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Conditioned Attic Assembly

Charles_B | Posted in General Questions on

New Construction build in Texas, Zone 3A.
low slope 2/12pitch Shed roof;
Galvalum Standing Seam Metal Roofing;
100% Ice&water Shield;
5/8 Plywood;
14″ BCI FJ1 24″oc;

Plan was to add 1-2″ of closed cell spray foam to underside of plywood decking & R30 Mineral wool batts between the 14″joists. (each attic space has 1 small hvac vent to help combat any potential humidity)  These attic spaces are very small due to shed roof & most the house is actually just a Cathedral Cieling assy. (note* i bought a moisture meter to check sheathing before it gets sprayed)

Is it okay to install closed cell spray foam to the underside of roof decking for a conditioned attic? is there any concern with the 2 air barriers on either side of roof decking? I had a local spray foam guy mention he would not do “closed cell” foam against the roof decking without me signing a waiver. He recommended installing “open cell” instead & said it was so the roof decking could breathe to the inside……(due to having full ice/water barrier 2/12pitch & standing seam metal roof which of none will breathe”

Please Help! is my original plan okay/safe? whats the best plan of action with the current situation (all done except insulation on inside/roof decking)

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Replies

  1. Expert Member
    ARMANDO COBO | | #1

    Installing 2” of ccSPF under a roof decking is accepted by code, but I would use a permeable WRB on top of the roof decking. I normally install the metal roofing on top of 1x4 purlins to allow for ventilation and drying potential.
    Having said that, I prefer to use a WRB (I&S is ok now), then taped 1.5” R10 polyiso rigid foam on top of the roof decking, then 1x4 purlins and finally the metal roof. The reason for the 1.5 R10 rigid foam is to match the height of the 2x4 nailer at the roof perimeter edge.
    The 1.5" R10 iso on top of the decking also allows to fill the 14" cavity with cellulose to R49, total R59, or reduce the I-joist to 9.5" and R33 for R43 total, etc., etc. This is all easier to install and probably costing less.

    1. Charles_B | | #2

      Thank you for the response,
      unfortunatly we've already have the standing seam metal roof installed and taking it off to then add 1.5" exterior rigid foam is not an ideal option at this point. all framing is done & installed with Ice/water barrier full shed roof (2/12pitch)

      attached a interior pic i had of a section of Cathedral cieling/furdown over kitchen area. standing seam metal roof is already installed w/ ice&water barrier underneath. standard galvalume. not much of a attic anywhere in house, mostly cathedral ceiling. 9' on the low end, approx 14' on the high end under shed roof.

      so 2" closed cell is code acceptable for the conditioned attic with the unvented galvalum standing seam roof ontop of full ice&water shield? is there any merit to the installer that told me its better to use open cell? everything i've read & studied about rotting roofs usually had issues & was typically open cell that let the humidity through to condense on the underside of sheathing.

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