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Fixing old slab: Radiant wood floor ideas.

TAGTAG Posts: 103Member
My new project has an existing 1700sf barn/ utility building on the property. It's two buildings .. the older part is a 24'w x 32'L garage from the 60's with a rough poured sloped slab. Block foundation -- old slab is level to the top of the block in the back and maybe 4" down at the front where the main garage door was. The previous owner attached a new larger SIP building to the old garage back in the 90's -- remaking the outside to look like an old barn. The buildings are open to each other -- the large 8x8 hinged garage door is now on the newer building.

My plan was to make the older part into a studio/workshop -- ideally with a heated wood floor. Got a price: around 15k a few years ago to properly take out the old slab -- Prep/ VB/ Insulate/ PEX. That still leaves the installation of some type of wood floor over the heated slab ... really can't spend 20k

Back in the mid 00's I added a 16 x 22 addition to my house - block/ slab on grade. Took PT 2x4's or 6's on the flats w/ 1.5" insulation between -- Warmboard and 3/4 T&G on top -- tied it into the existing radiant system. Works great.

I'm wondering if anyone has ever tried to retrofit a wood floor and include radiant?

I know nothing about over-pouring concrete. But -- by leveling the old slab I could basically put down a plastic VB and build on top. Again using 2x6 on the flats. The property is in PA and we have local mills - 1.5" thick long length wide hardwood is available that people use (face nail) in studios and man caves -- is't cheaper (and easier) vs putting down plywood and 3/4 T &G . The problem would be the radiant -- if I only put down 1" foam between the sleepers -- place PEX 3/8 tubing on top of the foam. What do you think I would get as far as BTU's per foot?

The old building has no windows -- one 32" typical door. The other building being SIP -- its like a cooler. Because of the construction and loss of propane -- I'm currently heating the whole thing with a 4k 240v electric heater. I'm using Warmboard in the main house -- that would add 6k.

I can't put this off too long because by summer time the ability to get equipment and cement truck to back of property will be problematic.

Comments

  • hot_rodhot_rod Posts: 11,259Member
    The output would be better and at lower SWT if you can get some conduction, aluminum transfer plates around the tube somehow.

    You would need to deduct all the area that the 2X6 on the flat take away from the available square footage of the radiant installation, maybe 2 tubes between each sleeper.

    Even the suspended bare tube systems could get you low 20 BTU/ sq.ft if you ran them hot enough. The limit is the temperature the wood can tolerate without stress.

    Ideally knowing the BTU load for the room would help determine how workable. Without any, or very little insulation below, you pay in operating costs.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GordyGordy Posts: 9,264Member
    Does the existing concrete floor sweat at all in the summer?

    Is the slope the 24’ dimension?

    If it is rip PT 2x stock wedges from nothing to 4” basically frame the floor over existing. Pinch nail blocks to the concrete floor every 4’ along each joist so you can nail it down firmly to the existing floor. Insulate, and do a sleeper system over the top. Color it done.
  • hot_rodhot_rod Posts: 11,259Member
    Construction adhesive and Tapcons is another fastening method for sleepers to concrete, shim as needed.

    The adhesive helps, should a fastener ever work loose, you still have a tight bond to the slab.

    With treated lumber, get it down quickly, it seems to twist and cup more than non-treated lumber as it lies around on the job site.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • TAGTAG Posts: 103Member
    It's a rough poured slab -- I have no idea how thick or what's under it. My guess - they graded it from back to front .. let the concrete fly. There are no expansion joints -- it's cracked but not a loose mess .. it's not a smooth flat plane in any direction.

    Have tried to think how to make a rafter system - one thought was to lose some height and place 2x4's on top of the block foundation and uses 2x4 down to the old slab for support -- maybe Sprayfoam and then the floor. I would only lose height at the outside walls as the building has cathedral ceilings. The side wall in that building are 8' the newer building has 10' sidewalls.

    The old slab is not wet -- its an extremely tight building and I do need to have a dehumidifier running. I'm sure some moisture comes through ..... I also have no idea how the newer buildings slab was done.

    When I did the addition w/ Warmboard over the slab I made sure I did a proper VB under the slab. That way I could tack the 2x with nails -- but also use proper adhesive to hold the PT lumber to the slab. We also watched for any real wet lumber -- I wanted the Warmboard down to hold it all together.


    To do the space would require about 25 made up supported 2x4 rafters -- each 24' wide. Not sure how much that would cost vs doing an over pour. Trimming and cutting the rafters would be time consuming and could be problematic as they got thinner and thinner.

  • hot_rodhot_rod Posts: 11,259Member
    I have poured gypcrete over old slab that are real wavy, that would at least give you a level surface to start with. Sleepers or Warmboard dry system over that
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • TAGTAG Posts: 103Member
    have never used -- guess it's self leveling ?

  • GordyGordy Posts: 9,264Member
    edited February 27
    If it’s just going to be a work shop/studio with out hard loading I’d do that. Put up a header on the low side to match the high end elevation, or 1” or 2 higher fill it up done. Could even use a DS mix which has small chips for aggregate.

  • hot_rodhot_rod Posts: 11,259Member
    TAG said:

    have never used -- guess it's self leveling ?

    Yes it self levels, that was one of the main selling features. Originally designed for fire and soundproofing in hotels, etc.

    Back in the early 90's Infloor promoted it as an ideal radiant system, it can feather down to 1/4" or less and does not require a lot of depth over the tube. We did a lot of them back in the day over framed floors, stapled 1/2" tube and poured a 1-1/2" gyp over it. Double plate all the walls so you have a screed depth and also nailing for wood baseboard :)

    Mostly engineered hardwoods or carpet over the top.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • TAGTAG Posts: 103Member
    VB and then over-pour would be ideal for building the floor -- I don't know how important it is for the old and new slab to be in contact. My guess is not having the VB between would be better.

    If I can't have the VB under the new slab .... I have to deal with the VB on top of the slab and I can't glue down the flat side sleepers to it. Start shooting nails into a thin slab does not should like a good idea.

    I need the VB.

    As I mentioned -- I can heat the whole space with about 15k BTU and this is only half. While I would like to use Warmbaord -- the added 6k may not be practical.
  • GordyGordy Posts: 9,264Member
    edited February 28
    For fastening sleepers use tap cons, or wire nail. Use a hammer drill with a 3/16” masonary bit put a piece of tie wire in the hole, and drive a 16 penny nail in it. Holds great, and is cheaper than tap cons.
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