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Community and Q&A

Floor radiant heat

home2 | Posted in General Questions on

I am building a 4 season room addition where radiant heating will be installed above the subfloor.  The basement below the room is currently not insulated and is unheated; eventually could be insulated and  heated (perhaps in a couple of years).

The subfloor is 3/4 inch AC2 treated plywood.  Hydronic radiant floor heat will be installed above the subfloor with PEX tubing, plywood sleepers and aluminum emitters.  I have available left over black roofing membrane that is either neoprene or EPDM (don’t know which).  I am wondering if that membrane can be laid overtop of the plywood subfloor to act as the vapor barrier (and sound control) with the PEX tubing, sleepers, and emitters placed on top of it.  I am looking for a reason to use or not to use it.  If installed what impact will it have on future heating the basement below?  Thank you for your help.

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  1. Expert Member
    Peter Engle | | #1

    Roof membranes see in-service temperatures well above 140F, so the temperature of the water won't be an issue. If the basement is dehumidified and eventually conditioned, there's probably no need for any vapor barrier though. With or without the VB, you will have to insulate below the subfloor.

  2. Expert Member
    Dana Dorsett | | #2

    Do you really WANT a vapor barrier above the subfloor, blocking the drying capacity of the subfloor & joists to the interior? (I don't think so!)

    Where are you located? (For climate/weather purposes- just the US DOE climate zone, or postal code is good enough.)

  3. home2 | | #3

    From what I have read on the topic of radiant floor heat above an unheated basement, a vapor barrier is recommended (as well as insulation on the underside of the vapor barrier). I am planning to put either rigid foam or batts underside between joists, R30 plus. I also understand in applications where the basement is heated, a vapor barrier in the basement ceiling is not needed. But, if the roof membrane offers noise reduction, then would it still be acceptable to install it? Or will it cause problems that I am not aware of?

    I have seen installations where rigid foam is installed above the subfloor after which the PEX tubes are installed. The rigid foam acts as a vapor barrier in those applications, similar to the roof membrane. This house is located in Zone 5, northern Illinois.

    Thank you for your help.

    1. Expert Member
      Dana Dorsett | | #5

      There's absolutely no need for a vapor barrier here. Where is the moisture coming from, and where are you hoping to prevent it from going?

      Basements (even unheated) are inside of conditioned space, and the subfloor itself has vapor retardency sufficient to keep wintertime moisture from the upstairs from creating mold conditions in the joists or basement, even if the basement were to drop below 40F (which it might during a Polar Vortex event in zone 5 IL, but not for the whole winter.)

      There's a huge dilution factor with the basement, and the basement probably leaks enough air to the outdoors to stay dry in winter, but perhaps not in summer.

      If the basement were heated R11 would be enough for zone isolation between the upstairs and basement. If the basement is going to dwell below 50F for weeks on end during cold weather there could be a case for R19s, but not R30. Only if it were a pier foundation or vented crawlspace would R30 be called for.

      There is no rationale for using foam board between the joists. There is sometimes a case for a small amount of foam board above the subfloor to make the radiant slightly more responsive, and recover more quickly from overnight setbacks, etc when the radiant is above the subfloor. (Roth panels are pretty responsive due to that aspect.)

      The fluffy stuff between the joists needs to be snugged up to the subfloor. While kraft faced batts are easier to install, exposed kraft facers are a fire hazard. Unfaced fiberglass batts shed fiber leading to airborne glass particulates in the basement. The solution is either to use rock wool batts, or to take PERFORATED (and thus semi vapor permeable) aluminized fabric type radiant barrier and snug it up to the fiber, side-stapling it to the joists. A 48" wide roll can be cut (carefully) into three 16" wide rolls, which gives just enough at the edges for side-stapling to 16" o.c. joists. The radiant barrier has sufficient fire ratings to be left exposed (it's done all the time in attics), and the perforations are small enough to keep the glass particulates at bay. (The is one of the few real uses I've found for this stuff.) There is also a modest improvement in thermal performance, due to the l0w-E facer pointed at the air space.

  4. GBA Editor
    Martin Holladay | | #4

    In my opinion, you don't need a vapor barrier in this location. The plywood is already a vapor retarder.

    One worry no one has mentioned is the possibility of odors. Roofing is designed to be installed outdoors, and it often stinks when it gets warm. For me, that's reason enough to scrap your plan.

  5. home2 | | #6

    Thanks everyone for the information, I think I now know how to proceed. I won't put down the rubber roof membrane, and I will put in the mineral wool between joists, probably R15.

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