Zip or Rigid Foam or Both (& HVAC)
Hello – we are building in Cincinnati (4800 sq ft). Our builder is a fiberglass batt person with no exposure to any of the techniques on the forum, but he’s willing to try new things.
The house will be a 2×6 construction with 10ft first and 9 ft second. We have 2 questions:
1) The thermal envelope – the basic current solution is to use the Zip system with 5.5″ of cellulose in the walls and more in the attic. The improvements I am considering are rigid foam on the exterior and spray foam in certain areas to increase the air tightness. Cincinnati is definitely a moderate but not really cold climate. Am I safe with the above values? I’m going to use an energy consultant to inspect the work and conduct blower tests. He’ll also write in specs for high quality workmanship.
2) One system with 2 zones or 2 systems? – With an air tight home, do you think it will be possible to run on just one 5-ton system with 2 zones for the two floors or is this just asking for trouble?
Thanks!
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Replies
Guy,
Q. "Am I safe with the above values?"
A. I'm not sure what you mean by "safe." If you decide to install exterior rigid foam, you need to be sure that the foam is thick enough to prevent moisture problems in your wall cavity. In your climate (you are either in climate zone 4 or 5), your rigid foam needs a minimum R-value of R-3.75 (if you are in zone 4) or R-7.5 (if you are in zone 5). For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "With an air tight home, do you think it will be possible to run on just one 5-ton system with 2 zones for the two floors or is this just asking for trouble?"
A. The best way to calculate your home's cooling load is to perform a Manual J calculation. For more information, see Calculating Cooling Loads and Saving Energy With Manual J and Manual D.