Insulating flat roof with 7" cavity with roof tear-off

Hot-dry climate. Roofer removing all sheathing. Insulating from above while cavity is open. Plan for 5.5" open cell foam to the drywall and also covering dropped soffit area that houses ducts. There will be a minimum air gap of 1.5" (7"-5.5") between the foam and the deck at normal ceiling heights, about 1 ft over dropped ceilings.

Any concerns with this installation? electrical, mechanicals. performance etc?

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whoa didn't see all the comments until after i posted. sounds like you are on top of things. The OCF is an effective air barrier and will stop most all convective sources of moisture, and I don't believe you need to worry about vapor diffusion in that climate... So leaving the air gap sounds good to me. I'm a cold climate guy so I'd like a second opinion as well. Where's David Butler when you need him?

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Brad Cook replied to David Butler's discussion How would you insulate this roof?
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Bill Spikowski replied to David Butler's discussion How would you insulate this roof?
"In an earlier response, I noted that we COULD haul drinking water and use the water we collect off…"
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Bill Spikowski replied to David Butler's discussion How would you insulate this roof?
"As to the elevations on the cupola – the existing windows up there might survive adding…"
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Bill Spikowski replied to David Butler's discussion How would you insulate this roof?
"The purlins are 1 3/4" thick. I believe the top chord of the trusses are 10" deep (but…"
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Bill Spikowski replied to David Butler's discussion How would you insulate this roof?
"The cupola windows are the only place there would be a problem if 4" of foam were added above…"
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Bill Spikowski replied to David Butler's discussion How would you insulate this roof?
"The purlins are 1 3/4" thick. I don’t know about the fire dangers of PIC or other foams.…"
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