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Another garage heating question

canadianexpy | Posted in Mechanicals on


So I’m building my dream shop/garage, then I going to build my house.. So my wife is making sure I stay on budget.

Building is 64’x 40′ x 14′

ICF frost walls that will be 4′ below grade and 2′ above grade then 12′ of 2×6 walls on top for 14′ ceiling.
Walls will be taped and sealed OSB best I can and have 3″ (reused) Polyiso on exterior and filled with Roxul.
Attic spaced will have blown in cellulose, R???
Floor will have 4″ foam under slab. If needed??

South facing side (64′) will have 4 large 5′ x 5′ windows, casement style on half of 2 windows.
West Side (40′) will have a 16’w x 12’h garage door.
North Side has a lean-to the length(64′) and currently no windows planned, just man door. East side no windows.

So what would you use to heat this oversize shop? No natural gas available.
Like to use electric, as the roof is going to have solar,( if incentives still available)

I don’t need t-shirt heat inside, and yes I will not always be in there.So turning down would be a nice option.

Any suggestions are appreciated.

Climate Zone 6 (18 miles SW of Peterborough ON)

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  1. GBA Editor
    Martin Holladay | | #1

    This is the perfect application for a ductless minisplit.

  2. 39Chev | | #2

    This will not be very popular on this forum, but I live in Minnesota and have built 3 shops. The first one had a hanging furnace...I was either hot or cold all the time. The second and third I built both had pex heat in the floor with a small electric boiler, and I LOVE it. The present shop has 2x6 walls with 2 inches of foam on the outside, so it doesn't take too much to heat it, but when it's COLD outside, it sure is nice to be working on a somewhat warm floor. Even when a shop is insulated well, it loses some heat through the overhead garage doors (really hard to seal well if they are used quite a bit). I keep the room temp about 55 and when it's really cold, the floor seems to be between 70-75, which is not really warm to the touch, but feels warm when you come in from 20 below zero F. Also, if you are on the floor working on cars/sleds, etc quite a bit, it is warm on the floor; with a hanger, the floor was always very cold...just comfortable heat for me.

  3. GBA Editor
    Martin Holladay | | #3

    I agree that one of the few places where hydronically heated floors makes sense is for garages or workshops built on slabs, especially if the garage or workshop is located in a cold climate.

    Of course, this type of installation is relatively expensive. And your approach (an electric resistance boiler) will have three times the operating cost of a ductless minisplit.

    (Electric resistance boiler = COP of 1. Ductless minisplit = COP of 3.)

  4. canadianexpy | | #4

    So I was looking at both systems, because I do like the idea of warm floors ( in-floor). and most people seem to lean towards this, but if the efficiency of the system is going to costs more over the long term then doesn't make sense.

    Also what problems could I have with a minisplit with regards to dust/ fumes or anything else in may encounter in a shop? mostly mechanical work, some wood, painting ..etc.. you know a do everything shop.

    So if I went mini split, what thickness of insulation in floor should I go with ?? only 2"

    I know again I should hire someone to do a heat loss calculation, but again budget(wife) . Anyone care to guess on sizing a unit? Not sure how to specify the garage door it will be insulated but we all know what that means.

    Thanks , very appreciated.

  5. chrisjri | | #5

    The Chiltrix air to water heat pump would be a great option. Make warm water at a COP similar to minisplit. Not sure about availability in Canada. Budget would probably have to shift a little from solar to radiant.

    Though I am still waiting for info from people who have installed that air to water system.

  6. GBA Editor
    Martin Holladay | | #6

    For more information on air-to-water heat pumps, which have disadvantages as well as advantages, see this article: Air-to-Water Heat Pumps.

  7. Jon_R | | #7

    A slab takes a long time to heat up, making intermittent use less efficient. It could even be that overhead radiant is most cost effective.

  8. canadianexpy | | #8

    Hi Jon

    That is one reason I'm leaning away from,slab heating. As for radiant I'm not sure how efficient these units are, I assume not. Also the mini-split offers cooling, which would be nice.

  9. canadianexpy | | #9

    HI Chris

    I'm a little skeptical of there efficiency, also the price seems high, from the little I looked into them up here.

    Thanks ,

  10. GBA Editor
    Martin Holladay | | #10

    Electric resistance heaters, including so-called radiant heaters, all have exactly the same efficiency: 100% (which means a COP of 1).

    You can expect your minisplit to have a seasonal average COP of about 3. And yes -- for intermittent use, the minisplit will respond faster than a hydronically heated concrete slab.

  11. canadianexpy | | #11

    Thanks Brad

    First hand experience is always best! What size are the shops?
    What do you do for cooling?
    The warm floor always sounds good......

  12. GBA Editor
    Martin Holladay | | #12

    You may also want to read this GBA article: How to Heat a Garage.

  13. canadianexpy | | #13

    Thanks Martin

    it seems to leave the door open to electric radiant ceiling mounts (experts) and still others people saying they love there in floor.

    No one mentions the Min-split.

    If it wasn't for the large leaky door it would be much easier, I think.

    I will attempt to price out a mini-split just to compare up front costs, vs. other choices.

  14. 39Chev | | #14

    My total shop is 32x64 with 12' high walls and a 2/12 vault, so about 14' in the middle. I'm heating 32x32 which is divided into 14x32 and a 18x32. I have a small window A/C unit that I run VERY occasionally in the summer (maybe 4-5 times a summer for a couple of hours)...more to remove humidity than needing stays pretty cool in the summer in the shop unless I have door open a lot.

    As far as recovery, it's true it can take a little while, but I keep it about 50 F all the time and then, if I'm going to be out there in the evening, I turn it up a little when I get home from work and an hour or so later it's ready to go.

    In my humble opinion, the in-floor heat just feels "warmer" in the winter. I know it's not as efficient as a ductless, but it only costs me about $30-50/month to heat in the winter and the extra $10-20/month is worth it for me.

    A couple of pics attached (hopefully) :)

  15. canadianexpy | | #15

    Wow Brad ...
    Great looking garage!!
    I going to have to price out both, which may help me decide.
    I always said I would do floor heat, but that was before I looked into other options, efficiency and this site.
    Did you put any in floor insulation, if so how much?.

  16. GBA Editor
    Martin Holladay | | #16

    Here's my advice: In your climate zone, for a garage, you need at least 2 inches of continuous horizontal rigid foam under your slab -- assuming you are heating with a minisplit or a ceiling-mounted radiant heater.

    If you plan to install hydronic tubing in the slab, I would double the insulation thickness to 4 inches.

    Remember: This is a job that you only get to do once.

  17. canadianexpy | | #17

    If I did go in-floor I have a couple questions?

    1. I think you always recommended 4" (+-R20) for Zone 6 if floor heating, if I do Mini-split would 2"(R10) do?

    2.I See lots of insulation with built in groves for tubing for in-floor heat, before this you would tie the tubing to wire mesh and pour cement, so tubing would be in the floor not under, do you see any advantages to wire mesh way, regarding efficiency?


  18. GBA Editor
    Martin Holladay | | #18

    I'm guessing that you were typing your questions before you had a chance to read my last comment.

  19. canadianexpy | | #19

    Wow you answered my post before I asked !!!

    Scary are you watching us....

  20. GBA Editor
    Martin Holladay | | #20

    I've never poured a slab with hydronic tubing, but most installers attach the tubing to the 6x6 mesh.

  21. GBA Editor
    Martin Holladay | | #21

    I have psychic powers.

  22. 39Chev | | #22

    I would agree with Martin. That's probably the only thing I didn't do as well as I should have...I only have 2 inches of foam under the slab...would have probably been smarter to do 3 or 4". Oh well, still really cheap to heat. The other thing I did is put foam filled plastic "2x10's" as thermal breaks below the garage doors. Someone mentioned them on here or Garage Journal a long time ago and they do a better job of insulating this problem area than just pouring the apron concrete right up to the garage floor. This was a huge heat-sink in my last garage. I wish I could remember where I bought it online, but someone on here might remember the stuff...

    Good Luck!

  23. canadianexpy | | #23

    Thanks Brad and Martin
    I guess I should figure out roughly the size of Mini-split that would be required , so I can actually compare the costs between the two options.

    The plastic 2x10 are on the inside of the garage door in the floor pour?

  24. 39Chev | | #24

    The plastic "boards" are actually about 3" thick x about 8" wide and they are pretty much below the garage doors and where we made the turn back to the outside (the garage doors sit inside the outside wall a little). When we formed up the floor pour, we just held that form back about 3 inches and then slid this insulated board inside the outside form and poured up to it. We left the form boards on the outside of the plastic board while the shop was built so they wouldn't get busted or pulled off and then removed the form boards when we poured the apron. They weren't perfect...kind-of warped, but by leaving the stakes in place until we poured the apron, we kept them pretty straight.

  25. user-6258531 | | #25

    If you have budget constraints, then a mini split with auxiliary electric resistance would seem to be lowest overall cost, both upfront investment and ongoing energy cost. In-floor heat is wonderful but not good if you want to turn down heat significantly when not using the garage. Having a mini split that you could somehow program to come on several hours before use would help minimize the use of resistance heating.

    OK, if it was my dream shop I'd have a wood stove as well.

  26. paul_miltenburg | | #26

    Dave B,
    This is a bit late, but may be useful still or to someone else. Somewhere online, I found an excel spreadsheet setup for calculating heat loss. You enter in surface areas and R values and approximations for infiltration. It's maybe not exact (no calculation will be) but it will give you an idea. I set it up for my shop (24x40x12) and then did it for different outside temperatures. Using average climate data, I could calculate total heating requirements for the season and then calculate the cost of different options. Maybe excessive for a shop, but it was an interesting and educational exercise.
    For the last few winters I have just used a woodstove, but I often have only a few hours to work out there, and I found a lot of time was spent cleaning the stove, getting it going and waiting for useful heat. It also took up floor space and is a safety hazard for young kids or combustible vapours. I moved it out this summer to reorganize things and don't plan to put it back in, but the chimney is still there. When I built, I did put pex in the floor and 2" of insulation under the slab, but the slow response time means I likely won't go that route yet. Just recently, I picked up a used propane high efficiency household furnace and plan to set that up for this winter. An air to water heat pump looks nice too though if combined with some thermal storage. The storage would allow it to run during non-peak pricing that we have here in Ontario and also allow the use of a fan-coil to quickly bring the air temperature up.

  27. lance_p | | #27

    Dave B.,

    I'm building next year here in Ottawa and recently had a discussion about insulating below slab in my attached garage/workshop. Here's the link to that conversation:

    Spoiler: A well insulated/sealed garage should stay above freezing with no sub-slab insulation so long as the perimeter foundation is insulated well below grade.

    I grew up in Port Perry just around the corner from you, and a buddy of mine is building a shop in Pontypool soon, probably next year. Let us know what you choose and how it works out!

  28. canadianexpy | | #28

    So trusses going up next weekend, and now more questions.

    1.My building inspector said I need poly and the inside studs before the drywall, I know this is usually a no-no, but I have been reading about potential if you have high moisture inside, which I might have some intermittent, cars in winter melting snow and or dripping after rain. Also possible washing down a car in winter. IF it's a NO for Poly, what do I tell the inspector?

    2. I'm trying to build this as airtight as possible(minus the garage door). so if I tape all OSB seams and want to strap 1x4 to the underside of the truss then drywall, can I just calk against the sides of the drywall to the top plate and use the drywall for an air-barrier on the ceiling?


  29. josh_in_mn | | #29

    You could use a smart vapor retarder, such as MemBrain. Looks just like Poly.

  30. canadianexpy | | #30

    Thanks Joshua , I did consider Membrain if inspector demands the poly.

    Lance I'm leaning towards in-floor, was talking to a neighbor who has similar size and he loves it.

    First back to figuring out the air sealing details ....

  31. lance_p | | #31

    I too was considering in-floor heating, but after learning that I could keep the garage above freezing without it I changed my mind. Was just offering my perspective. :)

    Question: If it's a detached garage and not considered "conditioned living space", why is the inspector even concerned about insulation and air sealing? Just curious. If you are insulating below the slab (and around the perimeter) you don't have to worry about it freezing... could you not pass inspection prior to insulation and drywall and just do your own thing later?

  32. RMaglad | | #32

    Lance - inspectors concerned about insulation and air sealing details is a good thing!

  33. canadianexpy | | #33

    Lance :I think the inspector is maybe more concerned, knowing it will be heated.
    I didn't get into it with him regarding needing or not needing poly.
    As for insulation the floor will be getting either 2 or 4" under slab, the frost wall is ICF so insulation is every place
    I think it will play out that the inspector will not need to return, after framing inspection. So i might not need to worry. .

    Ryan : I agree, and I think he just wants me to do the right thing , so I don't have issues down the road.

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