Does plywood over rigid foam meet the fire barrier requirements?

Question

I've seen a lot of discussion about using plywood instead of drywall over rigid foam insulation when finishing basement walls (here and here). As I look to insulate my basement, I have several reasons why I'd prefer plywood to drywall as a covering.

However, none of the discussions about plywood over rigid foam mention fire code concerns. There are separate discussions (here and here), but so far no direct answer to this question:

Does plywood over rigid foam meet the fire barrier requirements? Any specific type, thickness, or installation method?

I am in Wisconsin, USA, where our fire code is NFPA 1, Fire Code — 2012. I've searched the modifications and didn't see anything about fire block or barrier, so assume standard NFPA applies.

I am considering 2" foam glued to the cement, furring strips over that, and 1/2" plywood over that. Basement will not be finished, I'm just insulating for warmth. I want to cover the foam since it is used as a workshop.

Answer 1

Per Wisconsin Uniform Dwelling Code SPS 321.11, you should be good with 1/2” or greater wood structural panels (including structural plywood, note emphasis in quoted section below is mine):

SPS 321.11 Foam plastic.

(1) (b) Thermal barrier. Except as provided in par. (c), foam plastic insulation shall be separated from the interior of the dwelling by one of the following thermal barriers:

1. ½ -inch gypsum wallboard.

2. ½ -inch nominal wood structural panel.

3. ¾ -inch sawn lumber with tongue-and-groove or lap joints.

4. 1-inch of masonry or concrete.

5. A product or material shown by an independent laboratory to limit the temperature rise on the unexposed surface to 250°F for 15 minutes when tested in accordance with ASTM E-119.

6. For doors only, sheet metal with a minimum thickness of 26 standard steel gauge or aluminum with a minimum thickness of 0.032 inch.

For anyone else looking for the latest from the International Residential Code, it appears to be more restrictive to using 23/32” structural plywood (in addition to other materials). Per section 316.4 of the 2018 International Residential Code: (paraphrased) Approved thermal barriers are required for plastic foam insulation with exception of several situations which do not apply to the situation in the question based on the information given. Approved thermal barriers include 1/2 in gypsum board, 23/32 in structural wood panels (this includes structural plywood since some non-structural grades of plywood may be allowed to have more knots/voids in the glued sheets), and other materials that are tested to and meet the acceptance criteria of the Temperature Transmission Fire Test and Integrity Fire Test of NFPA-275.

Answer 2

There are 3 considerations: 1) Fire Walls, 2) Flame Spread Index, and 3) Smoke Development Index.

1) Fire walls prohibit (or slow) the passage of fire from one room to another. Fire walls are used to contain the fire to allow a.) occupants to escape, and b.) contain the fire to protect the structure. In residential construction the only fire walls are between a garage and living spaces. That is not your issue.

2) Flame spread is based on a materials ability to burn and spread through a building. All building materials have a Flame Spread Rating and all buildings are rated for the type of Flame Spread allowed within the building.

Residential buildings allow a “Class C” Flame Spread Rating. (See ICC Code Section 803 and Table 803.5.) A “Class C” Flame Spread Rating is between 76-200. (Just to give you an idea of ratings, oak has a rating of 100.)

So, if your rigid insulation has a rating of more than 200, it needs to be “protected”. Protected means covered. Drywall, plywood, etc. qualifies as an allowed protection.

3) Smoke Development Index is similar to Flame Spread Rating and requires a “Class C” Smoke Development Index too, which is between 0-450.

Again, if your rigid insulation is rated above 450, then it needs to be protected AND drywall and plywood qualifies as protection.

Summary:

Yes, plywood is allowed to protect the rigid insulation. Any thickness is allowed, but plywood is rated for various spans “for best performance.” Look on the plywood sheets and they have a grade stamp, like 24/16. The 24 means a maximum span of 24” on the roof and the 16 represents a maximum span of 16” on the floor. (They don’t rate plywood for wall coverings.) I’d use 3/8” on walls up to 24” on center spans in your case, because appearance is not important.

Btw, span is based on the exterior layer installed perpendicular to supports.