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I recently moved into a two-story home that has a storage cupboard under the stairs. There is no finish underneath the bottom part of the stairs, and the bare plywood is exposed, which is a fire safety hazard (since a fire that got into the cupboard could easily ignite the stairs and make it impossible to exit the home via the stairs).

Current building code in my area calls for the underside of the stairs to be protected by a sheet of drywall to protect the wood from direct exposure to flames. However, I'm looking for a quicker and cheaper temporary solution as a holdover while dealing with other important upgrades and renovations.

I noticed that the reflective bubble insulation at my local hardware store has a Class A fire rating, which is the same rating as drywall. It would be very cheap and easy to staple sheets of reflective bubble insulation to the diagonal framing (strongback?) along either side of the stairs:

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My understanding is that the purpose of putting drywall under the stairs is not to make them fire-proof (which is impossible with wooden stairs anyway), but to make them fire-resistant for long enough for anyone on the second story to safely move down the stairs and exit the house. Although it doesn't precisely meet the building code, is reflective bubble insulation with a Class A fire rating a reasonable temporary solution for protecting the underside of plywood stairs from direct exposure to flame?

One possible consequence I can think of when compared to drywall is that the reflective insulation might trap more heat in the cupboard and increase the speed at which the cupboard reached flashover temperature.

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    Yes, something fire-retardant is better than nothing. Plywood is a class III retardant but class I is obviously better. Unless you're inviting code guys to hang out and have a few beers you only have to make one person sleep easy at night; yourself.
    – MonkeyZeus
    Aug 9 '21 at 19:29
  • It's going to take stairs a long time to burn through to a point that would keep someone from using them. Don't' store flammable items or ignition source in there. Make the cupboard reasonably air tight to keep the whole area from burning (unless you got other huge fire anyway), or put a smoke detector in there until you drywall it.
    – dandavis
    Aug 9 '21 at 22:46
  • You may also consider fire escape ladders placed in each upstairs bedroom under the window. We have roll-up ladders with a steel bracket screwed to the window sill. Once installed, I did a test run out the window, then had each of the kids install it (with parental supervision) and climb down (the youngest was about 4 at the time), so they had the confidence they knew what they were doing and that they could make the trip. Of course, you'll want to fire-resist the bottom of the stairs, too.
    – FreeMan
    Aug 11 '21 at 15:35
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Wrong rating for the wrong job

The "Class A" fire rating you see on a package of reflective-bubble insulation is an ASTM E84 surface burning rating, measured using a horizontal apparatus known as a Steiner Tunnel and used to judge the fire behavior of surface finishes (although there's a good argument that a vertical test setup would be more apropos for the task at hand). It simply means that it won't burn super-quickly of its own right; plenty of other things that won't do much to stop a fire, such as insulating foamboards, also have Class A ratings.

As a result of that, you'll need to use drywall there, as drywall actually can stop a fire from getting through it for a period of time. A 1/2" sheet will do the trick under most building codes, fortunately, and isn't too hard to install, since a fairly simple fire-tape-and-mud job will do for under a staircase.

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  • That’s one of the best explanation of differences between Class A fire rated “materials” and Class A fire rated “system”. +1 for you. However, the real difference has to do with the material contributing (or resisting) fire and smoke to an event, while a system has more to do with slowing fire so that an item (like a wall or stair) can continue its ability to maintain its form and use. Systems will require size and spacing of the supports, minimum fastening requirements of protection (like gypsum board), etc.
    – Lee Sam
    Aug 10 '21 at 2:38
  • Good answer, thanks. Do you think the reflective foam insulation would be better than nothing, pointless, or worse than nothing?
    – Kevin
    Aug 10 '21 at 17:58
  • @Kevin -- pretty much pointless at best with a good chance of being worse than nothing Aug 11 '21 at 4:14
  • @ThreePhaseEel Thanks so much for your insight! Now that I understand the underlying concepts, I realize that many websites poorly explain the difference between "Class A flame spread rating" and "fire retardant", or even incorrectly treat them as synonymous.
    – Kevin
    Aug 11 '21 at 21:24
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I’d go back to your Building Official and remind him that the underside of the stair is not required to be protected as a system or a material until the building has 3 or more units. It only requires 1/2” gypsum board and it does not have to be rated. (See ICC R311.2.2.)

Of course you need to feel comfortable with what could happen in a disaster, so you’ll need to decide that yourself.

Edit: oh, btw, adding 1/2” gypsum board is only required in “enclosed accessible space under stairs” and is “required under the stairs and on the side walls “.

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  • I haven't had a code inspector look at the stairs or I'd be forced to put in drywall immediately. Keep in mind local jurisdictions can generally enforce stricter building codes; e.g. state can be stricter than national, city can be stricter than state.
    – Kevin
    Aug 10 '21 at 19:29

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