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Community and Q&A

Radiant Heat Overlay on Existing Slab-on-Grade

FishmanArch | Posted in GBA Pro Help on

I am converting an existing unconditioned garage with a concrete slab-on-grade to a conditioned mudroom with hydronic radiant floor heating and stone tile. The top of slab is approximately 7″ below the planned finish floor elevation. The project is in Pittsburgh, PA, and my main concerns are tile cracking, moisture management, and radiant heat efficacy.

My approach is to add the following layers, from bottom to top: One-coat epoxy resin system for moisture management, adhered rigid insulation (2″+) for insulation and to build up the floor level, plywood layer for attachment of PEX tubing, ~2″ gypcrete slab for encasement of tubing, floating/uncoupling membrane, thinset, tile.

Am I missing something? Is any of this overkill? Simpler way to do this?

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  1. Expert Member
    Akos | | #1

    The preformed EPS panels for radiant tubing tends to be the easiest to use, since it is not a large space, it won't cost a lot and save a bunch of labor. If you have the height, you can add a layer of rigid under the panels for extra R value.

    I would then pour a standard concrete slab over this to get it level from the sloped garage floor and up to the height you want. Tile can go right over this but a roll on decoupling membrane doesn't hurt.

    Sometimes for smaller spaces, resistance floor heat is simpler and doesn't cost a whole lot more to run. You can lay down the rigid and install tapered sleepers and plywood over it, something like the Schluter system can be installed over plywood and tile right on top. The benefit of this setup is that is much more response, you can put it on a thermostat so it only runs when needed.

  2. mr_reference_Hugh | | #2

    1. tile cracking
    I would speak to a person who does tile for a living exclusively. I would have concerns myself because of the foam insulation layer possible not sitting firm/flat on the existing concrete. I would get a high compressive strength foam to avoid cracking but this might be overkill given that the load on the floor will be relatively low.

    You could contact this Youtuber called the TileCoach if you don't have anyone you can reach out to. I really like his videos.

    Youtube channel

    This link is to his page where he provides the cost for various types of consultations.

    2. moisture management

    2 A) One-coat epoxy resin system for moisture management

    Be extra extra careful on the install to ensure longevity. I don't work with concrete but my understanding is that it can be very challenging to cover concrete and have the material stick over the long haul. Here is a Google search result for "epoxy resin peeling off concrete floor". It explains lots of the pit falls.

    2 B)) Moisture and plywood
    You don't need plywood to attach the piping! Just put down welded wire mesh (check with your gypcrete installer first).

    Consider what is said in this paper:
    "Poured gypsum will almost always have embedded reinforcing of some type. Welded
    wire mesh (Figure 2) is the more common reinforcement,"

    Besides, plywood is organic and will easily support mold. If you insisted on having a "board" type material above the insulation, then look at various paperless gypsum products. There are many on the market. Again, ask the gypcrete contractor or the tile contractor.

    3. radiant heat efficacy
    The efficiency will depend on whether the amount of insulation under the gypcrete and how you heat the liquid passing through the pipes. You might need to choose an antifreeze liquid in the pipes because the edge of your floor will abut a cold space (or maybe it does not). I was reading that the antifreeze liquids are less efficient because they can't heat up as much. If you don't already have a boiler (or equivalent), then this would be a very costly project for a small room.

    I know people who swear by their radiant floor in the bathroom. Sure, they crank up the heat to get the floor to be really warm and quickly warm their small bathroom. A family member lives in a house with 2300 sq feet of heated floors that I insisted on having installed and it is nothing to write home about. There is R10 under the slab in the basement but it is simply not enough. Unless the situation dictated that I needed heated floors, I would not do it again.

    Consider that one study found that "mean +/- SD awake temperature of 80.1 F..." for human feet.

    To feel the warmth under your feet, you would need to have your floor tile above 80.1F. But because you don't know when you are going into the mud room you would need to always keep the tile above 80.1F. The room would heat up pretty fast. But feel the warmth would not give much because we don't spend all that much time in the mud room.

    To back this up, you can read this GBA article. Skip to the section where they ask "Why isn’t my floor warm?"

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