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Community and Q&A

Can I use polyiso over cellulose if I paint ceiling with low perm paint?

brittershaus | Posted in General Questions on

I thoroughly enjoyed Martin Holladay’s “Musings of an energy nerd” article regarding “Are Dew-Point Calculations Really Necessary?” from 9/17/2010.

There is a little confusion on my end in that the article seems to speak heavily on the wall portion of a house and not so much on the ceiling. Can I substitute ceiling for wall as I read the article?

Also, can you comment on my approach of insulating my attic?

Plan (from inside out):

Vapor Retardant paint ceiling with .4 Perm rating (Bin or Kills). The house is from 1962, so there may be enough oil based paint on the ceiling, but it will help me sleep at night.
Air sealing of all cracks and crevices, drill holes etc.
6” of cellulose between joists (newly sprayed in)
Two layers of 1.5” poly iso (unfaced). Seams staggered and taped.
One layer of Plywood
Christmas/holiday decorations on top

The house generally has a RH of 55%. On the high side – I know. I will utilize a dehumidifier if necessary.


Thank you!



  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I substitute 'ceiling' for 'wall' as I read the article?"

    A. Maybe -- if you are performing a dew point calculation. If you are interested in complying with the building code, you will discover that the minimum thicknesses of exterior rigid foam for roof assemblies (or ceiling assemblies) are considerably higher than those for walls. You can learn more by reading this article: Calculating the Minimum Thickness of Rigid Foam Sheathing. (Check out the section that begins, "Is there a similar chart for unvented cathedral ceilings?")

    You didn't tell us your climate zone, so it's hard to know whether your proposed assembly will work.

  2. brittershaus | | #2

    Andover, MA 01810

  3. GBA Editor
    Martin Holladay | | #3

    You are in Climate Zone 5. The building code requires the rigid foam layer in this type of ceiling/roof assembly to have a minimum R-value of R-20. Your proposed assembly has 3 inches of polyiso, so I think that your assembly will work. You'll end up with a ceiling assembly R-value of about R-41 or R-42, which is less than the minimum code requirement (R-49), but which isn't too bad for a retrofit job.

  4. Expert Member
    Dana Dorsett | | #4

    In Andover the last 1.5" of polyiso will only operate at about R4-R5 (instead of it's rated R9-10) during the winter, which is important from a dew-point control perspective due to polyiso's odd derating curve with temperature. Making the outer 1.5" of foam EPS would deliver about R7 for the EPS, and keep the polyiso next to the deck performing at about R9, for a stackup total of about R16.

    A true 6" (rather than a 5.5" deep 2x6 rafter or joist) of cellulose would deliver about R21-R22, or at 5.5" would run about R20.

    Total stackup hits about R36-R39 for a cavity value. The R20 exterior-R prescriptive is based on a presumed R49 stackup, which means as long as about 40% of the total R is exterior to the roof deck, the roof deck is going to be just fine in most homes. A ratio of R16 / R39 is 41%, and a ratio of R16 / R36 would be higher still, so as long as you split the foam thickness evenly between EPS and polyiso you'll be fine. If it's all polyiso it may come up a bit shy. due to the severe derating of polyiso, which under-performs EPS when the average temp through the layer is colder than 45F or so:

    If you can spare the extra inch, you'll get better overall performance out of 4" of EPS (in two 2" layers) than you would get out of the 2- foam solution.

    Assuming you're not constantly cooking pasta in an open pan or have 27 fish tanks, in winter you should have no problem getting the interior RH down to under 40% @ 70F if you are ventilating adequately. A tight house will run on the high-humidity side, but when outdoor temps are below 35F the ventilation air is drier than 30% RH/ 70F air even if it's foggy out.

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