brand

You don’t build your reputation by saying yes to everything. You build it by knowing what to say no to—and why.

Spraying foam over existing insulation might get you paid quicker. But when that job backfires—when the smell lingers, the substrate fails, or the claim gets denied—it won’t be the customer paying for it. It’ll be you.

Use your expertise. Use your judgment. Use Foambid to separate cleanup, foam, and specialty prep in a way that’s clean and concise.

And remember: Spray foam is more than a product. It’s a system. It either solves building problems—or it hides them.

You get to decide which one you’re doing.

by Gage Jaeger, Owner and Founder of Foambid