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Possible attic floor insulation with vermiculite/fiberglass, then Foamglas resting on joists, then cellulose?

Richard_in_Maryland | Posted in General Questions on

I’m in Maryland, climate 4A, mixed-humid. I am thinking of a retrofit attic insulation in my 1930’s 2-floor brick house. The attic floor currently has 2″ vermiculite, then covered under my direction a few years ago with R-19 + R-11 fiberglass batts. I did not know about air sealing at the time, but must assume that there is air leakage. After reading at GBA and BSC, I worry that air leakage will accumulate water on the underside of the attic decking. I’d like to fix that.

Because of the vermiculite, which I must assume has asbestos, I don’t want to disturb the attic floor to seal properly. I initially thought of converting to a non-vented attic, with insulation boards to rest on top of the roof deck (the roof needs replacing anyway), but I believe this would be very expensive because this is a complex roof. I also cannot have a ventilated cathedralized attic because the attic has no soffits, with instead gable-end windows.

But before considering insulation between the rafters in a non-ventilated roof (e.g. rigid foam attached to rafters with cellulose blown behind it), I’d like to ask your reaction to this idea to essentially transfer the attic deck insulation (e.g. R-15 above and remainder below the deck) to do the same on the attic floor. From inside to outside:
1) existing plaster ceiling
2) 6″ floor joist space filled with 2″ vermiculite then R-19 fiberglass batts
3) remove the batts that cover the joists, and instead lay down and seal a layer of Foamglas insulation, resting on top of the joists. This would make a walkable floor on top of the joists, as an air and vapor barrier (perm 0.0). Construct Foamglas boxes over the ceiling light and plumbing penetrations to allow for easier future access from above. (No HVAC in attic). At periphery, seal between Foamglas and top of brick walls.
4) R-40 cellulose on top of Foamglas
In sum: ~R60 attic floor with ~R-23 below air/vapor barrier and R-40 above.
Weaknesses or concerns?

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  1. GBA Editor
    Martin Holladay | | #1

    Do you really mean Foamglas? Foamglas is rarely used and expensive -- and I'm not sure if you can walk on it unless it has structural support.

    If you are worried about your air barrier, why not establish an air barrier with plywood? Assuming that you have access to the attic that allows you to bring the plywood up there, just fasten the plywood to the top of the joists and seal the seams with high-quality tape. Then you have an air barrier, and you can blow as much cellulose on top as you want.

  2. Richard_in_Maryland | | #2

    Thank you, Martin. I'm glad to hear one can create the air barrier on top of the joists. Thanks for making me recheck the foamglas. Foamglas can be placed as roof deck directly on the rafters, as the roof sheathing, but apparently needs at least 3" thickness - which makes it uneconomical. My reason for considering it was that it would give me an integrated vapor barrier as well (comparison 3/4" plywood plus Grace Ice&Shield on top is ~$2.50 sq.ft. in my research). I thought the extra vapor control in the winter, as in the REMOTE system, might give extra durability to the roof (though I know that once you've got an air barrier, only ~1% vapor diffuses through). But if I put 12" of cellulose on top (total R-60) this hygric storage buffer (as RRiversong always pointed out) might make the barrier unnecessary. I'm also not in a severe climate.

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