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Community and Q&A

Self-Leveling Underlayment for Hydronic Radiant Floors

bucksbear | Posted in General Questions on

Hello GBA community – I’m a homeowner in Western MA working with contractors to get a hydronic radiant floor installed in our kitchen that is being renovated. We have new 3/4” AdvanTech subflooring but only realized how uneven the floor is after putting in the cabinets – we wanted the cabinets in before the finished floor and with no hydronic under them for overall height reasons. Across our longest run of cabinets (about 12’) there is a 2.5” difference in the floor (which is mostly flat, but definitely not level!). Our flooring plan is engineered hardwood over Viega Climate Panels (1/2” tubes with 7” spacing). After finding out about the level difference, I have been trying to figure out what our options are, and at this point, I’m resigned to either (1) let it go and deal with a very slant floor and wildly different toekick heights or (2) pour self-leveling underlayment. With the SLU, I wouldn’t be expecting to get fully level, as we cannot afford the 2.5 inches because of a full-height cabinet – we can max out at about 1 inch additional on the floor at the lowest point. Also, 2.5” of SLU would be a much bigger job requiring a mesh, if I understand correctly.

At this point, our plumber who is installing the hydronic system told us that we can’t do the SLU under the Climate Panels because he cannot float the Climate Panels and the SLU does not accept nails (which is true), so the other option is SLU over the Climate Panels, which would force the wood floor (either over that section or entirely?) to be floated or glued. The SLU product I have been looking at is Mapei Ultraplan Easy, which supposedly works well over wood if the correct primer is used and can feather down to 1/16” thick, and I estimate 12 50# bags would be needed to cover the ~110 s.f. that I estimate as low at a 3/4” depth.

I’m far out of my expertise here. Questions for you all:

– Is SLU my best option here or what else could I use? Is it not worth the hassle to partially level the floor?
– If we do go forward with the SLU, is using it over part of our floor (110 s.f. out of 450 s.f. in the room total) going to make the floor heat feel uneven? I imagine the concrete will hold the heat better than the wood.

Gotta love old houses. Thanks for any suggestions.

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  1. DC_Contrarian_ | | #1

    The SLU over the panels is going to defeat one of the major advantages of the Climate Panels, which is that they have low heat capacity. That means they heat up and cool down more quickly, which makes them more responsive to the thermostat and gives more even heating.

    I'd be thinking of taking the base cabinets out, taking the Advantech out, shimming the joists and doing it over. Is the Advantech nailed or screwed? Is it glued down? If it's not glued and is screwed it's not a big deal to get it up.

    I also think that if you leave in an inch-plus of slope it will still be noticeable.

  2. Expert Member
    Akos | | #2

    I'm with DC on this, take out the subfloor, shim and level. For a space that size, this would be about 1/2 day job, even with labor less than the cost of 30 or so bags of SLC you need. Ugly, but you can cut the subfloor at the cabinets and leave them in.

    I would take most of the full slope out unless there is a very good reason for it to slope.

    1. DC_Contrarian_ | | #3

      Bucksbear wrote: "With the SLU, I wouldn’t be expecting to get fully level, as we cannot afford the 2.5 inches because of a full-height cabinet – we can max out at about 1 inch additional on the floor at the lowest point."

      I'd have to hear more about that.

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