How to Derive Ceiling Temp from Roof Temp and R-value
My Zone 2A ceiling has exposed rafters, so my only insulation is exterior polyiso. I’m trying to decide how thick to make it — and my only concern is performance (mainly thermal comfort, as I’m already close to net zero), *not* construction difficulty (I’m the builder).
GBA Super Advisor Dana posted this in reply to my general post: “Even at a relatively modest peak temp of 120F on the metal roof the average temp through the [6″] foam layer would be around 100F.”
Can anyone explain how to perform this calculation? E.g., what would the ceiling temp be if the foam layer were 7″ or 9″ etc… I’d truly appreciate this as it’s the last design decision I have after a year of planning 🙂 I think this’d be very helpful info for anyone building in hot climates, in which rooftop SHG is a huge part of cooling load. Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part