Insulating under the staircase from the crawl space - faced fiberglass insulation ok?

Question

Our house is built on a slope, so the staircase to the bottom floor is right above the crawl space (vented; climate zone 3). The staircase is carpeted, and there's no insulation underneath. I can access the underside of the staircase from the crawl space. What is the proper way to insulate a staircase from below?

Can I just use three long batts of fiberglass insulation from top to bottom of the staircase, without cutting it to the size of each step? (i.e., insulation won't be flush against the steps) Or is mineral wool better?

What would be the best way to hold it up there, so they don't fall? I don't have to put a drywall under the staircase, do I?

Should I use faced or unfaced insulation? (I live in a very high fire risk zone, and I read the paper on faced insulation can burn)

Thank you so much.

Answer 1

If you use fiberglass bats, you need something to hold it in place. You could do something like that:

It's important to avoid air drafts between the stairs and the insulation, otherwise the insulation is useless, so the kraft side of the fiberglass should touch the tip of the stairs on the crawlspace side. I'm not sure this would work well.

I'd use polyurethane foam panels with built-in vapor barrier. They're a bit expensive, but you don't need much. They're easy to cut, and if your stairs are straight and the steps are all the same, you only need two sizes that you can pre-cut comfortably on a table outside.

You can use PU foam to glue the panels to the underside of the steps, and one screw per panel to hold them in place while it sets. Then you can seal the joints with a bit more foam.

...or maybe the simpler solution: