First time designing and installing radiant - feedback

My thoughts:

All electric resistance heat has the same efficiency, COP of 1.0. So an electric boiler won't be any cheaper to operate than resistance mats under the floor. The installed cost of the resistance mats will be cheaper, everything about it will be simpler. The only advantage of the boiler is that if you ever decide to switch to a different source of heat the plumbing will be there.

You're right to think about the responsiveness of the heat emitter (the floor). That's going to be a big determinant of your comfort — and heating is all about comfort. With a 6" concrete slab the responsiveness is going to be poor, especially in a low-heat-load building. At 16x22, that floor is 176 cubic feet of concrete. It will take about 13,000 BTU to change its temperature by one degree. If your heating load is 10,000 BTU/hr, you'd need the surface of the slab to be 14F above room temperature. To get the slab there from room temperature would take 182,000 BTU, or more than 18 hours of full output of the boiler. I'd be thinking about putting the foam over the slab and the tubing over the foam and then a finish floor over that. Does the Heat-Sheet allow that?

I'm glad that you're paying attention to insulation, I like to say an ounce of insulation is worth a pound of hydronics when it comes to comfort. In most buildings most of the heat loss is through windows and doors, you don't say in your description what kind of windows and doors you have. That would be helpful to know.

For the roof, it sounds like what you're doing is a cathedral ceiling, where the insulation runs along the underside of the roof. These are tricky, even pros often get them wrong, and if not constructed properly they are prone to moisture getting trapped in the sheathing which causes the roof to rot. This is especially problematic in cold climates like yours. This article gives good approaches for safely insulating:

https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work

If you are height-constrained, one of the approaches is to put insulation between the sheathing and the shingles. At least in my jurisdiction, the extent of a building is the exterior of the sheathing, exterior insulation and siding or roofing is not counted. Check with your building department. Another approach to add more insulation is to fur out the bottoms of the rafters.

It's possible to run the slab at a constant temperature. What you get is that when the heating load fluctuates (ie the outside temperature changes) the room temperature changes. This is different from what you'd typically do in a residential installation, where you try to vary the floor temperature to keep the room temperature constant.

So what would that look like? The diagrams show the design temperature of the floor at 75F with the air at 65F. They don't show what outdoor temperature that represents, let's say -25F so we can work in round numbers. If you kept the floor at 75F, when it's 75F outside the room would be at 75F too and the heat would be off. Between the design temp of -25F and 75F is 100 degrees, the room temperature is going to move 10 degrees over that range, so for every ten degrees the outdoor temperature drops the room temperature drops by one degree. So with an outdoor temperature of 65F you get 74F indoor, and:

55F→ 73F

45F → 72F

35F → 71F

25F → 70F

15F → 69F

5F → 68F

-5F → 67F

-15F → 66F

Since the air inside the building has a lot less heat capacity than the slab, it's a lot easier for the air temperature to move than the slab temperature.

Keep in mind it's your building, you can run it any way you want. But if that's not the behavior you want we can talk about other ways of operating it.

My suggestion is that you look at some kind of supplemental heat for when you want to warm the space quickly. The heated slab is fine for keeping the building above freezing, but when you want to warm up to room temperature it's going to take a day or more. You'll want something that can put out a lot of heat in a hurry to warm things up.

When the radiant floor is heating the space to 45F it will probably be around 50F. That's not "freezing cold" but it's probably not the warm toes feeling you're looking for either.

The way to have the floor at 75F and the room at 45F is to only heat part of the floor — about a third based on the discussion so far. If you know where you're going to be standing you could plumb one zone to cover those spots, and only run that one zone.

If you don't know where you're gong to be standing, or you want to be able to move around, I would recommend heated floor mats. They won't use any more electricity than an electric boiler for the same heat output.

I would still recommend some sort of high-output heater to quickly take the chill off. If it has a low setting for when the space is unoccupied, even better.

To get back to your original question of two loops versus three: I would normally default to three to make it easier and lower power draw for the pump (but for your small space and low flow rate it would not actually make a noticeable difference), and in case you have to abandon one in the future (exceedingly rare, but it does happen).

I suggest keeping the pipe pulled in from all the perimeter by at least a foot, or more if you know the outer edges will be populated with tools, cabinets, etc. If you recalculate the loop lengths that way you will probably find you can tighten the loop spacing to 4" for a portion of each loop, and I would do that toward the center of the room. Note that when you tighten up that spacing you will still flare out to do the end turn-arounds with a broader bend (like a shepherd's crook; see attached photo) unless you are using exceptionally flexible pipe.

But you have not said why, if your heat will be electric resistance anyway, you do not want to do electric wire or mat radiant. You could still use the Heat Sheet as insulation, but you will need to tie the radiant wire to reinforcement wire, so will not get advantage of its tracks. Or do you hope, as DC Contrarian mentioned, to use some other source in the future?

As far as quick heating for areas you will stand most often, consider electric radiant suspended ceiling panels, the advantage being you don't have to blow air and dust to distribute the heat.