Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Air space between sub floor and finished floor

Jay_22 Member Posts: 14
Thanks for the info. I thought that the air gap would create a greater R value or that the bottom side of the plywood would reflect the heat back down which it may, but if there is enough insulation under the tube it will drive it up ?
I have another question. How do you know whether to put your tubing 16"cc or use 1/2" tube as to larger tube ?
I am going to order one of Dan's books on radiant but I need to know something before the book arrives.


  • Jay_22
    Jay_22 Member Posts: 14

    I am looking at a radiant job to bid. The owner wants a staple up job. The floor is not level, he wants to use sleepers to level the finished floor. I need to know how much that will reduce the efficiency of my tubing.So, I will staple to 3/4 inch old board floor then there will be from 1/4" to 1 1/2" dead air space then the plywood and finished floor, I'm not sure what the finished floor will be at this point. I do know enough to insulate well below the tubing to drive the heat up but what happens with the heat when it hits the bottom of the plywood and say carpet or hardwood over the plywood. It seems that we have lost the conductive value ????

    Thanks in advance
  • Brad White_203
    Brad White_203 Member Posts: 506
    That air space

    Jay, that air space will add (if captive and held by normal building materials) an additional R value of about 0.97.

    Say your old board floor is 3/4" softwood (to which the tubing is attached), that has an R of 0.94, then the air space at 0.97, then plywood, (say another 3/4" AC softwood at 0.94 and then maybe a layer of rosin paper at 0.06 then 3/4" of finished hardwood at 0.68.

    I do not discount the sleepers for they and the air space have about the same R value you see...

    (Still with me? Jay? Jay? Wake up now...:)

    I get a total R value of the floor assembly of 3.59.

    (This does not include the interior air film coefficient of 0.68 but then I do not count that for radiant because it is about surface temperature.)

    If you want a surface temperature of 80 degrees (24 BTUH per SF in a 68 degree room) your water temperature will be over 155 degrees...

  • Phil_17
    Phil_17 Member Posts: 178
    Floor buildup

    Since you're already building up the floor, perhaps it would be best to look at using quicktrak, roth panels or warmboard instead of putting the tubing underneath? That will make for a much better system.

    More insulation underneath is generally a good thing, but it won't do anything to overcome the high R value you have on top: you are still looking at really high water temps...
  • jp_2
    jp_2 Member Posts: 1,935
    couple answers

    1.) the plywood ready will not reflect heat back down.

    2.) insulation really doesn't drive heat, it slows heat transfer, maybe a minor play with words.

    did you do a heatloss on the room or rooms you plan to heat?

    larger tubing can transfer more heat but doesn't conduct much more heat to the floor. -very,very small amount more.

    the customer may want simple stable up, that doesn't mean it will work and you'll end up getting burned.
  • kpc_40
    kpc_40 Member Posts: 62
    why not do an overpour?

  • tim smith
    tim smith Member Posts: 2,774
    It sounds like they do not have enough floor elevation for

    overpour or quicktrak, he said the floor leveling for finish floor was between .25 & 1.5" that would not provide enough floor leveling and the above sub floor methods. That's what I got out of it? Tim
This discussion has been closed.