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Flat roof insulation detail

Eric Mailhot | Posted in General Questions on

Good evening all,

I want to double-check my plan for my new flat roof rebuild. The flat roof will be unvented.

The plan is to have 8” of EPS slope insulation on the outside, and the membrane will be a polyurea coating by Huntsman (Demilec), so I can use the roof as a terrace. 

I have 16” trusses on the interior that I plan to fill up with blown-in cellulose with Siga Majrex as a water vapour control membrane. Siga Majrex is an “intelligent” membrane, where water vapour trap in the roof and trusses to dry out in the interior but block the vapour from the inside to enter in.

I’m in zone 6, and my understanding is that I need roughly 50% exterior insulation to 50% inside insulation.

The current plan put me roughly to R30 exterior EPS foam and R60 interior if I fill out the 16” trusses with blown-in cellulose. 

So, I don’t respect the 5o/50 recommendation, more like 33% outside and 66% inside. 

The fact that I use thick outside insulation, an “intelligent” water vapour control membrane, and a non-vented flat roof means it can safely use the current plan?

Many thx in advance for your inputs and advice.


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  1. Expert Member
    Rick Evans | | #1

    Eric, with R-60 on the interior, very little heat is getting to the sheathing to warm it up. It will be cool and risky with no outward drying potential. If you really make your smart vapor retarder bomber air tight and protected with strapping, then you might be able to get away with it.

    I would try to net the cellulose about 8" deep to keep the ratios closer if it were me.

  2. Jason S. | | #2


    Skip the cellulose and Siga membrane, and make sure to include a well-detailed air barrier at the structural roof deck. It's not worth the risk to exceed the 50% interior/exterior insulation ratio for the incredibly marginal efficiency gain. If you really want more R value, add more on top or switch to polyisocyanurate or use batts underneath the deck that don't exceed the ratio.

  3. Tom Woodson | | #3

    You can also spray open cell foam under the roof deck. No risk of it falling down (batts tend to do this) and I believe it's an effective breathable air barrier. I did ~10 inches under and 4 inches polyiso over in zone 5a

  4. Eric Mailhot | | #4

    Ok many thx for your advice, I will reduce the amount of cellulose so I don't break the 50/50 rule.

  5. Steve Knapp CZ 3A Georgia | | #5


    You might want to call Insulation Depot or another provider for a quote. Reclaimed rigid foam is likely to be much less expensive than the new stuff

    1. Eric Mailhot | | #6

      Hi Steve, I wish I could but I am in Canada and we don't have Insulation Depot in my neck of the wood. Plus, the slope of my flat roof will be made from the EPS and so it needs to be custom made by the EPS provider. Which is cheaper and faster than using my carpenter to slope the roof ;).


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