Insulating the underside of a concrete deck
I have a 2-story garage project where the lower level is built with ICF walls…R-25….and PEX radiant heated floors with an R-10 underslab insulation but the ceiling consists of the metal concrete pans and +/- 8″ of concrete and rebar. Assume that space above is unheated…..it might also get insulated and heated from time to time when the homeowner needs to do automotive work…..but in general it will be unheated.
The homeowner needs to maintain a 55 degree minimum heat in the lower level because it will be used for storing antique cars which do not play well in freezing temps. The dimensions of the space are 24′ x 34′ x 8′ high. Due to steel support beams the 34′ direction is broken into (2) 12′ sections and (1) 10′ section.
The question is how “best” to insulate the ceiling..both from an upfront cost and a long term energy performance cost? I had been thinking of attaching (3 rows) of PT 2×4’s flat to the steel pans using Tapcons and then installing regular 2×4’s…also in the flat….. in the perpendicular direction at 24″ OC to attach our drywall to. We would then run our electricals and piping as needed in this 2x space and then spray foam with closed cell to the bottom face of the regular 2×4’s. This would give us a minimum of 3″ of foam where we just have our 2x materials attached directly to the steel pan and as much as 8″ of foam where the steel pan goes up. But as I was reading various articles about spray foaming cathedral ceilings and roof decks, I became concerned about thermal bridging and water vapor getting into the 2x materials over time to cause rot and rusting of the screws and metal pan. We are trying to build “100 Year Buildings” so I am trying to be cautious without breaking the client’s budget. Maybe they are mutually exclusive?? Project is located outside of Philadelphia so we are a boarderline Zone 5. Martin had a table showing that IRC requires R-20 insulation to ensure water vapor does not condense on cold surfaces? I thought it was R-10??
Any comments are welcome! Thanks.
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