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How to insulate roof/deck over living space

trevorsoll | Posted in General Questions on

Ill be as brief as possible.

I have a new build that has a flatroof deck over a dining room.  22’ i josts are 12”oc     Space is 14’ wide. I joists are 11 7/8”.  
Building in zone 3 and avoiding spray foam. Deck top will be covered with duradek. What are the best options available to insulate?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Trevor.

    You have lots of options that could work. I suggest you read this article, which should point you in the right direction: Insulating Low-Slope Residential Roofs. And then post any more specific questions you have.

  2. trevorsoll | | #2

    Thanks for that. The information is great, but unfortunately doesn't provide the answer I need specific to my build. I believe the best option is to layer rigid foam in the joist space to achieve my needs. A few questions:
    a: will this layering of rigid work? (we'll also need vapor barrier under due to code in our area)
    b: I have to build a sloped 'overdeck'. 2x6s cut down over their length (11ft) with 3/4" g1s ply on top - to give a 2" grade. I don't have any extra space to be able to do insulation on top of the deck and then the sloped roof.
    With this deck having the waterproof membrane, I'm wondering if there's any issue on the 'dead space' from the insulation under to the waterproof layer. is this clear as mud?

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