GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Mini-split vs forced air and attic insulation

xmikew | Posted in Green Building Techniques on

So hopefully a simple question. I’m building a home and have not decided if I should use mini-split or traditional HVAC.

If I use HVAC, I was going to close-cell spray the attic.
Do i need to do this if I’m running mini-splits?

The house is 2400 sq ft so it would have a multi-zones. I presume the refrigerant lines would be running in the attic. do I need to worry about loss there that would warrant spending the cash on spray foam on the attic? Or could I go with traditionally insulated attic since I would have no ducts.

Couldn’t I just insulate the lines?

Thanks as always!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    Without preliminary heating & cooling load calculation numbers to work with it's really hard to advise, other than "Run some load numbers on the design, even if they're preliminary".

    You don't say where you are located, or what the intended insulation levels or windows are. It's a bit like asking how many horsepower you need, knowing little more than it's a car, and it might be red.

    Closed cell foam has a fairly heavy environmental footprint both in terms of polymer per R and the impacts of the blowing agents. Even if you need to run ducts it doesn't necessarily mean the expense and impact of insulating at the roof deck is "worth it". Framing the upper floor a foot taller and building a service chase under the structural joists to accommodate the ducts & electrical & refrigerant lines etc and blowing in 16-18" of cellulose above the joists can be cheaper than R49 in closed cell foam at the roof deck.

    Mini-ducted mini-splits fit nicely in ceiling service chases or under the floor in crawlspaces, and can reasonably serve multiple rooms or even a whole floor. A single 1.5 ton Fujitsu can deliver about 20,000 BTU/hr of heating @+17F, and 20,000 BTU/hr of cooling at 95F, which could be more than your whole house loads (don't know until you run the Manual-J), but multiple 3/4 ton or half-ton mini-ducted heads can break it up into zones without room-by-room microzoning.

  2. xmikew | | #2

    Dana thanks as always. Sorry for the lack of information, I incorrectly thought I was asking a general question but forgot that there is no such thing!

    I have run my calculations. beopt puts my max cooling load at ~ 20btu/hr and max heating around 18btu/hr. Manual-J (from beopt) states i need 24.7kbtu/hr.

    I'm in zone 3 (near charleston, sc)

    The house is on one floor and the trusses have already been ordered, I'm not sure if I can fit in a service chase now unfortunately? - i can ask the gc about it, it does seem like a better idea.

    Assuming the ship has sailed on the chase idea, everything i've read says if you are running ducts in my climate (hot humid - zone 3) then I should go with spray foam roof attic. I was curious if I went ductless if that logic applies to the minisplit refrigerant lines as well.

    The house details are:
    2x6 external 16" oc, R21 walls
    Windows are u-factor of .28 and SHGC of .19
    Encapsulated Attic, with CC spray foam (R-39) - at least originally planned
    Elevated Slab
    indoor cooling: 75
    indoor heating: 70
    outside design cooling temp: 91
    outside design heating temp: 28
    Shooting for 1 [email protected] tightness


  3. GBA Editor
    Martin Holladay | | #3

    Two points:

    1. You can create an unvented conditioned attic without using spray foam. It makes more sense to install one or more layers of rigid foam above the roof sheathing than it does to install spray foam on the underside of the roof sheathing. For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

    2. Refrigerant lines don't have to run in attics. They can run through walls, in joist bays, or along the exterior side of the house. Refrigerant lines are always insulated. There is no requirement that refrigerant lines need to be inside a home's conditioned space; in fact that would be impossible, because the compressor is always outdoors.

  4. Dana1 | | #4

    With a design heat load of 18,000 BTU/hr there are no gas fired solutions (other than a hot water heater and a properly sized hydronic coil in an air handler) that wouldn't be severely oversized for the whole house load, let alone loads that are broken up into zones. Just about anybody's 1.5 ton heat pump would work as a single zone, but if you really want it zoned, mini-splits are probably going to be the better option.

    Individual room loads matter, and it's highly likely that even a half-ton ductless head would be way oversized for individual room loads. It's possible to install mini-duct cassettes under the ceilings in closets to serve 2-4 adjacent rooms and make it work reasonably, but a lot depends on the floor plan and how you want it zoned. You can also run ducts in soffits under the main ceiling. For low-load houses with modulating systems the duct sizes can be pretty small compared to the typical 3-4x oversized HVAC systems, but you DO need a duct designer capable of dealing with mini-splits if the ducts are going to be very long.

    You're probably looking at a 2 zone 1.5- 2-ton multi-split , with either two mini-duct cassette or one mini-duct cassette + wall coil, or two separate mini-splits. A pair of 3/4 ton mini-splits is good for more than 20K heating or cooling at your design temps:

    The Fujitsu mini-duct cassettes have the handy option of being mounted veritcally, unlike most vendors, which means it can be installed in a fattened out wall in the back of a closet or end of a hall or something.

    The combined minimum-modulated output will be a significant comfort & efficiency factor. With an 18-20K load ideally you would want less than 5000 BTU/ hr of capacity with all zones calling for heat, to maximize the modulating range and minimize the amount of time the system(s) spend cycling on/off.

    Your window and wall U-factors probably aren't low enough to meet code for doored-off rooms with just one wall-coil head in a large open space.

    Is this a 1 story slab-on-grade, a 4 story tower with a full basement, or something in-between?

  5. xmikew | | #5

    Martin - thanks, it makes sense when it comes from you. Also only one of the lines in my 2007 built spec house is insulated. A big one connected to the air handler. There is a little one uninsulated that runs along with it. Anyhow, them both being insulated makes sense.

    I always just assumed insulating above the roof was much more expensive than spray foam because 1) you buy double the plywood, 2) double the labor. 3) my builder has never done it. I just assumed it was needed in really cold climates... Also it turns out I have no idea how much anything costs!

    Dana, I'm still unpacking your response. It doesn't have to be zoned. I should not have said that since I have no experience in HVAC design, just a lot of reading on GBA and hvac forums and asking some questions here. I said zoned because I figured one minisplit stuck on the wall somewhere central wouldn't really cut it as far as distribution. The house is 1 floor on an elevated slab (elevated 28 inches i think). I would not call it an "open" floor plan. It looks like the attached.

    My original question though was if I went with minisplits if I can just air seal the ceiling and then use traditional insulation in the attic which seems a lot less expensive than running ducts in the attic and then having to spray foam.

    Dana Said:

    You're probably looking at a 2 zone 1.5- 2-ton multi-split , with either two mini-duct cassette or one mini-duct cassette + wall coil, or two separate mini-splits. A pair of 3/4 ton mini-splits is good for more than 20K heating or cooling at your design temps:

    Annndd this is where I start trying to find someone that has experience with this stuff...

    Also this might be a separate question, what about the duct work for the ventilation system? Is there concern with running that through unconditioned attic? I'm guessing not.

    Thanks for all the help and the patience you all show. It is much appreciated.

  6. Reid Baldwin | | #6

    I recall reading an article that discussed using spray foam just around the ductwork in an unconditioned attic. Then, the ductwork was buried in cellulose. The author said this was a substantially improvement over common practice of ductwork in the attic but still inferior to ductwork in conditioned space. I thought the article was on the Energy Vanguard blog, but I cannot find the article now.

  7. Reid Baldwin | | #7

    I found the article I mentioned above. It is from Building Science Corporation.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "What about the duct work for the ventilation system? Is there concern with running that through unconditioned attic? I'm guessing not."

    A. The answer depends on what type of ventilation system you are talking about. If it is an exhaust-only system, you can run the exhaust ducts through an unconditioned attic (but in a cold climate, the ducts need to be insulated and sloped to handle condensation).

    If you are talking about supply ducts leading from the outdoors, headed to an ERV, HRV, or the return plenum of a furnace or air handler, those ducts could be installed in an unconditioned attic.

    If you are talking about the supply ventilation ducts leading from an ERV or HRV to a supply ventilation register, those ducts are best installed within the conditioned envelope of your house rather than through an unconditioned attic.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |