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Hydronic radiant heat over concrete slab

John Rooney | Posted in General Questions on

I’m building an addition that partly includes an extension for an existing room off one side of my house here in Connecticut. Existing floor is 2 1/4″ x 3/4″ oak strip hardwood floor. Addition floor will be slab-above-grade with poly and rigid foam underneath the slab. Need to “marry” the existing oak floor (conventional wood subfloor/structure) to new concrete slab using new oak strip flooring. Because I’m building this addition myself, there will be more than sufficient time for the slab to dry (perhaps a year or more) before new finish flooring needs to be installed. I have read on this forum where Marin advises on how to lay down a floating double-plywood subfloor before nailing down the finished hardwood. I like this idea, but a few questions:
1. Given my long drying time for the slab, is it really necessary to install a poly vapor barrier on top of the slab under the subfloor? My concern would be trapping any moisture between the poly under the slab and the poly on top of the slab. Better to allow any changing seasonal moisture variations in the slab a way to escape upward?
2. I’d like to do a top-side hydronic radiant tubing install to compliment my existing underfloor retrofit radiant heating (same zone/low mass). Can I screw down my 3/4″ sleepers (with aluminum fins) on top of a single floating 1/2″ or 5/8″ layer of T&G plywood (or Avantec?) on top of the slab? The sleepers would be the second layer of plywood but would also accommodate my heating system requirement without building up the floor height unnecessarily too much.
3. Realizing that it’s generally recommended to install 15lb building felt between finished floor and subfloor, I assume it’s NOT recommended to do this under a radiant-heated floor due to odors when the heat is turned on? Should I use anything in this regard between the finish oak hardwood and the sleepers?
Thanks
John Rooney

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    There are lots of issues here: getting the elevation of the finish flooring on your addition to match the elevation of your existing flooring; using hardwood flooring over hydronic in-floor heat (which can be tricky); whether the PEX tubing belongs in your slab or above your slab; and whether the R-value of the insulation under the slab is adequate.

    The most important question is: Have your placed (poured) the new concrete slab yet?

  2. John Rooney | | #2

    No the slab hasn't been poured yet. I need to figure out my DWV plumbing first. Let me clarify a few things. Part of this addition - the room being extended - will be the only part of the whole addition where I'm considering a "top-side" radiant install so that the heat source is closer to the top with all the wood that will be required with hardwood over a slab. Rest of the addition will have in-tube radiant in mid-slab.
    As for sub-slab insulation, my plan is for 4 inches around the perimeter walls and 2 inches everywhere else. As for floor elevation, you're right, matching the existing floor elevation will be the trick, but I plan to pour the extension slab at a lower level than the rest of the addition slab separated by 2x6 flush to the higher elevation. This lower slab level gives me the ability to build up with plywood for subfloor and radiant sleepers.
    I realize this is very non-conventional. Just hoping someone has gone through this and has a little experience enough to advise.
    thanks

  3. John Rooney | | #3

    Sorry, I'm new to this forum and just realized that I posted to the wrong place.
    No the slab hasn't been poured yet. I need to figure out my DWV plumbing first. Let me clarify a few things. Part of this addition - the room being extended - will be the only part of the whole addition where I'm considering a "top-side" radiant install so that the heat source is closer to the top with all the wood that will be required with hardwood over a slab. Rest of the addition will have in-tube radiant in mid-slab.
    As for sub-slab insulation, my plan is for 4 inches around the perimeter walls and 2 inches everywhere else. As for floor elevation, you're right, matching the existing floor elevation will be the trick, but I plan to pour the extension slab at a lower level than the rest of the addition slab separated by 2x6 flush to the higher elevation. This lower slab level gives me the ability to build up with plywood for subfloor and radiant sleepers.
    I realize this is very non-conventional. Just hoping someone has gone through this and has a little experience enough to advise.
    thanks

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