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HVAC options for “foamed” finished attic

user-270695 | Posted in Mechanicals on

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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  1. user-270695 | | #1

    Update: at 3:45 pm, the temp outside is 89f, the conditioned 2nd is holding at 75f, the 3rd fl or attic is 73f. Again, only one 8"? flex duct was pulled off the 2nd fl. system in attic and that the only source of cooling.

  2. Expert Member
    Dana Dorsett | | #2

    Try this way-better-than-a-WAG online freebie cooling load tool, see what it comes up with.

    Use 90F for the 1% outside design temp (for Philly area it's between 88-90F, depending on exact location.)

    A really crude rule-of-thumb that works for most newer-better construction is a ton per 1000' of conditioned space. You have a about 1000' of space on that third floor, with a roof that's probably ~R40 whole-assembly (after thermal bridging of the rafters are factored in.)

    The solar gain through the roof and the roofing temperatures (not outdoor air temperature) are likely to dominate the cooling load, unless you have a bunch of west-facing unshaded window area. You could be over a ton if you big windows on a west facing gables, but probably not over two. You could also be under a ton.

    Three tons is probably just plain nuts, but still under an old-schoolers "ton per 500'" crude rule of thumb. If they were honest on their manual J it might not be 2x the actual load.

    The heating load for that space is probably well under the output of 1 ton, so if you have to compromise, undersize the mini-split on the cooling end a bit from the calculated number. Going to 2-heads would only be needed if it's partitioned and doored off, and even in that instance it may be more optimal to use a single mini-duct type head and split short ducts to get the sizing closer to the actual load(s).

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