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Thermal Bridging
Justin Robert Milliet Rodrigues Taylor
Project Manager, CPHD, LEED GA
Asked a question last year

I should more than know this by now, but how do you properly account for the percentage reduction of studs in PHPP? I think that I have it right for wood studs, but for metal studs it's giving me an error message. My logic here is that the percentage calculation should be '(1.5 / 24)/100', if your wall assembly has 2x studs 24 inches on center. Is this correct? Or is there another reason why I'm getting this error message?

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You are getting the error message because you entered % for area 2 below 10%, which for steel studs it will be. It is in the "warning" column not the "error" column so the U value should still calculate. Remember the Lamda for steel is around 50 (metric). When I have entered steel studs I use a percentage of around 2-3%. It really does not matter as the high lamda makes the thermal bridging gigantic such that the insulation between the steel studs is reduces in effectiveness to about R1 to R1.5/ inch. It also does not fully account for the steel flanges on the steel studs. PHPP is not designed for use with steel studs as the thermal bridging is so bad. You could pay for a THERM model to confirm the value is you want. For wood the 1.5/24 is for perfect conditions, windows and doors often have double studs and you have the top and bottom plates made of wood as well. In my PHPP course we were taught that anything less then 10% is a lie, which is why PHPP gives you a warning if you enter a % less then 10.

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