Reflective bubble insulation for making stairs fire-resistant?

Question

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:

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.

Answer 1

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.

Answer 2

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 “.