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Exposed Intello Plus Membrane in Attic

jonny_h | Posted in Building Code Questions on

I know this is ultimately a matter for my local code official to answer, but I’d like to know if there’s any clear guidance on this before approaching them:

Is it acceptable for Intello Plus covering dense-packed cellulose to be left exposed in (a) a nearly-inaccessible attic space, and (b) a behind-kneewall space used for storage and mechanical equipment?

The specifications of Intello seem to be conflicting.  According to the spec sheet, surface burning per ASTM E48 is class A, 0 flame spread and 35 smoke developed index, which would meet the (Ohio) residential code requirements for both “wall and ceiling finishes” and “insulating materials” (though it’s not exactly either).  However, the same spec sheet indicates fire class E by a European standard, which google tells me is the second worst and is described as “high contribution to fire” — which makes it sound much worse.

So, which is it — perfectly fine to leave exposed, or gonna burn quickly and should be covered with drywall (which would be a real pain)?

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  1. the74impala | | #1

    Just for clarification, do you have the insulation up in the rafters vs ceiling joists, and intello is covering it on the underside of the rafters? Do you have a vent between the roof deck and the cellulose? Is this with trusses or stick built?

    1. jonny_h | | #2

      It's a stick-built 1950s 1.5 story house. We're doing a hybrid roof assembly -- exterior rigid foam on the roof (>50% of the total R value, which is enough for the zone 5 location), and the plan is to install intello on the underside of the roof rafters and dense-pack the rafter space with cellulose. Since it's 1.5 story, there's a behind-the-kneewall space, a section of effectively cathedral ceiling, and a very short "attic" space at the top. The behind-kneewall space will be used for storage, ductwork, and an HRV, and the attic space at the top will be used for ductwork.

  2. Expert Member
    BILL WICHERS | | #3

    Can you define "nearly inaccessible" attic space? There are some codes that are more loose in terms of allowing exposed stuff (like rigid foam) in "inaccessible" attics. The usual way that is defined is that you need to use a saw to cut a hole to get into the space. If there is a door or hatch, then it's an "accessible" space, regardless of how tight of a fit it may be to squeeze in there. The concern is usually if you can get in there, the space will be used for storage, and the code is worried about having exposed materials in spaces that may be used for storage.


    1. jonny_h | | #4

      The upper attic (above the 2nd floor ceiling / below the peak of the roof) has a maximum height of only about 2 feet -- I'd have to double check the measurement to be sure, but I think it's below the 30" height that requires an access by code (R807.1). It'll be full of ductwork and structural struts, and I don't expect anyone to ever go up there or use it for anything, but I am providing an access panel so I can stick my head up there and look around.

      The behind-kneewall space is definitely accessible and intended to be used.

      If I need to provide an ignition barrier over the Intello, it woudn't be too difficult to drywall the inside of the kneewall space -- but it'd be nearly impossible to do anything with the upper attic space.

  3. Expert Member


    If the material meets the flame spread and smoked developed levels for wall finishes, isn't that the end if the inquiry? It shouldn't matter whether the attic is accessible or not - or what the European standards are.

    1. jonny_h | | #6

      I suppose that's right -- I guess my confusion comes from the seeming disconnect between the classification based on the ASTM tests and the European classification -- one would have me believe it's totally safe, while the other would seem to indicate it's unsafe to leave exposed. You're right that all the local inspector should care about would be comparing code and the ASTM test results -- but I also don't want to knowingly create an unsafe condition.

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