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Basement Slab Best Practices Question

jb9 Member Posts: 104

I am currently working on a radiant design for my home and have some questions about how to run radiant in a basement. From what I understand, there are a lot of stories where folks experience regret that they didn't install tubing in their slab at the pour. I don't have immediate plans to finish the basement as the bulk of my efforts will be towards the house, which will be 2 stories. Does it make sense to spend the money on a roll (or 2) of pex-al-pex and run the tubing even if my basement will initially only contain my mechanical systems? Also, what are the basic "best practices" guidelines for the slab design? From what I have read, I need to make sure that I have a vapor barrier and then a layer of (rigid, I assume) polystyrene under the slab. How thick is recommended for the insulation? 2", 3"? Also, is it good practice to also have a layer of polystyrene on the perimeter of the floor slab running INSIDE the stem wall (in addition to another layer of insulation on the OUTSIDE of the stem wall)? How deep is optimal for the tubing to be placed? I know these questions are perhaps relevant to a foundation contractor, but I figured there are folks here who have been involved in designing thermal mass type systems.

I am hoping that I keep the basement at 55 or warmer with insulation and no heat, but I wonder if I am introducing a new load that I should incorporate into my boiler choice. Again, it will mostly be for storage and mechanical systems (DHW, HRV, Boiler, etc).

Thanks in advance.


  • Jamie Hall
    Jamie Hall Member Posts: 18,923
    Ideally the radiant tubing will be mid slab to slightly above the middle -- and therefore should be placed when the slab is, paying close attention to having it supported properly so it stays where you want it (concrete contractors sometimes get a little casual on this...). I'd recommend using fiber reinforced concrete, as well as the usual metal reinforcing. A little more expensive, but no cracks is a big plus!

    As to insulation and vapour barriers. You do need well compacted sand as the bottom layer. That should have a vapour barrier on top of it, and the vapour barrier should be continuous through the junction between the frost wall footings (and any interior footings). This is an often overlooked detail. Then I'd use at least 2" of polyurethane rigid foam on top of that -- 4" better. I'm not particularly convinced by an interior insulation break between the underslab foam and the frost walls, although there does have to be an expansion joint there. You certainly want insulation on the outside of the frost walls, though, right down to the footings.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England