Decide if the crawlspace is to be part of the house or completely separated from it. Then you can decide the route to take.

See Building Science Corp. on crawlspaces.

I'd place 10mil plastic on the ground. You can cover that with round pea gravel. I'd insulate the wall with Rockwool board. I'd insulate between the joists with Rockwool. You can choose other products.

Thick plastic on the ground for sure but no insulation is needed in the crawl space walls. Your crawl space should be well ventilated to the outside so the insulation would serve no purpose. You just saved a bundle here.

As far as a vapor barrier it is always applied on the warm side to prevent moisture from migrating to to cold side and condensing which would cause rotting wood and mold. Was a plastic vapor barrier applied on the floor joists before the sub floor was installed? It should in your case and if not I'm curious to see what others say. If not I suggest installing layer of foam board up tight to the PEX under the floor and sealing it with caulk to the joists. Then install fiberglass insulation with the paper side up to the floor or insulation without paper, and apply house rap. House rap witch will allow moisture vapor to escape but do not use plastic. If a plastic vapor barrier was placed on the floor joists before the sub floor was installed you can omit the foam board. The foam board would be used as a vapor and air barrier if no plastic was used. A final layer of a sheet good would be applied to keep out critters.

If you're doing a vented unconditioned crawl as suggested above, this is the method recommended by Building Science Corp.

To keep out animal pests, BSC recommends a fiber cement covering under the insulation.

Wouldn't the foam below the joists trap moisture inside the joist space? This one has always confused me when they always say one side should allow moisture vapor to escape. they use no vapor barrier under the sub floor, so the vapor migrates up and out?

There was no plastic barrier over the joists and under the subfloor. Our only thoughts on future use of the space was possibly to store vegetables and as emergency shelter in case of tornado since it's below ground. My goal is to make sure the PEX in-floor heat is radiating up, and not down to the crawl space, to make sure the plumbing down there doesn't freeze, and of course to avoid any mold buildup. I will definitely put down 6-mil plastic over the dirt. I just wasn't sure about the rest

Amy, ignore my suggestions, I really don't know what type of crawl space you have and don't want to give you wrong advice. Look into the link Worthy posted, it looks like good advice.

Your crawl space is 'below' ground?? What is the building code in your area?

Amy, Is this a house already built and lived in?

You have a bit of an unusual situation and I am trying to figure what you have now or are under construction. Because there is building code of course.

How much room in the floor joists below the heating system?

What does code say you need in the floor vs crawl wall? Even if code isn't an issue, it has at least some resemblance to common sense and financial impact.

In my area, we don't do both - either do the floor joists or do the walls. Generally sealing and doing the walls is preferable for many reasons. But the radiant floor may change that dynamic - in addition to your cold environment.

Insulate the walls and rim joist. I like foam boards like XPS or polyiso. 2 layers of 2" board should meet code in your area. Put polyethylene on the floor. It's much more effective insulating the walls than trying to insulate the floor and your crawl space stays warm. You should have a little ventilation in the crawl space to keep humidity down, but if you don't have that problem, see how it works without it.

Either approach is valid. A crawl space should have some ventilation. If you do the walls you should block off the vents to keep the cold out of the crawl space. Moisture build up in the crawl space can happen in the driest climates so provide some mechanical ventilation.

If feasible I like the rigid insulation nailed to the floor joists to drive the heat towards the house.

robin0919, The property is on a slight hill, so the crawl space is largely below ground. David Cary, the house is framed up with sheathing and a roof, so we are not living in it yet. This is new construction. The space in the floor joists is actually rather small; I read that if we insulate below the pex in-floor heat we should leave a 2" gap between that and any insulation, and we cannot do 6" insulation (R-19) with the size of the joists we have. We do not have building inspections where we are, just plumbing, electrical and septic inspections, though it looks like I need to go back and see what the code is in the general area.

I have to wonder why you did radiant if budget was an issue? Also have to wonder why you don't have a basement in the North? Where are your mechanicals? How tall is this crawl? The height of the crawl would help dictate the more economical solution. Obviously a tall crawl will cost more to insulated the walls.

I don't live in zone 6 but vented crawls have gone out of favor here in zone 3/4 even if technically allowed and done by tract builders.

R-19 would seem to me inadequate in a vented crawl in zone 6. That is our code in zone 4. And you can do it, you just need to use rigid foam - below the joists. While it isn't great, you could do r-13 between joists and a few inches of rigid foam below the joists.

Hi David, It's only a 4-foot crawl space. We tried to focus our budget on the structure and mechanicals (resale value down the road), because it's easier to replace cabinets and flooring than to do in-floor heat and 2x6 walls later. (We have a shop with in-floor heat that is attached to the house by an enclosed breezway. We went with in-floor heat and non-slab in the house for health reasons). However, unfortunately with the state of things right now (COVID) the cost of pretty much everything in our budget has gone up immensely so we are close to the cap of our budget. I want to do what is best for heating and without chancing moisture problems, but do have to keep the final budget in mind. Thanks for the suggestion.