Fiberglass a Non-Combustible Material for Underside of Wood Joists?

9/8/2021

I am building mid-rise wood framed multi-family buildings and am looking into some of the concealed space options available in NFPA 13.

8.15.1.2.17 permits the omission of sprinklers in the concealed space between the underside of wood joists and top of a limited combustible suspended ceiling as long as the joist space is filled with noncombustible insulation within 2 inches of subfloor above.

It is 8.15.1.2.17.1 that I am looking for insight on. It requires that the exposed bottom chord of the joists that would be exposed to the concealed space is faced with a noncombustible product installed as per manufacturers specifications.

Is there a product that has been recognized to satisfy this requirement?

I was thinking about overlaying the fiberglass insulation used to fill the joist cavities to cover the bottom of joist. While it would be full depth in the 2x6 joist, I would compress it and mechanically fasten it so that the joists would not be exposed to the concealed space below.

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7 Comments

Paul Pinigis

9/8/2021 08:17:08 am

What is your "limited combustible ceiling"?

Foil-faced fiberglass mat insulation seems appropriate here.

matt

9/8/2021 08:19:21 am

"limited combustible" and "non combustible" are fairly well defined by performance in various burn tests. Check to see what the required performance is in each test, and then check to see the rating of the product you want to use. Any company that goes to the expense of testing a product in order to certify it as "limited (or non) combustible" will generally not be shy about putting that information all over the packaging and marketing materials.

Jon Sullivan

9/8/2021 08:22:34 am

I'll let others weigh in on the specific application, but from a materials standpoint, you want to check out section 4.10 which defines non-combustible materials with respect to this standard.

Chad

9/8/2021 08:25:38 am

Usually Rockwool is what is used. Some will say fiberglass is limited combustible (without the facing), I’ve heard some argue against it.

As far as non-combustible mineral wool(Rockwool) is your only real choice for insulation

Jesse

9/8/2021 08:28:45 am

We run into this a lot. We tell the GC that in lieu of automatic sprinklers, non-combustible insulation must be used, and to refer to the insulation data sheet.

A couple pit falls though. 1) for batting, a lot of GCs like to use chicken wire or something similar to keep it in place. Most AHJs in our area no longer allow that as experience has shown it becomes detached. They have required other methods of sealing the insulation into the void space. 2) non-combustible spray foam insulation is used frequently, but often the depth of the concealed space is deeper than what the spray foam will fill, and they have to use batting to fill the rest which brings us back full circle to #1 above.

Good luck!

schulman

9/8/2021 09:28:30 am

dont compress insulation ... not designed to work that way
let it be .. use the right thickness or add double layers if need

Alex

9/8/2021 10:48:06 am

I agree with Schulman. Make sure that the insulation is installed per the manufactures specifications. Compressing may work depending on the insulation.

Fiberglass a Non-Combustible Material for Underside of Wood Joists?

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