What kind of "shop" is it, retail or sawdust?

Personal workshop. I should add that I'm in northern Minnesota. It will be used for storage,  light auto work, marine and powersports,  woodworking,  etc.  It will be spray foam insulated and sheeted inside

I can’t imagine not having these in Northern Minnesota! Ground temps will be C-O-L-D and permeate the space. And even if you skipped radiant, how could you even think about not doing enjoins under slab insulation to not try to isolate the slab from the frozen tundra???

I Didn't believe you until I looked up the price of 2" XPS 250 because I was watching the stuff up until last fall when it was half the price than it is now, sorry. I won't tell you to bite the bullet but with either heating source you decide on I would recommend some kind of insulation under the slab and will tell you my experience.

I live in Minnesota, My original shop was 30' x 40' with 12' block on 12" poured foundation walls with a 12" slab built by the previous owner (yes, over kill) without insulation under the slab. I added 30' with block foundation walls and put 2" XPS foam under the 4" slab, and XPS foam down and around the perimeter 2 feet. My heating method is a forced air furnace .

From now and into mid June the old floor not insulated will sweat and stay wet continuously unless I use some high powered fans and run a dehumidifier and a A/C unit. The new part with the insulation stays dry with no condensation at all even without fans and dehumidifiers. Without the fans and dehumidifiers my huge collection of tools and stuff start to rust up, and everything in storage gets musty and stinks like an old wet basement.

If you can find the funds and at least put down 3/4" or 1" foam it will work as a thermal break and help with the sweating, and running foam round the perimeter will stop the freeze from moving underneath the slab. When the shop isn't in use I keep it at 38 degrees and will find items left on the non insulated floors around the outside walls frozen or close to it. The Insulated floor has no issues.

For me the sweating floors suck and bought 2" foam years ago to cover the old part and will lay it out and pour 4" of concrete over it but haven't had the time yet. IMO the cost is well worth it.

You need sub slab insulation either way. But if you also need air conditioning, you will be paying for multiple systems if you choose radiant heating. Long term, it’s it’s not really worth the additional cost for a shop. Better insulation, with 2x6 or 2x8 construction, and under slab insulation, is always worth it.

Some great responses here!

My current shop has an electric boiler that runs the radiant floor heat. I can attest to Charles's statement about in floor heat being slow to respond. Mine is on off peak which in our climate, doesn't do me any favors either. When coupled with a propane hanger heater, it is fine, but on it's own, not great.

I feel that if I can maintain a constant temp vs the up and down that I will be fine without the floor heat. I do see now that I will need some sort of insulation underneath no matter what I do.

thanks for the feedback!

A big yes to all the ground insulation and vapour barriers but ‘cost notwithstanding’ the approach I would favour would be close spaced in-floor hydronic radiant for the lowest water distribution temperature and greatest efficiency, supplemented by hydronic reversible fan coil units that could be used to raise temperatures quickly in heating mode and provide air-conditioning and dehumidification in cooling mode. The response of the slab will be dependant on insulation, thickness of the concrete and depth within the slab at which the piping is installed, among other factors. Piping installed closer to the slab surface will respond faster if thermostat is bumped up; piping sunk deeper into the concrete results in a more stable but slower responding thermal mass. Take your pick.

With in-floor radiant you basically heat the first 6-feet from the floor up and who cares about the rest as long as you and your feet are warm. Although ceiling fans would also be useful. With forced air you have to heat the entire volume of the envelope!

Radiant is great for working on vehicles in winter as it keeps the floor drier and warmer. The room also recovers faster when garage doors are opened and closed due to the energy stored in the thermal mass of the slab. Also much more comfortable to work on your back underneath a vehicle - that is dry.

One integrated energy efficient system with either an air-to-water or geothermal liquid-to-liquid heat pump as an efficient source. The thermal mass of the slab can also be used as a backup during a power failure by just keeping the circulator(s) running continuously with a very small generator to keep the all pipes from freezing for most outages.

Your existing electric boiler can be a backup during extreme weather and for compressor lockout or other HP failures.

Something else I can tell you from experience, particularly if you plan to own this for some time is that what looks like an astronomical installation cost today will look like an absolute real bargain 15-years from now - if properly designed and installed!

IMPO

SR

With the advice in the previous post, you can rest assured that for $50,000 (or maybe more), you can take the chill out of the air in your garage.

And what do you base your esteemed cost analysis on if the OP is considering in-floor radiant anyway?

SR

My estimate is based on your always recommending the same approach for everyone, without asking where they're located, nor what their budget is, nor what utility rates are.

They are in northern Minnesota. They will LIVE in the shop while the house is being built. Without this investment, the year + they will live there will be miserable.

I'm also in MN and we built a shop in 2019, 2 stories with living area on 2nd floor. I'll reiterate what others have said, you need foam under the slab no matter what plus down and out from the slab if you're doing a monolithic slab instead of deep footings as is common in our area. We keep the shop area at a lower temp than the living area that functions as my office (and guest quarters)- 2 zones. I love the quiet, even heat. We have a propane "Mr. Heater" unit in the shop if we want to bump up the heat in the shop quickly for some reason and to qualify for a dual fuel rate from our utility for the boiler. I can't say enough good things about our Electro Industries boiler. The dual fuel can be a challenge though. Our power to the boiler is cut for ~4 hours in the morning on the coldest mornings of the year, which is challenging since I started working from home full time in 2020. The building is energy efficient but if it's -50F outside, my office will drop to maybe 55 degrees inside from where we keep it overnight at 62F and i need to supplement with a space heater by my desk until the load control comes off. That being said the dual fuel rate makes the electric boiler a really attractive option.

I see, the compulsive drive-by hit and run poster with less than nothing to contribute is back with his nasty fetish to post negative criticism without substantive backup, offering no new content nor any data or facts to back up his claim.

You do this compulsively, every time! Do you think the OP’s incapable of coming to their own intelligent conclusions without your nasty commentaries?

I would still like to see more about your $50,000 or more justification that you just threw out like a hand grenade and then took off.

IMPO

SR

Wet floor is avoided with a 6 or 10mil vapor barrier. depending on code requirements, if any. I'd choose full depth footing with one row of block. One-inch foamboard glued to the inside of the block for a thermal break before the floor pour. Vapor barrier under. With spray foam walls and ceiling you'll have a tight shell. South facing windows could help in the winter with the correct overhang to block the higher angle summer sun. Fixed glass insulated units are easy to frame and install. You can use standard patio door replacement glass. Consider water lines for easy drainage when you no longer continuously heat the shop in the winter.

"You do this compulsively, every time"

The irony of this comment gave me a smile. Look in the mirror, my friend.