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Insulation for I-joists with hydronic radiant system

diyer01 | Posted in General Questions on

We have installed staple up aluminum radiant heat transfer plates to 3/4” Advantec subfloor in our basement. There are engineered I Joists, 16” on center. We stapled kraft faced aluminum reflective barrier (Radiantec brand) 1” below the Pex tubing, kraft faced side down. What insulation is recommended below the reflective barrier. We are thinking of using Kraft faced R-19. If this is appropriate would kraft face be installed facing cement floor? The basement is heated. It will not be a finished basement. We live in Maine-  Zone 6.

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  1. Expert Member
    Akos | | #1

    The reflective barrier is not really doing that much, if you have installed already, it doesn't hurt, if not, I would skip it, not worth the money/time. With radiant transfer plates, the best is to have batts snug right against the underside of the floor.

    For batts, you can pretty much use anything. R11 is more than enough, even R8 rolls would work.

  2. diyer01 | | #2

    The reflective barrier is already installed. What are your thoughts re: using kraft or foil faced batts?

  3. Jon R | | #3

    Be aware of the fire implications of faced fiberglass. Use R11, R13 or R19 - whatever is cheapest.

    The gap is beneficial in that it slightly reduces temperature variation on the floor.

    1. Expert Member
      Akos | | #4

      Not quite, I've tried it.

      With a single heat transfer plate strip per joist bay, you'll get striping no matter the insulation gap. With two strips per joist bay, there be no striping no matter insulation location. About the only way to get around striping with a single run is to suspend the pex+plate bellow the floor and use it as small rad. In that case you do need a gap between the plate and the insulation.

      The reason you don't want a gap with heat transfer plates is that it creates a nice air channel. Unless your rim joists are well sealed up, you can end up loosing a lot of heat to air leaks.

      Even with the radiant barrier, I would stop the radiant barrier about a foot from the rim joist and staple it to the sub-floor there to block the channel.

      With a conditioned basement, I don't think facing on the batts you use matter. Friction fit mineral wool batts might be better as you can easy remove these in case you need to get in there to run wires or pipes down the road.

      With I joists, get batts for metal studs as these are slightly wider and fit snug between the webs.

      1. Jon R | | #6

        You seem to think I said that it eliminates striping, but I clearly said otherwise. Easily verified with THERM. Rim joists need to be sealed either way (air passes through fiberglass insulation).

  4. diyer01 | | #5

    Thank you for the replies.
    We have double runs of Pex tubing per joist bay. Using double grooved aluminum heat transfer plates.

    Good point about the fire code issue. Then unfaced is likely the best choice for insulation.

    My husband is hoping for an easier way to insulate joists without using batts. Your thoughts?

    We have ended reflective barrier and battened it down as suggested.

    A deflection of I joists led us to take down some of the R-19 insulation in joist bays. Finding Pex under I joist explained why the floors were never warm.

    With the completion of this project we hope the floors of our one story home will provide the warmth we anticipated initially. In fact even now before putting up insulation in the bays under master bedroom/bath, we feel warmth. This is encouraging.

    Thanks for your responses

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