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Foam insulation on top of Bitumen low-slope roof?

Andrew808 | Posted in General Questions on


QUESTION: Pros/Cons of spraying foam on top of roof for insulation, then coating with Acrylic or Silicone

Location: Hawaii (warm all year, very high UV, no heating)

Home: Single story 1960’s shallow pitch (2:12) with very low slope enclosed patio (but water does run off — not completely flat – no ponding). No air conditioning (yet).

Roof: ¾” Plywood + 2-ply Bitumen

PROBLEM: Home is HOT due to no insulation over high open beam ceilings

Assumption: We are not going to insulate the inside of the home. 

  • ½ of home is shallow drop ceiling and does have R19 fiberglass on top of ceiling (in between ceiling joists). 

  • ½ of home is all open beam. Looking up, we are looking at the roof.


Possible consideration for spraying only over the open beam section to reduce cost?

Concern — unsmooth finish. Sounds like the thicker, the higher risk. 2” sounds reasonable… maybe 3”, but certainly not thicker.

Quotes we should have soon:

  • We are getting a quote – 1.5” SWD Spray foam + SWD or APOC silicone. — Cost ~ $12/ft2  (Person spraying has been supposedly doing this for 15 + years)

  • I have another person that utilizes sprayers from the mainland and prefers acryclic coatings – ~ $9/ft2

    Thank you!

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  1. HertzR | | #1

    If you are interested in getting a new roof, which it sounds like you are, you’d be better off in my view installing two layers of 2” polyiso over your current sheathing (4” total). Then you could install new sheeting + ice/water shield over the polyiso and build up a new bitumen roof as planned.

    You’re in climate zone 1, which I believe calls for R20 continuous insulation above your roof sheathing - 4” of polyiso in CZ1 easily gives you that over the long term.

    And the thicker the polyiso layer on top of your roof the better.

    Spray foaming over your existing sheathing and then attaching a new roof to the spray foam seems complicated to me. I’m guessing there will be some number of wood framing supports to provide structure for the foam, which would create thermal bridges not found using the polyiso method. Plus with polyiso you don’t have to worry about keeping the spray foam flat and level.

  2. Andrew808 | | #2

    Thanks Richard.
    Sort of — I have lots of regrets with our current roof. However, multiple roofers and architects have stated “you have a good roof.” They estimate 30 years or so as is. However, we have to solve the heat.
    I like your idea a lot, and it seems near perfect. But... the cost seems very high.
    $30k gets me spray foam+silicone — I’m told this would be a near forever solution.
    At some point in the wind lift is an issue with any Build up.

    If you build up with polyiso...
    Do you need have air gaps?
    Or, just bitumen+poly+cap sheet+Bitumen (or torch down/similar) continuous?

    Thank you for your comments,

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