My basement walls are covered with a few inches of polyiso foam board with interior stud walls being built over the foam board. I have some extra pieces of foam board from the installation that I am going to add to a couple of the stud bays (a little r-value, and use them up in a productive way). A couple of days ago, I happened to receive a shipping box that had a few relatively small pieces of shipping Styrofoam, and it occurred to me that rather than just tossing the styrofoam in the trash, I could add it to the foam board scraps in the stud bay.

Is there any reason not to do so? This isn't going to make any substantial r-value difference - I would do it just to use the styrofoam rather than trashing it. But I want to make sure there's not a risk or danger in doing so (e.g. risk of fire, etc).



2 Answers 2


Ask the folks in Grenfell Towers about this. Most could not escape because they were incapacitated by the poisonous fumes from when the polyiso burned.


The first problem is that plain old polystyrene packing foam is a fire accelerant. Which means if a fire gets going, this will spread the fire like a rocket, greatly shortening your viable escape time, which is already too short as it is due to the proliferation of plastics in consumer goods.

Here's an example of how different grades of insulation materials react to fire. The stuff in the #2 bay is polyiso, and the #1 bay is several grades of spray foam - all architecturally rated product by the way.

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Don't miss the surprise at 13:20.

Poison / toxicity of fumes

I don't know if you've noticed, but in the last 30 years a new cause of death has rocketed up: Smoke Inhalation.

Foam's propensity to fire collides catastrophically with its tendency to emit poisonous fumes when burned. These instantly degrade your brain's capacity to make decisions and keep legs working as they should, and you don't make it out of the fire. And then your death is recorded as "smoke inhalation" since the fire didn't actually burn your body.

And the toxic smoke came from plastics. People had far less trouble with this when household items were made of wood, metals, natural cloth and other natural substances, that did not produce large amounts of hydrogen cyanide and numerous other chemical toxins.

What is packing foam?

Most foam is made of Polystyrene, aka "6 PS" on the recycling symbol. The exact same material is used in model airplanes. The difference is that the polystyrene is foamed by making it into sand, saturating it in liquid pentane, injecting the liquid into a mold, and flash-heating the mold to make the pentane violently boil. The molds are hollow, and are heated with pressurized steam then cooled with cold water to reset the mold.

What is pentane?

methane - ethane - propane - butane - pentane - hexane - heptane - octane.

These are basic hydrocarbons, with 1-8 carbon atoms per molecule. Methane, you know as natural gas. Propane is an easier-to-handle "natural gas" used for camping, because it can be kept liquid in a modestly pressurized tank. Butane can be kept liquid in a simple plastic lighter. As you can guess, Pentane's boiling point is higher still and naturally wants to be liquid below 97°F/36°C. So it's the perfect blowing agent for molds using steam to heat and cold water to reset. Octane you know is gasoline. So it is in that spectrum.

Pentane is highly flammable of course, but its flammability characteristics are not used in manufacture. Non-flammable non-toxic blowing agents exist, but unfortunately they are Freon, and so are largely banned. Relatively little pentane remains in the foam; most of the flammability is from the polystyrene. Which is really a problem, because polystyrene burns aggressively and emits much more poisonous smoke. Pentane burns cleaner than gasoline.

  • A very fast google search showed interesting stuff made with styrofoam and gasoline.
    – crip659
    Sep 6, 2022 at 20:16
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    @crip659 since gasoline/octane is pentane + 3 more carbons in the chain, it too is a solvent of polystyrene, allowing you to get a nice polystyrene/gasoline emulsion. And the gas wouldn't evaporate instantly, so could potentially carry the styrene some distance before sticking it to things. I'd be afraid of the stuff. Sep 6, 2022 at 20:20
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    choosing this as answer for thoroughness. I will not be adding even the small pieces of polystyrene. And I will also hopefully not be kept awake at night worrying about the polyiso sheets that are already on the walls. ugh...we have created some amazing things as humans, and also some real problems. often at the same time... Sep 8, 2022 at 13:11
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    I added a screengrab for attention. That is a great demo video you found. Feel free to undo my edit if you don't like it.
    – P2000
    Sep 8, 2022 at 16:24
  • @P2000 I'm likin' it! Sep 8, 2022 at 18:51

That packaging foam is EPS or similar to it. EPS is the white bubbly styrofoam you're likely familiar with, from appliance and electronics packaging etc... This foam is open-cell which allows for ready access of oxygen to help fuel a fire. In peanut form it is even worse.

I don't believe there is a fire rating requirement for in-wall XPS or EPS generally in homes except where a rating is specifically stipulated e.g. between adjacent dwellings, in-house rental suites etc...

XPS is rated for commercial wall insulation, EPS is not. (See Dupont). XPS is denser and closed-cell which makes it more difficult for air/oxygen to fuel the fire. XPS is the coloured foam board (often pink or blue) found in construction/hardware stores.

So, with EPS the smoke and spread is worse compared to fiber glass (best) or XPS (rated and permitted in commercial). Such fire rating considers further elements of the wall assembly (e.g. gypsum boards a.k.a. drywall sheets) that separate the insulation from the inside space. Also embedding the EPS or peanuts with fiber glass insulation probably helps moderate the fire, but it won't be as benign as without EPS altogether.

The packaging foam might also contain additional components not present in EPS, thus possibly worsening toxicity, flame spread and smoke.

I would avoid EPS, and thus also packaging foam. Bring it to recycling or pass it on to someone else who could use it for packaging, e.g. offer it on an on-line give-away site.

Ref: https://www.dupont.com/content/dam/dupont/amer/us/en/performance-building-solutions/public/documents/en/meeting-the-fire-code-with-continuous-foam-plastic-insulation-43-D100637-enUS.pdf

  • Artsy type people might be interested in it, to make things.
    – crip659
    Sep 6, 2022 at 19:09

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