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I have a walk-in attic off my bedroom that goes above the garage. The attic wasn't venting at all so I recently installed a solar fan to help reduce the heat in the summer. Since it wasn't venting properly before the fan install all my fiberglass insulation on the backside of the interior wall needed to be replaced. The attic itself is not insulated - only the walls on the bedroom/bathroom. I pulled all of the fiberglass insulation from between the studs and I was going to just replace it with newer fiberglass insulation. But I'm thinking about just going with foam board insulation since it's more resistant to mold growth (?) and not just replacing the fiberglass insulation.

My plan was to just stack a few pieces of 2" foam board over the existing foam board between the studs and use spray foam to seal the gaps around it.

Can I just use foam boards vs fiberglass?

There is existing foam board on the backside of the interior drywall. Can I just stack a few layers of 2" foam board on top of the existing reflective boards or do I need to create space between?

If I need space between them - Can I leave it empty or do I need to fill it with fiberglass or something else?

Or Do I just go with fiberglass insulation and place it back in between the studs and call it a day?

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Thank you for any help.

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  • Are you OK with putting drywall over the insulation, or are you wanting to leave the insulation exposed? Apr 18, 2023 at 2:08
  • Thanks for the reply. I'd rather just leave the insulation exposed since this is an attic. Apr 23, 2023 at 20:27

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You'd need to drywall over it

The issue with leaving bare foam boards between your studs is that the foam will smolder and eventually start burning outright if an ignition source is present for a significant length of time, producing a nasty, smoky mess in your attic. As a result, the building codes require a layer of ½" drywall or an approved equal as a thermal barrier over foam insulation. For spray foam jobs, some places approve of using an intumescent coating that "puffs up" when heated strongly, thus providing an insulating layer that keeps the foam underneath from burning further for a length of time, but I doubt that will work with foamboards.

If you want a mold proof fiberglass replacement: mineral (rock) wool

Instead, I'd use a modern mineral wool/rockwool batt as the insulation here. They're fireproof, highly mold-resistant, and bug-proof, and also much less prone to sagging than fiberglass batts are, making them easier to handle and install.

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  • Thank you for the response. The mineral wool/rockwool batt looks like a great option! So if I use that I would not have to cover it with drywall correct? So I could use a single layer of foam insulation to fill in the gaps between the existing foam boards that are against the interior drywall then layer the wool/rockwoll over the top of that since it is fireproof? Apr 24, 2023 at 22:57
  • @user1150977 -- you'd have to run the rockwool-over-foam thing by your AHJ -- also, depending on the stud depth, you may not have enough room in the cavity for both the batt and the foamboard without causing the batt to stick out a bunch and not have nearly as good a friction hold on the studs as a result Apr 25, 2023 at 3:49

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