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Sizing/placing ductless mini-splits for whole house with cathedral room

alexinnc | Posted in General Questions on

Hi there,

Another Manual J question here that I can’t quite find similar situations on in the forums.

We’re wondering where to place two ductless mini-splits in a two-floor farmhouse addition (of a second floor)  that now will have one big double-height cathedral room. Any best practices based on this floorplan?

We are heating-dominated and want to place these with that in mind. House stays relatively cool here with two mountains on either side, and a creek running past, despite full sun from 11-4:30 in the summer.

This house used to be only one level. At that time, we ran a 15K BTU Fujitsu unit and a woodstove and were fine. Fujitsu struggled below freezing to do the whole house, but was perfect for bringing 40˚F up to 68˚F without the woodstove. All the new walls, including in the great room, will be double 2×4 stud walls with 1/4″ air gap, rockwool insulation. 18″ of rockwool to go in attic.

In the roof plan you can see that the “false dormer” part of the house will have 18″ of blown rockwool attic insulation, and the steep cathedral portion of the roof will be 18″ of spray foam.

L1 floor in the crawlspace will be flash and batted with 6″ rockwool batts. Thinking a vented crawl that’s not part of the thermal envelope.

We have not determined if a solar attic fan would be helpful or not.

Thanks for insights or best practices! Wondering if this is a paid job for a Manual J contractor. In which case, I’m open to suggestions.


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