My child's basement walls have been sprayed with foam insulation. The home inspector says it is highly flammable (some web sites call it solid gasoline) and must be covered with fire retardant paint ($280 a can) or drywall. They hate the texture of it so they don't just want to paint it. But building stud walls in front of it and drywalling it will be expensive and time consuming.

The house is 130 years old. The basement walls were the exposed fieldstone foundation. Someone decades ago sprayed them with this stupid foam, probably to make the basement warmer when they put in walls and made a bedroom down there. While studs and drywall might be easy and cheap in an all-square situation, this is very much not that.

You can see the horrible texture in this picture, taken for a different reason, but the two yellow walls you see are coated with this stuff.

basement walls

They are wondering if they could use drywall mud to smooth the hated texture and also achieve the required fire retardant properties. Is this feasible?

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    Drywall mud by itself will not last. Just a light touch will probably cause cracks and/or pieces to come off. I do not know how long(during a fire) the paint will protect(some fire retardant chemical are quite nasty(health wise) themselves).
    – crip659
    Jun 12, 2023 at 14:33
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    IMHO It shouldn't cost too much to install studs and drywall and you'll end up with a much better appearance/safety product. I'd bite the bullet and do it right. Ask your self, how much is your child's safety worth? This comment is opinion based so I'm not posting it as an answer. Jun 12, 2023 at 14:44
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    The basement was likely insulated with the expectation that walls would be installed at some point before occupying the space. Yes, studs and drywall will cost something, but as far as time-consuming goes, I can't imaging how long it would take to try and build up a solid, smooth covering with plaster... As crip659 mentions, drywall mud does not have the strength; it will crumble with the lightest touch or building motion, especially when installed over something flexible like insulation.
    – spuck
    Jun 12, 2023 at 14:44
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    The good thing about stud walls is that they do not need to be up tight to the foam/fieldstone. If a few inches of lost floor area is okay, then the walls can be placed where they will be straight. Those new walls are mainly just to hold drywall and paint, not to hold up the house.
    – crip659
    Jun 12, 2023 at 15:24
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    Pull a small piece off and try to light it on fire, outside. does the foam actually burn/smoke? It might not be the deathtrap you've heard about online; there's many kinds of foam.
    – dandavis
    Jun 12, 2023 at 20:34

4 Answers 4


It needs drywall.

I can't find anything allowing anything else in a living space. Regardless of your inspector's allowance of paint, I certainly wouldn't do it for a bedroom. There are allowances of assorted thermal barriers for crawl spaces and attics, but not living spaces.

2603.4 Thermal barrier. Except as provided for in Sections 2603.4.1 and 2603.9, foam plastic shall be separated from the interior of a building by an approved thermal barrier of 1/2-inch (12.7 mm) gypsum wallboard or a material that is tested in accordance with and meets the acceptance criteria of both the Temperature Transmission Fire Test and the Integrity Fire Test of NFPA 275. Combustible concealed spaces shall comply with Section 718.


Drywall mud, no. Not without drywall, first.

Plaster, yes, if they wanted to do plaster. But if they hate the shape of the walls, not much point - drywall will change that quicker and cheaper than plaster will. It would be worth considering if the non-planar fieldstone wall look was a positive for them - but you say it's not.

Drywall does not need to be square, plumb, or level. Just more-or-less planar, or sweeping curves can also be managed (tighter ones may take wetting and double layers of 1/4") and the planar sections can be faceted to work with the underlying shape without eating 20% of the room to make it all square, plumb, and level.

High points of the foam can also be ground down to improve the "fit" if not overdone to the point that it's completely removed, exposing rock.


I'm the first around here to shout from the rooftops the horrors and hazards (see the 13:00 surprise) of spray foam insulation. From the Sunshine Mine to Grenfell Towers to Browns Ferry nuclear plant, the non-flammability of the stuff is wildly overstated. And I paint LPU and 2-part epoxy paints, so I know all about mixing errors and isocyanate. That 'smell that won't go away' is a genuine health threat; look up the MSDS's.

But I'm also quick to calm undue fear (e.g. asbestos; it's not plutonium). Not all spray foam is excessively flammable and this doesn't look like 500 cans of (bascially napalm) Great Stuff - it looks professionally installed. The professionals have access to 2-part field-mixed foams, and in exchange for the isocyanate risk of a mis-mix, they get much better characteristics - including possibly much better flame resistance.
So the right answer is to chisel off a little bit and do a flame test and see what it does. If it resists ignition, it might be made out of the right stuff (as opposed to "Great" stuff).

So I'm pretty sure it was pro installed, probably to manage thermal issues in the building. I would hope that a pro would have selected a foam with the proper fire resistance for the application. And probably closed-cell foam as well. That might not be bad at all.

I'm sorry you don't like the aesthetic, but this is unfinished, utility space, not "habitable space", and does not count toward the square footage on which the house is priced and assessed.

That thing... where a homeowner looks at such unfinished spaces and says "Let's finish this into habitable rooms and get a kick on my square footage and home value" - that is called "Finishing" a space. It generally involves treatments on all walls, ceiling and floor. Studs and drywall looks perfectly appropriate here, obviously you'd need to level the studs one way or the other. Shims maybe? I would want to fill the voids with something to avoid chimney effect if it does catch fire. The standoff space would also allow for fitting electrical, however, I would seriously consider metal conduit and metal boxes as a means to prevent fire. Something like FMC or MC cable with appropriate cable clamps into steel boxes. It will need to be done to code in all respects.

Such work is considered a remodel, and you need to do the usual permit pulls, plans, inspections, etc. Refusing to pull a permit can backfire later when you try to sell the house and get credit for the additional square footage, especially if the work was done sub-par. Lastly, when putting people in a basement, always test for Radon. Last thing you want to do is give them lung cancer.


I've had to deal with that flame retardant paint for my attic insulation.

If it's closed cell foam, what I understand is that it's not that the foam bursts into flames but that it puts out some really toxic smoke. The flame retardant coating delays that smoke by an hour or so which gives firefighters time to fight the fire without having to deal with the nasty black smoke. The stuff isn't intended to save your house from fire, it's intended to save the fire-fighters saving your house from the smoke. That doesn't make it less important for you: if there is a fire, you'll have more time to get out before being affected by the toxic emissions.

The flame retardant coating itself (assuming it's the same stuff) is matte white. The foam has that bubbly texture because it expands as it's sprayed on. Trying to make these flat with plaster or drywall mud would not work: I doubt it would adhere well to the foam, the foam isn't that stable, and it would just take a lot of work even if it did stick and then stayed. I think it would probably be less work and expense to put up walls in front of the foam.

Putting up the walls should not be that difficult. You can use metal studs which are fast and easy to work with (although that's subjective I guess). They are straight, light weight, cut with shears, join with a crimp tool, cost less than wood right now, and come in 3.5, 2.5, and (I believe) 1.5 inch widths to let you save some space. You'll have a nice finished room with normal looking walls. This really sounds like the better solution.

  • "they" is my adult child and his wife. The foam is bumpy as can be, as though literal popcorn had been applied to the wall before foaming it. It is also not white, presumably having been painted with ordinary paint. There is no flame retardant coating yet. Jun 12, 2023 at 17:11
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    That looks like unpainted sprayed on closed cell foam to me. The coating is white (but could be tinted), the foam is yellow and has that texture. It has that texture because it expands as it is sprayed on. Looks like it was pretty neatly applied. If it's to look nice, the best option is really to put up drywall. Trying to skim that flat is going to be a fiasco. Jun 12, 2023 at 17:50

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