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raised floor question

I have a situation where i need to raise the floor 8 inches.  there is an existing 4' slab with no insulation under it.  what is the best way to come up that 8 inches (there will be tubing in a 2 inch slab at the top)?  6 inches of rigid foam under the new slab? to much? too expensive?  4 inches of foam and a 4 inch slab with embedded tubing? any advice or suggestions please....


  • ZmanZman Posts: 2,534Member ✭✭✭✭
    4 and 4

    Ideally I like 6 and 2. I think you will need 3 if not 4 of concrete to prevent cracking. I assume there is ground under all this? Be sure to space the tubes so they are in the center of the slab.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • thanks

    yes, the existing slab is on the ground.  So I could put down sleepers and plywood and pour on that, but I am assuming insulation is better than an air space and that the additional cost will be won back over time...?
  • SWEISWEI Posts: 5,554Member ✭✭✭✭
    air space

    Will resonate like crazy when you walk on it.  Insulate.
  • McMasterMcMaster Posts: 28Member

    [quote] Be sure to space the tubes so they are in the center of the slab.


    Truer words have never been spoken
  • BobAlsBobAls Posts: 7Member
    How big is your building and where is it located?

    The further north you are the more important the insulation becomes. Is it a large commercial building or small house? If it's a small basement slab, the extra cost of more XPS won't kill you. Also, very important: Do Not forget to insulate the slab edges from the walls (if present).

    Also, do you have adequate drainage in the present basement? If not, you could use some of the available space under the foam layer to add crushed rock and drainage piping to drain off any unwanted water. Don't forget the poly layer.

  • thanks!

    Thanks to all.   It is a 3000 sf footprint.  residential application.  existing slab is fine but slopes to a drain.  what would be a minimum amount of insulation if I used crushed rock as a base/leveler, then xps, then slab? 2" stone, 3" insulation, 3"slab? agreed on the insulation around the perimeter.
  • BobAlsBobAls Posts: 7Member
    Utilize existing drain...

    If you integrate the existing drain by sloping towards it and carefully not blocking off anything you could be fine. Is the basement dry at all times? If it's not dry, now is the time to fix it.

    2 inch should be your minimum for the foam (XPS).

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