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Warm roof insulation: Ratio of air-impermeable (polyiso) insulation above deck to air-permeable (cellulose) insulation below

2430cortland1 | Posted in Energy Efficiency and Durability on

i have decided to convert my vented cold flat roof to an unvented warm flat roof to alleviate current moisture issues. The current placement of the scuppers will limit the amount of polyiso right foam insulation that can be placed on the top of deck; figure r20. We live in Chicago (zone 5) so this is the bare minimum required. Below the deck are 14 inch joists that are filled with loose fill cellulose insulation; I estimate the r value below deck to be r42. I will make sure we add a vapor barrier above the wood deck (but below the polyiso) and will seal the perimeter of the joist with closed cell foam. I read a previous article (D Dorset posted 9/17/205) hat stated the r-value of the insulation above deck needs to be atleast 40% of the total R value. My question is would it be best for me to remove the loose fill insulation below deck as leaving it will not get me to the required 40% ratio?

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  1. Expert Member
    ARMANDO COBO | | #1

    The code doesn't use percentages, as its a bad practice to use made-up rules of thumb. 2015 IRC requires you to install R49 min. in your roof. R806.5 requires you have R20 min. above the roof decking (3" R6.7 Polyiso), and the easiest way to install insulation under the roof decking is 8" OC foam R29. To insulate full cavities with cellulose, you need to perform a dew point analysis to know if you need more rigid foam over the roof decking, which you said you may not have.
    If you use TPO and foam over the roof decking, you don't need to use vapor barrier. TPO is your vapor barrier.

  2. GBA Editor
    Martin Holladay | | #2

    Your instincts are correct: you don't want to have too much fluffy insulation under your roof sheathing, because excess fluffy insulation raises the risk of moisture accumulation in your roof sheathing.

    As noted in one of my articles (Combining Exterior Rigid Foam With Fluffy Insulation), the rigid foam layer for this type of a roof in Zone 5 needs to provide at least 41% of the total R-value of the roof, with no more than 59% of the remaining R-value coming from the fluffy insulation. Somehow, you need to reduce the R-value of the fluffy layer so it is close to R-29, not the R-42 you have now.

    Armando may be right that the easiest approach is to remove all of the existing loose-fill insulation and replace it with R-29 of open-cell spray foam.

  3. Expert Member
    Dana Dorsett | | #3

    The magic number is a minimum of 40% for zone 5.

    With the polyiso on the cold side of the assembly for dew point control purposes it's prudent to derate the polyiso to R5/inch for to be sure, independent of the labeled value, due to it's lower performance when the average temp through the foam is cold. (The labeled performance is tested at a mean temperature through the foam of 75F.) So it really takes 3" for an R50 nominal roof, it's 4".

    For higher performance retrofits in zone 5 it's the Mass Save Deep Energy Retrofit Builders Guide developed by the Building Science Corporation for the state of Massachusetts (which is entirely in zone 5) is a useful document to have on hand:

    For your purposes, study the Roof 1: Unvented Attic with Insulation Exterior to Roof Sheathing drawing on page 50 carefully. (Note the "At least 40% of the total R-value" prescriptive in the drawing.)

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