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New PEX loop to garage: Add Thin Slab or Add Subfloor ?

Hello Everyone! I am a long time Dan Holohan fan, from way back in the early 2000s’ when I got my hands on his Antique Steam Radiator ID and Steam system books. The Wall (website design) was a lot more basic, but just as informative back then.

I am moving into a 1972 construction house which was built with a two car garage on the ground floor, but the PO never installed a garage door! Boston suburb ...imagine the heat loss :(

I have a potential radiant floor loop design to consider in my garage (and have to do this before the garage door size is finalized). The house is presently heated with an oil fired forced air furnace. The garage floor area is about 340 ft. sq. There are no underfloor water issues; the house sits at the top of a hill.

I have a 30 tube thermal solar setup that I will be installing for the DHW. The radiant loop in the garage will be there to absorb the excess heat, when generated by the solar system. I’d like the garage slab loop to be potentially useful as a heat source zone (or at least some useful thermal mass) and may convert more of the house to radiant heat.

As I see it, I have one obvious and one less obvious way to add a hydronic loop (1) an additional thin slab pour with an embedded pex or (2) a plywood subfloor in the garage with pex under and some HD floor surfacing to accommodate the cars and an occasional floor jack.

Thin Slab - Design Considerations:
  • Minimum additional thin slab thickness: I can raise the slab by up to 5,” but would prefer to minimize the elevation to save the headroom.
  • Vapor barrier between the existing and new slabs?
  • Thermal barrier between the existing and new slabs?

Subfloor - Design Considerations:

I know next to nothing about the plywood option, except that I think I’ve seen the routed plywood or composite panels with pex channels. If this is an option here then the design questions are:
  • Suitable in the residential garage floor setting?
  • Vapor barrier below?
  • Thermal insulation between the slab and subfloor: yes or no?
  • yes - to prevent heat loss into the slab below, but then no thermal mass
  • no - no insulation, then thermal mass for storage might be possible
  • Potential material stack:
  • Vapor barrier
  • Thermal insulation
  • Pex panel
  • ¾” Plywood Layer 1
  • ¾” Plywood Layer 2
  • Vinyl Tiling suitable for light garage use
  • Seems to me that I can get away with about as little as 3” total additional thickness
  • The plywood/PEX subfloor would start 20” inboard of the garage door with a short piece of concrete (new pour) extending from the subfloor and outside for an additional shoulder in front of the garage.
There is a lot I don’t know ...except that it’s a long way from theory to practice. What do you guys think of these ideas?


  • acwagner
    acwagner Member Posts: 505
    Regarding the subfloor option, probably best to contact the manufacturer of a plywood/composite PEX panels and see if they rate them for garage use, or if they have recommended assemblies for garages. Passenger vehicle garages floors are required to withstand a concentrated load of 3,000 pounds over a 4.5" x 4.5" area.

    Also, have you already considered option 3--removing the existing slab and re-pouring?
    Burnham IN5PVNI Boiler, Single Pipe with 290 EDR
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  • hot_rod
    hot_rod Member Posts: 16,312
    Radiant ceilings are nice in garages also, easier to install often, and less "stuff" covering them so you have a lot of surface area as you radiant panel.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • MichaelRent
    MichaelRent Member Posts: 5
    Thank you for the tips! I think I outpaced myself with this a bit.

    I am now doing a full heat loss and hydronic loop calculation to get a baseline. I am still hoping to pour an additional thin slab (how thin might depend in part on tube spacing). I considered repouring the entire garage floor, but would rather not if a thin addon is a viable option.

    I do have about 5” to 6” of additional headroom to raise the garage door height and still be over 7’ so the “thin” slab can be 4” or 5” thick, if need be. I’ll talk with cement guys around here for clarity on this.
  • nibs
    nibs Member Posts: 470
    If the slab is not insulated, you need a layer of insulation, EPS can be had in quite high densities, and as long as you are not driving a tank or large truck, a 4 inch high strength slab with en trained fiber should be strong enough, You might want to run it by a structural engineer. Be sure to place not cut your control joints, and sleeve the pex where it goes under them. Cure the cement properly, and give it a month before heating the floor, or you may get edge curl and some cracking.
    I would not hesitate to staple the pex to the foam insulation and pour over.
  • hot_rod
    hot_rod Member Posts: 16,312
    At least a 2" insulation, and 2 or more around the edges, plus a 4" slab works out to the 6" you have to give up. The front edge will be a tough detail to insulate and cost you some loss.

    Regular 250, (25 psi) Dowboard, Owens, etc is fine for residential under slab garages. Even the 150 15 psi is adequate. I'd stay away from the white beadboard, it soaks water.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream