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How should I heat a fossil-fuel-free superinsulated garage/workshop?

Irishjake | Posted in General Questions on

So I’ve got this super insulated detached barn/garage that I’m building. There is lots and lots of insulation, below slab, both interior and exterior wall insulation, between the garage area and the apartment above, roof and the garage doors are as insulated as I could find.

There is a bathroom in the garage too – so I don’t have to traipse my dirty self back to the house. This means the garage is heated….yeah for me, my workshop, and mantown!

I’m in central NH (Zone 6A) – 8000HDD (Not including this winter), there is no fossil fuel heating on site. I’m struggling to figure out what my heat load requirement will be to heat it. My hunch is to go with an electric radiant ceiling panel in the bathroom and one above the workshop area. I don’t think it will ever get near to freezing, unless of course we leave the garage doors open in the winter. We are using a mini-split HP to heat the apartment above. We have lots of PV too.

Do any of you have a recommendation for what I should use for my heating needs. I was also thinking about just simple electric resistance baseboards – but was trying to avoid them. Is this ridiculous? If I go with it in the bathroom – what would you use in the garage/workshop area?

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  1. GBA Editor
    Martin Holladay | | #1

    A ductless minisplit is between 2 and 3 times as efficient as an electric-resistance heater. So if you can afford a ductless minisplit for your garage, it will lower your electricity bills.

    You say that you don't think that your garage will ever get close to freezing -- and it won't, if you leave a heater on. Without a heater, I think that your plumbing pipes could freeze (because garage doors leak a lot of air and have a low R-value). If it is -30 degrees F and windy, your garage will get cold.

  2. Expert Member
    Dana Dorsett | | #2

    The first thing to do is calculate the actual heat load, at the 99% outside design temp, from which one can figure out the reasonable solutions. Without that it's a bit of a WAG.

    The dimensions and U-factors of the walls, windows, and doors would be a good start. There isn't an agreed upon definition for "... lots and lots of insulation...", so you're going to have to share the construction stackup including the insulation locations & R-values to come up with a reasonable U-factor estimate for the wall .

    With a ZIP code we could estimate the 99th percentile temperature bin.

    Any particular temperature you'll be keeping the space, or shall we assume 68F?

    All that said, unless it's a gia-normous space with very tall walls &/or lots of windows it's probably going to be within the capacity of a 3/4 ton cold-climate mini-split like the Fujitsu -9RLS3H or a Mitsubishi -FH09NA (or maybe even the half-ton -FH06NA.)

    But as I indicated, without more information that's just a WAG. The real load numbers matter as to which you might choose though.

    What type of work would be happening in this workshop?

    Are high ventilation rates a requirement?

  3. Irishjake | | #3

    Martin & Dana.....thanks as usual!

    So zip code is 03216.
    There is apartment above which is why it is so well insulated, and it is heated by a mini-split.
    Insulation below the apartment deck.
    subslab = 10.5 EPS
    walls = R37 (R15 interior dense pack fiberglass & R22 exterior (2 layers of EPS))
    floor joist = R 48 (16" of fiberglass batt)
    roof = R54 (5 layers of EPS)
    It is REMOTE technique, I&W on the roof, Prosoco on the walls.
    Integrity dble hung windows U-0.28
    Therma-Tru doors
    Garage doors (R-18...RIGHT!!!!!)

    I'll have an exhaust linked to CO/timer for the garage.
    Bath exhaust.
    Garage/Workshop will be everything from just tinkering, to woodworking, to welding, automotive, etc.
    Ceiling height is 10', and 8' in the bathroom.
    overall dimensions are 26x32 for the garage area and the left sided bump out is 8x 24
    here is a section view from SU.

    I was going to go with a mini-split, but wasn't sure if that was the best choice......

  4. Irishjake | | #4


    I think 68 is a bit much for the design temp. I think 55 or 60. I know some mini-splits have a minimum low setting right?

  5. Expert Member
    Dana Dorsett | | #5

    Most mini-splits bottom out at 60F for a normal heating mode, but I believe most Fujitsu's have a "minimal heating" mode which keeps it about 50F, give or take. Let's assume an inside design temp of 60F, even if you normally let it run cooler.

    The 99% outside design temp for 03216 is probably about -5F, comparable to Laconia (-5F), cooler than Lebanon (-3F) or Concord (-2F) see:

    So at design condition you're looking at a design difference of 65F.

    The garage plan PDF has no dimensions, you gave no dimensions for the windows, doors, or garage doors, so I'll just outline a methodology.

    When you know the U-factors of the different exterior surface types, it's:

    Area x U-factor x 65F= BTU/hour

    If the windows are typical not large not small New England-y double hungs it's probably 10 square feet per, so for 6 of them call it 60 square feet total, so for U0.8 windows you're looking at roughly

    60' x U0.28 x 65F= 1092 BTU/hr of window losses.

    The doors probably have a published U-factor too, and the garage doors probably do as well. Looks like 2 doors, probably 18' per, call it 36 square feet. If they're insulated R4-ish doors that's about U0.25.

    36' x U0.25 x 65F= 585 BTU/hr for door losses

    The "R18" garage door are probably about 7' x 9' or 63 square feet each for a total of ~125'. Without published U-factors we can probably come up with reasonable estimations if we had a better description of their construction, but for now with thermally bridging internal bracing & hinges etc, let's call it R8 "whole assembly", or about U0.13.

    125' x U0.13 x 65F= 1056 BTU/hr of garage door losses.

    With your partially specified stackup and some guesses about interior gypsum, exterior sheathing, and siding, the wall area U-factor is going to run about U0.030. Measure up all the exterior wall area of the garage & bathroom, and subtract out the area of the windows & doors. Then:

    Area x U0.030 x 65F= BTU/hour load from wall area. With 10' ceilings and a perimeter of about 130 feet, that's a gross area of 1300', less 125' of garage door, 60' of window, and 36' of door leaves 1079 square feet of wall.

    1079' x U0.030 x 65F= 2104 BTU/hr of wall losses.

    With fully conditioned space above, the heat load of the ceiling is zero.

    With R10.5 under the slab the heat load of the slab is going to be "in the noise" of whatever we guesstimate the air leakage to be, so for a rough cut, assume it's zero too.

    Add it all up and you're at 4837 BTU/hr of conducted heat load @ -5F.

    Assuming it's leaking 100 cfm in a wind, that's 6000 cubic feet per hour. The specific heat by volume of dry air is about 0.018 BTU/cubic foot, and you're looking at a temperature difference of 65F so 100cfm of infiltration or ventilation would be a load of

    6000 cf/hr x 0.018 x 65F= 7020 BTU/hr

    Added to the 4837 BTU/hr of conducted loss that's a heat load of about 11,857 BTU/hr. That's the nominal capacity of a 9RLS3H

    If the seals on the garage doors are pretty good and it only leaks 50cfm it would be under 9000 BTU/hr, which is within the range of a FH09NA @ -5F.

    The capacity with an indoor temp of 70F and an outdoor temp of +5F is about the same as the capacity when it's 60F indoors and -5F outdoors, so the Mitsubishi probably still covers it. If you don't care if it drops to 50F when it's cold out, the better choice would be the FH09, since with an interior design temp of 60F the heat load at +47 is only about 20% of the load at -5F.

    So if the heat load is under 12,000 BTU/hr @ -5F, the load at +47F is about 2400 BTU/hr, which is below the 3100 BTU/hr minimum output of the 9RLS3 @ 47F but still above the minimum output of the FH09NA.

    If the load happens to be more like 8000 BTU/hr @ -5F, the load at +47F is only about HALF the minimum output of the 9RLS3, but right at the minimum modulation range of the FH09NA.

    A mini-split that isn't still modulating at +47F won't hit it's HSPF efficiency numbers, so unless the load number estimated at -5F using the real areas comes in above 15,000 BTU/hr (not likely), the FH09NA will likely edge out the 9RLS3H on efficiency based on it's lower modulation range.

  6. Irishjake | | #6


    THANKS - Apologies for just not giving all the info at once, so folks can give the most educated answer. Garbage in - Garbage Out. I attached an additional plan of the garage/workshop area with the data you were asking about.

    I had originally planned to put a FH09NA in but was throwing around different ideas. I felt as though I should probably put electric resistance in the bathroom area, and in the apartment entry vestibule. Since I was doing that, I thought maybe the radiant ceiling panels would be good too, (maybe even better at heating the space more quickly, or at least feeling warmer) instead of the minisplit. I've heard that radiant heat (specifically like a Modine unit works great....but I'm not using propane.

    The difference is about $2500 in up-front costs - between the minisplit and the ceiling panels, even less if I just went with floor baseboards (but those are no good in the garage area).

    I've already spec'd a FH12NA for the apartment above., and we have spec'd all Mitsubishi MSHP for the house too.

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