GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Staple-up insulation

rhl_ | Posted in General Questions on

I’m working on my first floor “staple up” hydronic job. I’m using thermofin C plates. They recommend a 5/8” air gap between the plate and the insulation.

the question is, with an old house. Is approximating this with batt and insulation the best way to get the air gap and achieve desired level of insulation? 

too big of an air gap degrades performance of the floor heat and is less insulation. 

after I get the floor heat + new electrical done I’m going to air seal before I insulate and use an intello or equiv vapor open product to air seal again then put up the drywall ceilings.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Brian Pontolilo | | #1

    Hey Ryan.

    How deep are the floor joists and what is the goal for R-value?

  2. rhl_ | | #2

    its a ~ 100 yr old house, the beams are ~10" deep but its 10.5" in some places and 9.5" in other places. "As much as possible" is the answer, but, its less about total-R and more about how do you actually achieve this level of detail on the air gap between the stapled up tube and the insulation?

    So to lay it out, you have a Thermofin C plate: which has a height of (5/8"+5/80") at the top of the joist bay, plus an idealized 5/8" airgap, meaning you want the insulation to begin about: 1.3125" off the subfloor. Then you want to fill up the remaining cavity with insulation.

    If you leave too much of an airgap you dont heat the floor, as the heat goes down through the air, if you put the batt in contact with the pipe, you heat the insulation, and not the floor.

    It's a bit forgiving, the thermofin experiments show a 2" airgap is only a bit worse than a 5/8" air gap, but, if you want to nail 5/8" can you do it?

  3. Expert Member
    Akos | | #3

    The air gap is there to reduce striping on the floor. You won't loose heat to the insulation if close. As long as you have some gap, it should work.

    I've found with heat transfer plates that you get a bit too much striping on tile with a single run in each joist bay though. With double row heat plate you also don't need any insulation gap.

    The important part is to seal up the end of this insulation gap before you get under our exterior walls. Even with well sealed rim joists, there is usually a lot of air leaks in this area and you can loose a lot of the floor heat efficiency.

    With the thermal bridging of dimensional lumber, there is very little assembly R value gained by insulating to the max. I would just go with 2x8 batts and a layer of taped rigid underneath.

    If you must have your idea gap, you can staple up some housewrap to 2x2s nailed to the side of the joists. This is a lot of work for infinitesimal efficiency improvement.

    P.S. If this is over a conditioned basement, then you don't need crazy insulation. R14 mineral wool batts are more than enough to provide zone isolation and they will stay put from friction without any extra support.

  4. rhl_ | | #4

    Thanks Akos. Yeah, plan is for around ~5" of closed cell HFO foam on the rim joists to the exterior walls, and on the walls which run parallel with the joists the floor is stopping 1 joist bay in from the wall, to save my own sanity while installing. I have done two runs in PEX-AL-PEX, and, i'm now quite fed up and ready to switch to hePEX.

    When the pipes are perpendicular, actually the loops are often stopping short of the exterior wall, the plates may go a bit further. It's actually just too difficult to get in those areas.

    The plates are almost entirely doubled up per joist run, except where space doesn't permit.

    WRT the rigid foam, is that safe? will there be any moisture issues?

  5. Expert Member
    Akos | | #5

    Pex-Al-Pex is problematic enough for straight runs, trying to fish it between joist bays is not fun. My new favorite is PERT pipe. More flexible than most PEX.

    If you are running double row of plates, I can't see what benefit if any the gap will get you. Simplify your life and push the insulation tight against the underside.

    I can't think if any assembly where a layer of rigid under the floor would cause any issues. It always works. Best way to insulate overhangs.

    Having said that, the best location of rigid is on basement/crawlspace walls. Unless there is a good reason to go unconditioned, you get more energy savings by insulating the perimeter and only installing minimal thickness batts to the floors.

  6. rhl_ | | #6


    I’m going to use 2x8 batt insulation, I think it’s safer to have an air gap per manufacturer instructions.

    I guess the layer of rigid would prevent drying of any moisture under the floor. I was planning to use an intello like product as an air barrier, but rigid would also do well.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |