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Community and Q&A

Desperate help, minisplit sizing

aaronmiiller | Posted in Mechanicals on

I’m a GC building my own home. I live in an extremely rural area with very few knowledgeable hvac professionals within 300 miles. I have decided to go with mitsubishi ductless mini splits to heat and cool the entire house. Basically I need help choosing what size units to purchase, how many heads, and to me most importantly where to place the units. This home will be occupied by just me and the wife so interior doors will be open most of the time. Walls will be a double 2×4 wall system spaced 1″ and filled with wet cellulose, along with a 1/2″ layer of polyiso on the exterior of the walls bringing the r value to around R31-R32. The ceilings are 9ft tall and will be an R60. Any help is greatly appreciated. Below is a rough layout of the home.

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  1. aaronmiiller | | #1

    Almost forgot to mention I'm in zone 3, hot and humid.

  2. GBA Editor
    Martin Holladay | | #2

    First, if you are building a double-stud wall, the wall should not include any exterior rigid foam. Here is a link to an article with more information on this issue: Exterior Rigid Foam on Double-Stud Walls Is a No-No.

    Second, you need to perform a room-by-room heat loss and cooling load calculation. That's the first step when you are designing a heating or cooling system. More information here:

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Calculating Cooling Loads

    If you don't want to perform these calculations, you can hire an energy rater or a mechanical engineer to perform them for you.

    Third, you should read this article: How To Buy a Ductless Minisplit.

  3. GBA Editor
    Martin Holladay | | #3

    You may also be interested in reading this article: Rules of Thumb for Ductless Minisplits.

  4. aaronmiiller | | #4

    Thank-you for the response, I appreciate the recommendation on the rigid foam. Other than that none of my questions were answered, anyone else?

  5. Kenny78 | | #5

    What Martin is politely saying is that you haven't provided enough information.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Basically I need help choosing what size units to purchase."

    A. We can't do that until you complete a heating and cooling load calculation.

    Q. "I need help choosing how many heads."

    A. We can't do that until you complete a heating and cooling load calculation.

    Q. "I need to know where to place the units."

    A. It would be helpful to know the results of your heating and cooling load calculation, and to know something about your window specifications, to answer this question.

  7. STEPHEN SHEEHY | | #7

    Aaron: To size the units, you need cooling and heating load calculations, as Martin mentions. It would help to label the rooms. Is the room on the upper right a laundry/ mechanical room or a bedroom? If the former, you may not need cooling or heating, but if it's a bedroom, you might.

    Are you still in the design stage or have you started building?

  8. aaronmiiller | | #8

    I'm about to start building. Yes I should've labeled the rooms, the room in the upper right corner is the master closet, which is also where my wife has located the washer and dryer. You guys need to understand, I'm a GC with little knowledge of hvac and loads. The closest qualified person to perform a load test is roughly 4 hours away. Windows are double pain low e Argon filled, they work well in our southern humid climate, the low e is especially dark, very little solar gain.

  9. aaronmiiller | | #9

    There are 2 companies actively installing Mitsubishi mini splits within 1 hour of the job site. Being a GC I have dealt with both companies and let me tell you, I wouldn't wish their sloppy work on my enemies. Both companies couldn't even install the register covers of a ducted system in the ceiling straight, is that a company you would have work on your house?

  10. user-2310254 | | #10

    You don't need a local person to have your load calculations completed. I worked with David Butler at Optimal Building Systems ([email protected]). Give him a call. He can also help you to specify your system.

  11. GBA Editor
    Martin Holladay | | #11

    There are lots of issues here.

    In general, it's usually best to finish the design of your building (including the design of your HVAC system) before construction begins. Since you're a GC, you know that. So don't rush into things.

    Second, if you don't trust the HVAC contractors in your area, you have a problem. Here at GBA, we can't advise you to go forward unless you can find an HVAC contractor you trust.

    Third, if there are no competent minisplit contractors in your area, you may not want to install a minisplit. A conventional forced-air system might make sense, if that's the type of HVAC system that can be easily installed and maintained in your area.

    Fourth, a heating and cooling load calculation really is a good idea. It's also required by all building codes.

  12. aaronmiiller | | #12

    Thank-you Steve, I will certainly give him a call. I have weighed the decision of going ductless or forced air for 2 years, my decision is made. In the climate we live in it only makes sense.

  13. GBA Editor
    Martin Holladay | | #13

    The plan shows that the master bedroom doesn't have a window. Since that would be a code violation, I'm guessing that this house will have more windows than shown on the plan.

  14. user-2310254 | | #14

    Aaron. I was very wedded to the idea of using split minis on my new house until David pointed out that they really were not a good fit for the floor plan. A conventional heat pump ended up being more efficient, affordable, and practical.

    As Martin suggested, I think you need to hit the pause button and make sure you have most of the design details nailed down before proceeding. As a GC, you know how projects tend to become very expensive once you start problem-solving in the field.

  15. aaronmiiller | | #15

    Martin we don't have code where I live. Not having a window in that room being a code violation is ignorant regardless, closet, which is not separate from the master bedroom has a window. Sometimes you code junkies just need to chill out. This isn't Holmes on homes. I've never been on a forum like this one. All I asked was a couple simple questions and I'm being borderline flamed. I had no idea the installation of ductless splits were such a big secret. Steve I appreciate your input. Would you rather have a ductless system in my poorly executed house plan, or a forced air unit with all the ductwork installed outside the building envelope consisting of flex duct only, no rigid? The reason I ask is because for as long as I've been working I have yet to see anything other than that installed in my area. I'm no hvac guy but I know thats not correct.

  16. GBA Editor
    Martin Holladay | | #16

    I brought up the window because it affects heat loss calculations.

    I'm not a code junkie, but I know that having an operable window in a bedroom, big enough for emergency egress, may save your life (or the life of a loved one) during a fire. If you ever decide to sell your house, you may find it difficult to sell if the master bedroom doesn't have a window.

    But of course the decision is yours.

  17. GBA Editor
    Martin Holladay | | #17

    Q. "I had no idea the installation of ductless splits were such a big secret."

    A. It's not a big secret. GBA has been providing lots of information on this topic for years, and I've been trying to share the information with you. Again, these two articles (see links below) are good places to start -- but there's a lot of information on GBA on this topic. If you type "ductless minisplit" into our search box, you'll see what I mean.

    How To Buy a Ductless Minisplit

    Rules of Thumb for Ductless Minisplits

  18. aaronmiiller | | #18

    I knew what you were getting at when you mentioned the window, and my point was, I couldn't step 10 feet West and climb out the window should there be a fire? You see common sense plays a part in code violations such as these. You do make a valid point about selling the house though. That being said, home inspections in our area are rare and even if there would be a home inspection, the guys licensed in our area to perform home inspections don't even know code, because once again it's not enforced. That's not to say I wouldn't appreciate some enforcement. Can you imagine in the 10 years I've been a GC and the 40 years my partner has been a GC we have not ONCE been inspected. And that goes for general construction, plumbing, and electrical. We're living in the past big time, our only building center for miles just started handling house wrap about 4 years ago if that gives you an idea how behind we are.

  19. aaronmiiller | | #19

    I've read both those articles and dozens of others, I still haven't found the answers to my original questions.

  20. STEPHEN SHEEHY | | #20

    Aaron- There are lots of folks here who are more than willing to offer good advice. You asked a few simple questions, but didn't provide enough information for anyone to give a useful answer.
    There's no code where I live either, but that doesn't mean a bedroom with no windows is a good idea.

  21. user-2310254 | | #21


    I think you can build an affordable, efficient, and healthy house by following a few best practices. It is fairly easy to bring your HVAC system inside the home's conditioned space. With one exception, none of my subs were really into "green" building. Nonetheless, they were able to help me complete a fairly tight, efficient, and healthy house. If you have similar goals, you might want to read this article ( Also be sure to read the articles under "Related Links."

    The folks here tend to recommend data-driven methods to achieve specific goals. Martin isn't trying to insult you. He is simply saying that you need to provide a number of data points for this board to offer good guidance.

    So far you have learned that a double 2x4 wall assembly, while a good idea, should not include a layer of exterior foam and that a HVAC system's design (mini split or conventional) should be based on an accurate Manual J calculation (which can be developed by David Butler or another HVAC designer for a reasonable fee).

    If you stay here and continue to learn, you will end up with a much better home than would be likely otherwise possible.

    PS. I agree with Martin on the master suite window. If you like a really dark room for sleeping, you can always buy blackout drapes.

  22. aaronmiiller | | #22

    OK so give me a valid reason to put a window in there and I will. I like sleeping in the dark man, Windows suck. I guess I'll have to just wing it on the ductless. I'm thinking a 12k in the living room and a 9k in my bedroom.

  23. aaronmiiller | | #23

    As for the foam on the exterior wall, while I am reconsidering, in that article it does say in hot humid climates such as zone 3 (where I am) it is acceptable practice to install foam on the outside. So really the only thing I've learned there is something that applies to the area 500 miles north of my location.

  24. STEPHEN SHEEHY | | #24

    You can do what you want. Most people like windows for fresh air and to bring in light so they can look at the outside world when they wake up.

    According to the floor plan, your house has five small windows. Most people would find that gloomy, but it's your house.

  25. GBA Editor
    Martin Holladay | | #25

    Good point about exterior rigid foam in a hot climate. You're right -- in your climate, the rigid foam should be OK. Sorry for the red herring.

  26. aaronmiiller | | #26

    In our area windows are rarely opened, most of the year our A/C is running. In the house I'm living in right now I had to break the windows just to get them out to replace them, the former home owners had painted them shut and hadn't opened them in 30 plus years. Most window replacements we find this same circumstance.

  27. aaronmiiller | | #27

    Martin do you think it would be worth it to install 1/2" of rigid? My house plan right now is 2 2x4 walls spaced 1", I know that's not much but that's as much room as I could afford to lose inside the house. I thought a layer of rigid foam would be a good addition to r-value while also serving as the moisture barrier.

  28. GBA Editor
    Martin Holladay | | #28

    Your 8-inch thick walls provide a higher R-value (about R-30) than most walls in your climate zone. There is no economic justification for a higher R-value.

    If you omit the polyiso, you may be able to use the money you saved to pay for Manual J calculations.

  29. aaronmiiller | | #29

    Many thanks Martin. Why is it that they insulate more in the north and not the south? I'm guessing due to the extreme temperature differences from outside to inside the home? I don't guess we have quite as much temp differences when considering for 8 months of the year were between 90 and 100 degrees and there is only maybe a 30 degree difference?.

  30. GBA Editor
    Martin Holladay | | #30

    Yes, you guessed correctly.

  31. Kenny78 | | #31

    On sizing the hvac, everyone will gladly help when sufficient info is given. Your question would be like someone calling and asking how much a kitchen remodel(etc...) is going to cost. When you ask them sqft, cabinet type flooring options they want they don't know. However they want a fixed price right now.

    There are online resources to perform manual j calcs, free and paid. Some will give you an anuerism but even I found one that allowed me to downsize to 3 tons from the existing 4 ton.

    What zip code is this project located?

  32. aaronmiiller | | #32


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