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Community and Q&A

Choosing Between Two Roof Assemblies

mikeysp | Posted in General Questions on

Hi. I am in zone 4a (Nashville area).

I would appreciate a sanity check on my shop building/in-law suite building. (Pic below)


  1. If I go with the polyiso over the roof, how do I install it?
  2. Do seams need to be taped?
  3. What goes on bottom layer and what goes on top layer?
    1. My 2″ thick polyiso has a white skin on it. (Pic below)
    2. My 1.5″ polyiso has a black felt (Pic below)
  4. Is it foolish to only have 3.5″ of foam and I should go with vented attic and blow in cellulose?

I am weighing two roof assembly options:

  1. Blow-in cellulose over ceiling with vented attic
  2. 3.5″ Polyiso on top of OSB

I have the recycled polyiso on hand. The cellulose will be about $1400 (very affordable)

The polyiso route will give me a much more spacious feeling with 5 extra feet of space to peak. The trusses are 8ft OC (post frame).

The cellulose will give me a much higher R-Value

I will be making a pretty air tight assembly. Sealing with 3M 8067 and Dymonic 100.

I also have 2″ of recycled Fomular 250 25psi foam board under concrete.

Thank you.


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  1. user-2310254 | | #1

    Hi Mike,

    My reading on GBA suggests that rigid foam on the roof is normally installed as a sandwich (sheathing + foam + sheathing + roofing). If you decided to go this route, you could combine your 3.5 inches of polyiso with air permeable insulation applied inside against the sheathing. (See

    Going with a vented attic and cellulose would probably be a lot easier and cheaper. But you'd want to avoid putting the HVAC equipment or ducts in this space.

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