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New cape with simple roof. Type of cathedral ceiling insulation.

SSHabal | Posted in Energy Efficiency and Durability on

Hello everyone. I am building a small cape (22×32′) in zone 5 in NJ. 

I want to ask your advice about the most “practical” roofing assembly. After reviewing multiple types of articles about cathedral ceiling insulation and discussions, I have concluded that it has to be done very well to work. And I have my doubts local contractors posses attention to detail and skills to do that.

I build a simple 12/12 pitch roof. I have three top options so far. I would be very grateful for any feedback, criticism, or suggestions. I would rather be stupid hear and ask dumb questions than in 5 years replacing a roof. 

a) I’m thinking closed cell foam (R49, 7”) inside. Ice shield outside and standing seam roof. No ventilation barrier. Seems to be straightforward and common to contractors. 
b) same as a, but with 2x4s over the heating to allow for ventilation to prevent ice. Does it make sense? 
c) 4” of EPS over heating (R20), open cell foam (R29) inside. Drywall and spacle in interior to air seal bit allow for drying towards interior.


  1. mr_reference_Hugh | | #1

    Don’t cover the entire roof with ice shield, just the edges. There are many synthetic underlay materials that are peal and stick and others that are not.

    I would avoid open cell on the inside against the sheeting because it can hold lots of water vapour.

    Option A is a problem. I recall one article asking the question “what if you get a leak? The answer was you are in a tough position.

    My go to solution would be 1 of 2 options.
    1. No interior insulation
    Roof deck with screws not nails for wind resistant (see FEMA recommendations)
    Taping seams of the roof deck with high quality tape to protect with from water damage if layers above are pulled off by high winds.
    Ice and water shield - up 3feet from the edge of the exterior wall (and of course on the overhang)
    Synthetic underlay
    Polyiso (however think is required )
    Roof deck on top of polyiso
    Synthetic underlay
    Roof covering resistant to strong winds and hail

    2. From interior
    Polyiso under trusses (or rafters, or I-joists) that is taped air tight
    Cavity filled with dense pack cellulose
    Vapour open rigid roof deck like wood fibre insulation board taped at all seams
    Vapour open house wrap - higher quality
    2x4 standing on edge for air gap
    Zip sheeting or 5/8”CDX screwed and taped at seams to prevent water from getting in at seams if there is damage to roofing materials
    Synthetic underlay taped at seams
    Metal roof screwed down with longer screws to resist wind and thicker gauge to resist hail

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