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clarification on types of Thermax and fire rating

ben_riegel | Posted in General Questions on

I live in climate zone 6a (coastal Maine). I am wanting to install insulation in my crawlspace because it currently is uninsulated. I was able to find some second-hand Thermax insulation, which is my preferred choice because I would rather not have to put drywall over the insulation.

The second-hand Thermax in question is metal building board. I found this document from the state of Maine ( saying that only Thermax sheathing and white finish products have been approved for use without a thermal barrier. I had been under the impressing that all Thermax didn’t need a thermal barrier. Is that not the case? The product specification says that it can be exposed to the interior without a thermal barrier and is class A fire rated (

I contacted my town’s building department for clarification, and they didn’t answer my question, telling me that they don’t enforce the Maine Uniform Building and Energy Code, which includes the 2015 IRC. So it sounds like no one is going to check? I just want to make sure that I’m putting safe insulation in my house. Is Thermax metal building board as safe as the other “approved” Thermax products? 

Can anyone provide clarification on this?

Thanks for your help.

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  1. Expert Member
    Michael Maines | | #1

    There's no reason a thicker foil facing would have any effect on condensation, or thermal performance in any way. The thicker facing is tougher than the standard facing, which tears fairly easily. That's rarely a problem, but neither is there a problem with using the thicker facing on your foundation walls.

    However, for the state and municipalities to protect their liability, they can only accept products and assemblies that have been tested to meet requirements, and that testing is expensive, so if the Thermax you found is not listed as complying with ASTM C1289 Type 1, Class 2, and the code official is not willing to grant you an exception, then there's not much you can do.

    The Thermax product you linked says it meets those testing standards, so I don't see why you couldn't use it. Are you worrying about the words "sheathing" vs "building board?" Those are just marketing names and should have nothing to do with acceptance, though of course your code official has the final say.

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