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Best product to tape underlayment in radiant application?

TorBu | Posted in Green Products and Materials on


I am installing an underlayment that does not come with adhesive on the seem, so I am looking for a recommended product that works well in a radiant application. (Radiant tubes in heat transfer plates that are attached to the bottom of the subfloor. Water temperature will be in the low 90s.)

The manufacturer of the underlayment recommends using duct tape that is being used in HVAC applications, but doesn’t the reflective barrier create cold spots/lines in my radiant application?


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  1. user-2890856 | | #1

    Torsten ,

    Who is the manufacturer of the underlayment you will be using and what type of product is it ?

    Will your radiant floor assembly have an air gap between the R layer and the subfloor ?

    Foil faced bubble wrap type barriers do not provide the necessary resistance to direct heat energy upward through a subfloor and finish floor in almost every case I have seen . They can be added above the real insulation but really have little effect . If , in fact this is what you are looking at using you must still utilize some sort of insulation underneath that . You are spending the money to do radiant for several reasons and I often end up looking at these jobs performed and designed by others using just a similar product that are not providing the comfort or efficiency that was expected . Downward losses can only be negated or lessened by higher R values .

  2. TorBu | | #2

    I will be using Roxul COMFORTBATT® R-30 in-between the joist underneath the heat transfer plates that are mounted to the wood sub-floor. So no air gap between radiant and sub-floor. I can put the Roxul up against the heat transfer plate or leave and air gap, depending on what the best practice / recommendation would be for this kind of application.

    As underlayment on top of the sub-floor, I have picked the Sponge Cushion DeciBlok MC.

    The engineered hardwood floor will be nailed down. (I realize that will impact both, sound control as well as moisture barrier.)

  3. user-2890856 | | #3

    No airspace below subfloor . You want every square inch to have upward flow immediately .

    Did you account for the resistance of that product in your heat loss while determining supply water temps ? Important to do so .

  4. charlie_sullivan | | #4

    The tape won't impede the heat flow at all. If the tape was facing an air space, you'd need to consider the particulars of the type of tape, but it's in direct contact with solid surfaces so there's nothing to worry about there.

    And I agree with Richard that it's better with no air space underneath either. I think it gives less opportunity for some inadvertent air flow to carry the heat away to somewhere you didn't want it through some unplanned path.

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