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Community and Q&A

Garage apartment mini-split feasibility and options

minute_basset | Posted in Mechanicals on

First – thank you to all who so actively participate here. This site is an amazing resource of practical, no nonsense information and superb advice from the forum community.

We’re close to finalizing our plans to finish our garage/apartment project. We’d like to “go more green” and use mini-splits to provide primarily heat to these 2 spaces, but would like advice to determine if this is the best solution for this scenario. 

The Existing Garage Shell:

  • Engineered, pre-fabricated, built in 2011 
  • 30′ x 40′ standalone New Englander style garage – 1.5 story with a 13′ gabled dormer.   
  • 10/12 pitch with architectural shingles.
  • 5″ slab with 2″ rigid foam (R-10) and stego vapor barrier underneath on top of 4′ frost wall w 2″ (R-10) exterior foam to the footing
  • 2 x 6 construction with advantech exterior sheathing, tyvek, and vertical shiplap siding.
  • One wall partially built into hillside – all the rest are above grade.
  • Simon ton windows, (3) 9×9 garage doors (R-12 and weatherstripped)
  • 10′ ceilings in garage, 9′ ceilings in center flat part of second floor. Roof trusses are set on 3′ exterior knee walls

The Insulation/Sealing Plan

  • Minimum R-22 in all exterior walls (most likely just open cell)
  • R-49 under the roof sheathing (2″ closed cell flash coat to get vapor and air barrier and the remaining 9″ of open cell to also cover rafters where possible)
  • 2″ closed cell (R-13-14) in the garage ceiling to provide an air and vapor barrier so there’s no garage smells infiltrating the living space above 

Initial Manual J
calc using the BuildItSolar Home Heat Loss calculator recommended in another article:   

41000 btu/hr for design temp of -6F for CZ 6.  I also have a call into Eff VT to see if I can get support getting to a more accurate Manual J calc for the 2 separate spaces. 

The apartment/studio space will be mostly open plan – maybe a 3/4 wall  partition to make a bedroom divider, and of course, the bath. It won’t be occupied full time – just short term rental and kids so we’ll need to be able to set back to 50 or less.  Could we get away with a single zone/single floor mounted unit to heat the space given that it was sized correctly?  

The garage space is partially divided by the stair well that runs perpendicular to the long wall and is offset from the center point. We need to keep it above freezing and able to raise the temp when we want to work.  I’ve been thinking that the Mitsubishi ceiling cassette that distributes air in 4 directions placed in the center of the space would be better able to distribute heat “around the corners’ of the stairway into the shop  and garage areas. Or ,would a single wall mounted head be better?  Since the ceilings are 10′ high, would it be prudent to add a ceiling fan as well.  

Hot Water Options
We’d also plan to use an on-demand tankless water heater to provide DHW to the bath and kitchenette in the apartment.

The other option is to go with a combi-boiler (Bosch or Rinnai) to both heat the garage and apartment and provide DHW. We could run slant fin upstairs or use a hydronic radiant panel.  What would be the best way to distribute heat in the garage space?   

Any thoughts or advice would be greatly appreciated..

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  1. Expert Member
    Akos | | #1

    The single central floor mount should work. I would add in supplemental electric panel/baseboard/floor heat in the bathroom and bedroom. These can be set for freeze protection when unoccupied and the mini split doing most of the heating.

    Mini splits are not the best to quickly heat up a place, you really need to oversize a lot if you want fast. For most workshop/garage type of places, you can't beat a propane celing heater or a radiant tube heater for quick heat.

    You can go with an over sized wall/celing mount mini split, just make sure it can be set to low enough temperature, not all can be set bellow 60. It should be on its own outdoor unit as multi splits don't modulate low enough when the head is over sized for the load.

    If the place is not going to be used full time, propane tankless heater adds a lot of cost for not much benefit, stick an electric tank. Your propane costs work out to about $0.10/kWh electric equivalent, if you power price is close to that, there is no cost saving there.

    The manual J infiltration number seems very high. I think the volume of the structure is over-estimated which is throwing off your heat loss calculation. Garage doors can be quite leaky, so maybe not far off.

  2. minute_basset | | #2

    Hi Akos,

    Thank you for the feedback/input. I will look into electric DHW options.

    I've double checked the volume calculations. The garage floor is 30W'x40'Lx11'H= 13,200 ft3. Second floor ranges from 7920 ft3 when calculating the volume from the 3' high knee wall to 9' ceiling height to 12,420 ft3 if calculating all the way to the roof peak:
    Total 21,120 - 25,620 ft3

    I wasn't sure what to use for the air changes/hr factor. There's tyvek between the sheathing and siding. I'm not sure if the sheathing got taped back then so we probably won't have a perfect continuous air barrier. There are 3 insulated garage doors (R-12 and weather stripped). Although the foam will be tight, as you say the doors are leaky. So I used 0.6. What do you think? Hoping to get a more accurate manual J this week.
    Thanks again..

  3. Expert Member
    Akos | | #3

    Without a blower door test, getting a better value for infiltration losses would be hard. Your guess is a good as mine, but 0.6 ACH (I'm assuming this is natural air change per hour, not @50PA) seems high for an all SPF construction. Might be worth to get it done after the SPF goes on, it would make a big difference in the equipment size.

    Since you'll have a door between the two spaces and they should be fairly well separated, it would be good to treat them as distinct space for your heat calculation. Most likely the upstairs area would need a lot less heating.

  4. Expert Member
    Dana Dorsett | | #4

    >"Initial Manual J calc using the BuildItSolar Home Heat Loss calculator..."

    That's not a Manual-J calculator, and it's pretty crummy overall. Even a using a spreadsheet tool for an I=B=R calculation using parallel path model for calculating the U-factors would do quite a bit better job than that tool.

    A 30' x 40' apartment (1200 sq.ft.) build to IRC code minimum should come in well UNDER 20,000 BTU/hr, maybe even under 15K @ -6F if it doesn't have much window area.

    Are you really planning to heat the garage to 70F ever?

    >"9×9 garage doors (R-20 and weatherstripped)"

    There is no such thing as a true R20 garage door. Does the manufacturer publish a U-factor for that door?

    >"Minimum R-22 in all exterior walls (most likely just open cell)"

    That's below IRC code minimum for zone 6. It needs R5 continuous on the interior or exterior side to meet IRC

    >"R-49 under the roof sheathing (2″ closed cell flash coat to get vapor and air barrier and the remaining 9″ of open cell to also cover rafters where possible)"

    That needs an interior side class-II vapor retarder to prevent frost build-up in the open cell foam by the end of winter.

    >"2″ closed cell (R-13-14) in the garage ceiling to provide an air and vapor barrier so there’s no garage smells infiltrating the living space above "

    That's a very expensive (both environmentally and financially) method of air sealing the garage ceiling. A plywood or OSB subfloor is already a class-II vapor retarder, and very easy to detail as an air barrier. R13 of open cell foam would use less polymer than 1" of closed cell, and even that is tough to rationalize from an environmental point of view.

    >"The other option is to go with a combi-boiler (Bosch or Rinnai) to both heat the garage and apartment and provide DHW. We could run slant fin upstairs or use a hydronic radiant panel. "

    Maybe- but the minimum modulated output of a 199K combi-boiler is usually well above the anticipated heat load here, and each zone has to have sufficient radiation to not short-cycle the beast into an early grave when operated at condensing temperatures, which is a lot more baseboard/panel than you might be thinking. Run this napkin-math on it:

    >"The garage space is partially divided by the stair well that runs perpendicular to the long wall and is offset from the center point. We need to keep it above freezing and able to raise the temp when we want to work. "

    A wall coil type would work. I suspect you're looking at a Fujitsu 9RLS3H or Mitsubishi FH12NAH. Both have a means of operating to a +50F indoor set point- the Fujitsu "MINIMUM HEAT" mode is straight up set to 50F, or Mitsubishi "SMART SET" allows dropping the setpoint to +50F (but not lower.)!/product/25333!/product/26102

    Oversizing it to heat it up quickly would be bad for average efficiency, if you're using it primarily for freeze control most of the time. With a wi-fi control on the thing you could give it several hours head start even when you're not there, if planning to work in that space later in the day.

    1. minute_basset | | #5

      Thank you Dana..

      To follow up on your comments:

      Absolutely need an accurate heat loss calc. My gut was that a 15-18k mini split might work for the space. When working on a small, mostly open plan, high performance Habitat house this year, I was shocked at how many different heat loss calcs and design configurations I got from both contractors and suppliers. The Efficiency VT calc was (12k btu). We got things ranging from 18K-28K. Most quotes were for 2-3 multi-zone configs which really aren't recommended for cold climates. One was for 3 single zones - 2 9k's and a 6k. So, I'm concerned about getting the best recommendations for the space and the climate. It starts with that heat loss calc.

      The garage doors are R-12 (didn't catch that error). Even then, I know that's probably a stretch. I haven't been able to track down the original spec for the U-factor. Also, we wouldn't heat the garage to 70F - more likely high 50s - mid 60s depending on the requirements of materials we were working with.

      As for R-22 in the walls - can't do a continuous R-5 on the exterior of the building. I understand the $$ are best spent on the shell, but it is a garage and a sometimes short-term rental. I'd like to do a "good job" on the garage and "better job" on the living space. Would it be an option to stick to the open cell in the exterior garage walls and then upstairs, build a second interior wall on the gable ends and knee walls, spray enough open cell to get the air barrier and then fill the rest of the cavity with fiberglass or cellulose?

      On the roof, one of the foam quotes I received indicated that 1.5" closed cell on underside of the roof sheathing was R-10 and a class II vapor barrier. Installing another 7.5" of open cell on top of that brings us to R-49. So we should be ok.

      I didn't realize the sub-floor met the criteria of a class II vapor barrier. To air seal it, would that require taping all the seams on top, foam sealing or caulking the perimeter of the sub-floor from the top and then foam sealing under the sheathing along the rim and exterior floor joists? Then finish up with a batt insulation in the bays?

      Since there's rigid foam on the foundation perimeter, would batts in the ceiling be ok or should rigid foam be applied to the rim joists between the TJIs and foamed into place?

      Regarding heating. Thanks for the pointer to the article on boiler sizing. An Energy Star contractor had recommended a combo-boiler and when I started looking into them I saw they were huge in comparison to the load we'd need. So, I'm ruling those out. When we lived in Denmark, our heating system looked like a small on-demand hot water heater that provided heat and DHW - it was a thing of beauty and I wish I could remember what it was..

      As much as I'd like to go with the mini splits both up and down, the total cost of the project is making my head spin. Years ago I would have thrown faced fiberglass in the walls and ceiling and gotten a Modine Hotdog and called it done.

      I'm going to get some quotes for Mitsubishi Hyperheat units anyway. If you use one of their wifi thermostats, you can set the temp down to 40F. The lowest set point with the remote control that comes with the unit is 50F.

      I was looking at one of these recessed ceiling units for the garage as it can be centered in the space and it blows in 4 directions - might distribute the heat better around the stairwell. Only question is with a 10' ceiling, will the heat make it to the floor... The Hyperheat version of the compressor to work with this head was supposed to come out last month:

      And then the floor mounted interior unit for the apartment:

      That just leaves meeting DHW needs. I've reviewed the articles on tankless vs tanks, electric, gas, hphw, etc. Since the apartment won't be used year round, and we'd limit occupancy to 2, I'm leaning towards a small electric hot water heater with small tank installed in the bath to provide HW to the shower and sink. There's a bosch with 4 gallon tank that might work. I'd also like it to provide HW to a kitchen sink that could be up to 30' away, but that might be pushing it. We have a 100amp service in the garage. The challenge will be getting something that doesn't take up the whole panel - especially since the mini-splits are going to need their own dedicated circuits. I don't know what I'll have left to run my tools!

      thanks again for your critique and input!


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