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Is it worth the cost to add polyiso on top of my EPS roof insulation stack?

user7370719 | Posted in Energy Efficiency and Durability on

My plan is to use 2 staggered 4″ layers of EPS.  Note that these are from ICF forms and are only 12″x 48″ boards – They do have interlocking tabs, but there will be a LOT of seams.  As I already have my screws to connect my OSB to the bottom layer, I only have an additional 1/2″ of space to add polyiso.  Is it worth the cost/effort to add a 1/2″ layer of polyiso on top of the EPS?  Foil faced?  Foil up or down?  Thanks for any advice

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  1. Expert Member
    BILL WICHERS | | #1

    I’m going to say no, it’s not worth adding the polyiso. 8” total of EPS is around R32+. 1/2” polyiso is a little less than R3, so you’re not adding much for your efforts — less than 10%. I’m assuming you will be using batts or something else inside to bring the overall assembly up to code minimum insulation levels for your climate zone?

    Make sure to stagger all the seams so that you avoid 3D air pathways. If you’re really worried about those seams, you could put a bead of foamboard adhesive in the gaps as an extra seal, but I don’t think that’s really needed.


  2. Expert Member
    Peter Yost | | #2

    Bb -

    sounds as though you are using salvaged EPS? I would definitely tape the seams of the joints. You need to make this topside insulation assembly airtight. EPS moves over time and I think that taped joints deal with this movement better over time than joints sealed with liquid sealant.


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