Conditioned Attic – Fireproof Material after Insulation
Climate zone 6: In a conditioned attic (only used for storage) which now has 4″ of closed cell spray foam covered with intumescent paint (all under the roof deck), if I put additional insulation, for example mineral wool batts, or fiberglass batts, do I have to cover those batts with yet another layer of fireproof material or coating? However, I think having any type of batt (vs. sheetrock) as the finish layer can introduce dust or fiberglass into the air.
Sheetrock is labor intensive, is there a sheet good or staple-able material I can apply over the batts?
Goal is to bring the attic section of the house to the same R49 or so of the cathedral ceiling living areas.
Thanks
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Replies
Mineral wool is considered a thermal barrier under most codes, so you can install the batts as they tend to say in place just from friction.
For keeping dust at bay drywall is the cheapest, it doesn't have to be fully finished as rough taping is good enough. One of the variable perm membranes or permeable radiant barrier (the ones that are perforated) can also work. The important item here is the surface needs to be somewhat permeable, at least a class II or higher vapor retarder.
Mineral wool alone is fine for fireproofing purposes. You could use plywood over the batts for support, and to seal them in place, but drywall is probably cheaper. As Akos mentioned, you only need to mud and tape the drywall to air seal it, and to make it act as a fire barrier, but you don't need to sand or fully finish it. My recommendation would be to at least rough sand it and then prime it though. If you don't prime drywall, it tends to yellow and look crummy over time. Priming will also make it easier to do any future finishing work if you choose to down the road.
Bill
You aren't required to have any covering over batt insulation in an attic. If you're worried about dust and fiberglass particles, you could staple a permeable house wrap over it. You may even find a fabric that is cheaper, like flat sheets.
We have rockwool over the foam in our rafter bays (but drywall on the eaves walls). We use our attic for storage. We have not found any issue with dust over the couple years since we installed. We did have to fur out our rafters to get the batts to fit. Picture is here: https://www.greenbuildingadvisor.com/article/a-vexing-question-about-heat