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Ductless minisplit retrofit question

jdchess | Posted in Energy Efficiency and Durability on

I live in southeastern NC, near the coast. My house is just under 1900 sqft with a bonus room over an attached two car garage. My HVAC ducts are in the unconditioned, vented attic. I understand the problems with this. Would a ductless minisplit make sense as a retrofit option? If so, can you just remove the ductwork and patch the drywall where the supply registers are? How do you determine how many head units you need and the best places to install them? I was thinking about this as an option for when I eventually have to replace my heat pump and/or air handler.

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  1. Expert Member
    Dana Dorsett | | #1

    You have to run a room-by-room cooling load calculation to know how many and what size mini-split heads make sense.

    You could just do a "ton per thousand feet" estimate and not be insanely off for whole-house cooling loads on a post 1980 house in most instances, but on a room or zone basis that could be WAY off. Rooms with west facing windows have huge late-in-the-day solar gains occurring when the air temps are still high and the roofing/siding have been cooking in the sun all day, leading to very high peak loads.

    In an NC climate it's worthwhile assessing both the heating & cooling loads when contemplating replacing your AC equipment (with mini-splits, or ducted heat pumps), since high-efficiency heat pumps have gotten to the point that they are competitive even with natural gas for space heating in many markets with moderate climates. (It sounds like you're existing unit is a heat pump?) The up-charge for a heat-pump mini-split or multi-split that still delivers the heat at NC type 99% outside design temps isn't that much, and could be worth it to retain options as fuel & electricity prices evolve. A right-sized multi-split with an HSPF of 10 or better will blow the socks off a typical 10 year old ducted heat pump system, using barely more than half the power. This is only partly due to the very high part-load efficiency, but also the lower load of being rid of air-handler driven air infiltration, and duct gains/losses. (With perfect ducts fully inside of conditioned space some of the variable speed ducted heat pumps would do that as well.)

    With mini-splits it's important for both comfort & efficiency to not oversize by more than 50%, so that it's spends most of it's time modulating it's speed up and down with the load at extremely high efficiency, rather than cycling on/off with start-up and idling losses. The very smallest ductless heads run about 6000-7000 BTU/hr for cooling, and may be 2x oversized for the loads of smaller doored-off spaces. The smallest individual mini-splits run 9000 BTU/hr cooling, which are oversized for MANY individual rooms, but can reasonable for larger or high-solar-gain rooms. It can often be more efficient to install a mini-duct cassette in the top of a closet space between two or three rooms, and split the output (& returns) 2-3 ways with extremely short duct runs to size it to rooms with sub-5000 BTU/hr peak loads.

    Give the room-by-room load numbers a shot using sorta-real tools, not some rule of thumb. It's not rocket science. These online tools use standard Manual-J methods:

    You can look up or estimate your 99% & 1% outside design temps using the ACCA Manual-J listed figures:

    Don't be surprised if your existing equipment is 2-3x oversized for your calculated cooling load. Just having the ducts in the unconditioned attic adds something like a full-ton of compressor load in many homes, and all of the rule-of thumb HVAC guys use have fat thumbs, which reliably oversize it.

    My gut feel based on your location and the house size is that a 1.5-2.5 ton compressor and 2-3 heads/cassettes would likely cover both your heating and cooling loads without much trouble, unless your house is unsually leaky or you have a lot of single-pane clear glass windows or sliding doors to deal with. (In which case some upgrading of the building envelope would be in order.)

    It's likely that your cooling loads will call for more ductless compressor & head than your heating loads, but if that's the case split the difference if going with the bigger system would oversize the heating by more than 50%. NC is still a heating dominated climate, and oversizing by more than 50% results in less heating-season comfort & somewhat lower efficiency. On the days when the slightly undersized cooling doesn't quite keep up with the sensible load, the extremely long modulating operating cycles will dry the air out to where it's still pretty comfortable, even if the room temp is warmer than the setpoint.

    It almost goes without saying, but you "...patch the drywall where the supply registers are...", don't forget to patch over the return registers as well.

  2. charlie_sullivan | | #2

    Also, if you are considering envelope upgrades, doing them first can allow you smaller cheaper HVAC equipment. Particularly if you run the numbers and the cheaper option is almost but not quite within reach.

  3. jdchess | | #3

    Dana and Charlie,

    Thank you both very much for the info.

    I should have mentioned that the house is only 5 years old. I know that it is somewhat leaky and I'm starting to work on air sealing. My heating and cooling power usage has just been much higher than I feel like it should be. The ducts in the unconditioned attic are only R8, so that's obviously a problem. The attic floor has 12" of CertainTeed Insulsafe SP blown in fiberglass (R30). Exterior walls are R19 I believe. The bonus room over the garage is about 250 sqft and requires a lot of energy to heat and cool. I have a 3.5 ton Goodman heat pump and air handler (13 SEER). Everything is running fine and the charge is correct. I thought that your comment about a 1.5 - 2.5 ton compressor was interesting. So if the house was sufficiently tighter, I wouldn't need a 3.5 ton compressor, or was this specifically in regards to switching to a minisplit?

    Air leakage is the only other thing I can think of that could be a problem. I know that in the winter, especially if it is slightly windy outside, you can easily feel cold air through the electrical outlets on the exterior walls, and there are several places along exterior walls where you can feel cold air from underneath the baseboard. Obviously, these things need to be addressed.

    Is there a way to determine roughly how much retrofitting a mini-split would save over the current duct system? Aside from a blower door test, what else can I do to test what I currently have?

  4. charlie_sullivan | | #4

    You can get duct leakage tested--you might find an auditor that can do a blower door test and a duct leakage test and crunch the numbers for you to predict how much you could save by sealing air leaks and by getting rid of and/or sealing the ducts.

  5. Expert Member
    Dana Dorsett | | #5

    Having the ducts & air handler in the attic adds about a ton to the compressor requirements for cooling due to the combination of direct gain and air-handler driven infiltration, even with R8 duct insulation (which meets code, BTW.) So if the 3.5 ton ducted unit is keeping you sufficiently cool/warm, you don't need more than 2.5 tons of ductless, and may do just fine with 2 tons of ductless or even less. If the ducts were run in interior soffits or a service chase inside the pressure & thermal boundary of the house the sizing would be the same as with ductless, since they are not exposed to the super-heated/cooled attic space, and any duct leakage would still be inside of conditioned space.

    The R30 fiberglass is on the thin side, and is pretty lousy during the cooling season due to the infra-red translucency of fiberglass. If there is room to blow another 5-6" of cellulose on top of it (whatever it takes to bring the depth up to 14-15" everywhere) it'll be worth it over the long term. But before you do that it's important to air-seal both the ducts and the attic floor / ceiling plane, taking extra care to air seal the register boots to the ceiling gypsum.

    R19 batts are pretty much junk- such low density and poor air retardency that they are more like air-filters than insulation, especially when the sheathing is not air-sealed to the framing (which is why you have the breezes coming through the electrical sockets. If you can't air-seal it well enough, one solution would be to dense-pack cellulose or new-school fiberglass over it drilling from the exterior. That's a pretty expensive proposition though, not to be taken up on a whim. If there are insulation contractors specializing in air sealing, that would be a more appropriate & cheaper first-step.

    FWIW: R19s are the same weight per square foot as R13s, the same amount of material, basically a stretched out fluffed-up R13. When compressed to 5.5" in a 2x6 cavity it only performs at R18- the R19 labeling is at it's full manufactured loft of 6.25". They're cheap, better than nothing, but not something to use if performance & comfort was part of the goal.

    From a priority point of view air-sealing what you have (ducts & house) come first, to maximize the system efficiency. Since it's only five years old you have time to work out the rest. In the meantime should electricity prices go sky high you'd still have to do the math on whether it makes more sense to spend $7-10K on ductless heat pumps vs. rooftop PV solar. But rarely is it economic to scrap HVAC equipment this early in it's service life.

    If you seal the house and ducts it should cut quite a bit out of the power bill. From there going to best-in-class ductless might save you at least another 1/3, but probably not 1/2. Air sealing is usually pretty cheap compared to buying a multi-split though, and is a necessary step even if you later went ductless.

    Basement, crawlspace (vented or unvented?) , or slab on grade?

  6. jdchess | | #6


    It's a slab on grade foundation. Also, half of the exterior walls are 2x4 and not 2x6, which I assume means they have R13 batts and not R19.

  7. Expert Member
    Dana Dorsett | | #7

    If the slab edge doesn't have insulation, it's still worth digging down a couple feet and installing at least R8 EPS all the way up to the bottom edge of the siding, and installing Z-flashing behind the bottom edge of the weather resistant barrier (felt or housewrap), and finishing the exterior with a vapor permeable protective finish like Quickrete Foam Coating stucco-like material, but that's a lower priority than air sealing.

    The 2x4 walls with the R13s are probably less breezy-leaky than the R19 walls due to the higher density of the matrial, but that's not to say they're air-tight. A blower door test will find all...

  8. jdchess | | #8


    The slab has no insulation to my knowledge. On installing R8 EPS, would you typically dig down to the bottom of the footer and just lay it against the slab and throw the dirt back in, or is it more complicated? Does anything go in between the EPS and the concrete? What is the purpose of the Z-flashing behind teh bottom edge of the housewrap?

    Thank you again.


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