HVAC options for “foamed” finished attic
I am finishing a clients large, 6 year old home’s 3rd fl. The 80% efficient heating cooling system for the 2nd floor is up there.
We installed 9″ of .8 Lb. open cell foam to the underside of roof slope, covering rafters much of the way. The gable ends have 5″ of same.
Today, it is 85f outside. Their 2nd fl. tstat is set to 75f and the 2nd fl is 75f. With one flex duct removed in attic or 3rd floor the temp up there is 75f.
My HVAC guys are coming up with 3 tons for this 3rd fl. I think they are way off the mark and that their programs (manual j) are vastly underestimating the aggregate value (air seal and thermal) of the foam.
How can I achieve an accurate assessment of heat loss/gain on this space which measures approx. 20’x 55′ ?
What general reccomendations would you have to condition this space?
I am leaning toward a multi split with 2 ceiling cassettes. I will also provide make up air for the existing 2nd fl unit that is up there via barometric damper to exterior.
This is a suburb of Philadelphia.
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