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Hybrid Roof Ratio – Zip Roof Sheathing – Climate Zone 6

17Whippoorwill | Posted in Green Building Techniques on

Based on this recent article and chart attached: https://www.greenbuildingadvisor.com/article/the-ratio-rule-for-hybrid-roof-insulation
In climate zone 6, if I only use 1/2″ Zip roof sheathing, and roof code is R49, then I should not use any insulated roof deck sheathing at all, because I would not meet the hybrid-ratio minimum R25 suggested for above roof deck and R24 below roof deck.

I thought any amount of exterior insulation would be good, but it seems not.  Am I missing something?

Thanks

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Replies

  1. Patrick OSullivan | | #1

    > I thought any amount of exterior insulation would be good, but it seems not. Am I missing something?

    You’re missing the why. “Any amount” (meaning not enough) of exterior insulation doesn’t keep the sheathing above the dew point. This means that the sheathing can accumulate moisture through sorption. Because the roof is unvented, you then risk that moisture lingering in the sheathing for too long and facilitating rot.

  2. Jon R | | #2

    You have to meet (or exceed) the requirement. Not "if you don't meet it, use none".

    You could move to a vented under sheathing design.

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