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Want to install minisplit — need advice

jgraeff | Posted in Mechanicals on

Hi everyone!

Our master suite has only 1 small ac vent, none in the closet or bathroom, this makes it very uncomfortable in the summer in Florida. We have a large slider window and a really small double hung window in the bathroom. This makes windows units not a good choice as I’ve tried to install many without success.

We have a flat roof over this part of the house so we can’t run more ac to the other rooms.

3 walls are outside walls, one wall is concrete the others are basic construction, minimal insulation and no insulation on the roof. Both windows are double pane.

It’s roughly 20×20. I was thinking 9000btu would be more than enough but that’s just a guess.

Honestly I’d prefer to leave the bedroom door open and let it run more to cool the living room as well as I spend more of the time there anyways.

Any advice is welcome.

I’m looking at daikin and Fujitsu for units by the way.

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  1. GBA Editor
    Martin Holladay | | #1

    This type of job should always start with a Manual J load calculation. For more information, see Saving Energy With Manual J.

    You might want to investigate whether it's feasible to insulate your ceiling.

    Most HVAC contractors should be willing to give you advice. If you can start the process by performing a preliminary load calculation -- perhaps using a simple online tool -- you'll be better equipped to talk with your HVAC contractor.

  2. Expert Member
    Dana Dorsett | | #2

    A flat uninsulated roof in FL is a ridiculous unglazed solar collector over your head that will radiate heat at you for hours after sunset. Comfort is about the average radiant temperature in the room, not air temperature- lowering the flux of heat radiating from the ceiling is more important than cooling the air.

    Blowing the rafter bays full of fiberglass or cellulose and re-roofing that 20 x 20 ' section with an inch or three of cheap reclaimed roofing polyiso under new high solar reflective index (SRI) "cool roof" roofing a may be 2x as expensive as a cheap cooling only 9000 BTU/hr mini-split, but it'll deliver a lot more comfort, and may even bring the load to within whatever your currently under-sized duct is delivering.

    How deep are the rafters?

    Where in FL are you- US climate zone 1, or climate zone 2?

    The reason for asking is...

    In zone 2 IRC 2015 code min calls out R38. If the rafters are as skinny as 2x6 you'd be able to blow ~R20-R21 in there, and to meet code on an R-value basis would require 3" of reclaimed polyiso. But zone 1 only requires R30, so even 1.5" of polyiso above the roof deck gets you there.

    If 2x8s the fluff would be R28, so in zone 2 you'd still need at least 2" of foam, but in zone 1 an inch will do.

    If 2x10s or deeper the fluff would be R33-R34. An inch of polyiso would get you to the necessary R value in zone 2. The R33+ would meet code-R in zone 1, but it would also still be need at least R5 above the roof deck for dew point control, per chapter 8 of the IRC, so you'd need an inch of polyiso (or 1.5" of EPS) even with 2x12s.

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