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Way above code attic insulation

rhl_ | Posted in General Questions on

I have a Tudor with a funny crawl space.

To first approximation think a cathedral ceiling, where the ceiling of room below leaves a triangular (extruded) attic space,  the height of the attic area is approximately 4’.

im considering doing 2-3” of ccSPF on the roof rafters, air sealing the floor, and then filling the entire cavity with cellulose, since no one ever needs to access the area. 

I don’t want to leave an air gap because of typical moisture issues. 

this would certainly be way overkill on a thermal insulation basis, and clearly un-necessary, but my interest is in aggressively reducing air leakage.

If we can put aside the absurdity of 4’ of cellulose is there anything wrong with this? 

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  1. Expert Member
    BILL WICHERS | | #1

    You'll probably exceed the capacity of the attic floor (assuming it's drywall for the ceiling below) to support that much insulation. Drywall is rated as to how many pounds per square foot it can support. If you need more than drywall can handle, you'll have to build a floor in the attic to support the weight of the insulation.


    1. rhl_ | | #2

      Yeah, contractor didn’t think so, but, either wood or intello or both should do the trick.

      1. Expert Member
        BILL WICHERS | | #4

        I don't know if intello could handle that much insulation, either. A 4 foot thick layer is about R-151, which is way more than is commonly installed. A quick check of one manufacturer's data sheet says you'll be almost 6 pounds per square foot for that insulation layer too, and that it will settle almost 6 inches with time.

        I think this might not be the best idea to do... Keep in mind that there is a ratio of fluffy to foam that you need to meet for vapor control too, and the more fluffy you have, the more foam you need. With a four foot thick layer of fluffy, especially mositure-buffering fluffy like cellulose, things might be OK. I've never used such a thick layer of cellulose so I can't say for sure. I would definitely beef up the attic floor though.


        1. rhl_ | | #5


  2. Jon_R | | #3

    > 2-3” of ccSPF on the roof rafters
    Is this the right R value ratio for your climate?

    > I don’t want to leave an air gap
    Unless you dense pack, it will settle and leave a gap.

  3. the74impala | | #6

    You can run strapping over the intello to support the weight.

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