Warm roof insulation: Ratio of air-impermeable (polyiso) insulation above deck to air-permeable (cellulose) insulation below
i have decided to convert my vented cold flat roof to an unvented warm flat roof to alleviate current moisture issues. The current placement of the scuppers will limit the amount of polyiso right foam insulation that can be placed on the top of deck; figure r20. We live in Chicago (zone 5) so this is the bare minimum required. Below the deck are 14 inch joists that are filled with loose fill cellulose insulation; I estimate the r value below deck to be r42. I will make sure we add a vapor barrier above the wood deck (but below the polyiso) and will seal the perimeter of the joist with closed cell foam. I read a previous article (D Dorset posted 9/17/205) hat stated the r-value of the insulation above deck needs to be atleast 40% of the total R value. My question is would it be best for me to remove the loose fill insulation below deck as leaving it will not get me to the required 40% ratio?
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