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Question about insulating under Warmboard-S on 2nd floor

jb9 Member Posts: 104

I am aware that it is important to use insulation (EPS perhaps) underneath WB. I definitely plan to insulate underneath the 1st floor, but my question is about the 2nd floor. I am going to have exposed (timber) floor joists on the first floor ceiling so I am curious how to approach this. Can I use EPS inside the joist bays and then cover it with sheetrock? Since the 1st floor is a heated space, does there need to be insulation under the 2nd floor WB? I am hoping some of the pros here have seen this detail on a job site.

Thanks in advance.


  • Rich_49
    Rich_49 Member Posts: 2,624
    edited March 2016
    Short answer is , YES , ALL RADIANT applications require
    R esistance on the opposite side of the space being heated . No ifs , ands or buts about it
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • SWEI
    SWEI Member Posts: 7,356
    Slightly less short answer: You can use blown or batt insulation in the joist bays below it. If the ground floor is slab-on-grade, XPS is the best option.
  • rick in Alaska
    rick in Alaska Member Posts: 1,244
    You definitely have to insulate under the second floor as the others have said, otherwise the radiant heat from the second floor will want to also radiate down to the first floor, and overheat it.
    Since you said in your description that you are having exposed floor joists, then your idea of using eps and covering it with sheetrock is most likely your best bet.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    A basic rule of thumb is 2 times the R value below that heat source as will be above it. Wood has an R value of roughly 1 per inch.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • jb9
    jb9 Member Posts: 104
    Thanks for the recommendations. If I do the EPS and then cover it with sheetrock, does one just use longer screws or is the EPS glued to the underside of the WB and then the sheetrock is screwed to horizontal strapping on the EPS?
  • rick in Alaska
    rick in Alaska Member Posts: 1,244
    Don't know if I would trust the glue with that weight, not alone the fact you would have to have some way to support it until it dried. I would go with long screw all the way through the EPS and the sheetrock. BUT! you have to make sure they are not long enough to penetrate the floor and hit the tubing. Furring strip also work, but then you still have to worry about the screw length, so just do it in one shot.
  • jb9
    jb9 Member Posts: 104
    Good suggestion Rick. I totally agree about how critical it will be to spec'ing out the correct length fasteners. I will probably do all this particular drywall myself just to be sure. And I suppose I could even do this particular drywall before laying the 2nd floor Pex. When I used the term horizontal strapping, I am assuming that is synonymous with furring strip. I feel like there is probably an established best approach to doing drywall over EPS over WB... I'll probably use a drywall stand an transfer all the tubing lines on WB to the drywall.