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Is a reflective surface necessary under radiant floor heating?

JoshEngle | Posted in Mechanicals on

I have a situation in a house with a full basement where there is radiant heat in the floor joists of the first floor, accessible from the basement.
There are unfaced fiberglass batts in between the joists under the radiant piping. My business partner thinks we should replace the fiberglass with foil faced foam, but I think it’s not necessary.
My partner’s rationale is that there needs to be a reflective surface and there is none now.
In my understanding, foil facing ads one half of an R value to the insulation if there is an air space next to it. Using that rationale, I would think that foil facing is not necessary in this situation since the R value of the 6 inches of fiberglass is greater than that of the one inch of foam plus reflective surface.
Am I missing something? Is the reflective surface doing something other than boosting the R value?
The basement is semi finished and semi conditioned.

Josh Engle

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  1. Dana1 | | #1

    There is no point to having a reflective surface, which requires an air-gap between the radiating surface and the reflector to have any effect at all.

    The emissivity of the radiating surface has to be pretty high too. If the tubing is in aluminum heat spreaders the extremely low emissivity of the aluminum means the radiated component of the heat transfer downward is almost nil.

    You only need about R10 of fluffy stuff to deliver reasonable zone separation between two conditioned spaces (the semi-conditioned basement and the room above), but most will install cheap R19 batts, which is more than enough.

  2. user-2890856 | | #2

    Your assumption is correct Josh , Reflective surfaces add no measurable value . R value (resistance) is the only factor that needs be addressed .

    Is there any type of heat emission plates in the install or just tubing ? If there is just tubing I STRONGLY suggest and recommend adding extruded aluminum heat transfer plates to enlarge the conductive surface to the subfloor . Tubing alone with an airspace is very ineffective and requires high water temps .

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