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.

Pex bottom of slab vs center

Sukhoi29SU
Sukhoi29SU Member Posts: 77
4 in. concrete slab planned in home with 2”XPS foam underneath, as well as perimeter insulation.

How important is it to lift the pex into the center of the slab? Vs staple it to the foam?

I’ve read mixed opinions, and most recently the local plumber who seems experienced with radiant and might take on my project told me not to waste my money on lifting pex into slab- and that it will perform perfectly well stapled to the XPS foam.

I understand floor might be less responsive to temperature changes. I’m not overly concerned about that as much as I am losing a lot of performance in the radiant application.

Any data available regarding the performance of one vs the other?

Thanks in advance!

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,010
    The ones I have helped with had mesh on the foam and pex attached to the mesh with ty straps. Others with more radiant experience will respond
  • Gordy
    Gordy Member Posts: 9,514
    Personally I think it’s splitting hairs with a 4” slab.

    What I see beneficial is that the stripping will be less defined than if it were 2” from the top. However you lose a bit of output.

    There is data to mull over.

    https://www.hpacmag.com/features/hydronics-radiant-floor-tubing-depth-siegenthaler/





    BrewbeerGroundUpSukhoi29SU
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 77
    edited March 2019
    That article gave the exact info that I was looking for.

    In order to understand the significance of those numbers for my application, I’d need to determine how much efficiency I would lose with my wood boiler. I.e. how much more wood would I need to burn or , with batch burns, how the required time between burns would be reduced in order to get the higher water temps through the pex to achieve the desired temp.

    If I went from a burn every 48 hours down to 36, for instance - just to achieve the same output because I decided to put the pex in the bottom of slab on top of the insulation vs 1/2 way into the slab- that would be a significant consideration. With a natural gas boiler the difference might be negligible.

    Interesting stuff..

    Thanks for the link.
  • Gordy
    Gordy Member Posts: 9,514
    You really won’t know where you stand with out a heat loss to run the numbers at. From there it is btuh need verses output. Which is a floating number through out the season as outdoor temps change.


    With a program you can run your heat loss numbers at different outdoor temps to see the load variations through out the season. That will help put into perspective how much fuel consumption would vary.
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 77
    One thing I’m confident about is that regardless of whether the pex is at the bottom or center of the slab, it’s going to heat the house so much better than what I had before ...

    I’m looking forward to it!
  • nibs
    nibs Member Posts: 470
    With not much run time we to give a base line. We stapled our pex to the foam, probably r20+/- most of our slab is 2" but some is 3" and in one room 5", I cannot tell any difference. There must be some efficiency loss, but I cannot tell. With XPS foam if you use high strength concrete with fiber you could likely pour a 3" slab with no problems at all, talk to your cement supplier.
  • Sukhoi29SU
    Sukhoi29SU Member Posts: 77
    @nibs thanks for the response. How is your 2” slab holding up? I’m planning on a suspended concrete floor for the main floor - after beefing up floor joists, and was initially wondering what the minimum slab thickness was. I was planning on 2.5 inches after reading 2 was min for radiant application. What sort of high strength mix did you use for 2” slab??
  • nibs
    nibs Member Posts: 470
    Mix design was 1part cement, 2 sharp sand, 2 gravel, by carefully measured volume (not shovels full) fiber and super plasticizer to minimise water content. If we had fly ash, would have added 1/4 part to the mix. Site mixed with barrel mixer, hand carried to protect pex when required. Control joints cast in and pex sleeved at joints. Made our own staple pusher 2x4 with a half hole to fit staple on end, cut 8" long to act as measuring stick while fastening pex, I prefer the more flexible fiber but either kind works well, we find if we screed carefully and tamp well down after steel trowel no fiber shows on surface.
    One shrinkage crack due to being short one bucket of cement and forgetting the fiber at that one spot.
    Sukhoi29SU