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Community and Q&A

Air Gap for Foil Vapor Barrier

tundracycle | Posted in Energy Efficiency and Durability on

Purveyors of foil radiant vapor barriers ( say that an air gap is needed for this to work. True?

When used in the wall of a sauna for instance (I think primarily because foil can withstand heat that other vapor barriers cannot) they suggest installing the foil to the studs then 1/2″ furring strips and then the interior wood (typically T&G cedar or similar). The walls are typically insulated (batts, blown or sprayed) to the outside of the foil but the inside is facing a sauna that could be 200°f or more so like Martin’s steel roof example (, perhaps does offer some energy benefit?

So a typical wall might be exterior cladding + sheathing + studs & batt insulation + foil radiant vapor barrier + furring strips + T&G hemlock.

Is some way of venting this space needed? Both supply and exhaust venting? Or will radiant heat simply be reflected back to the interior sauna walls?


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  1. Expert Member
    BILL WICHERS | | #1

    Yes, you need an air gap for radiant barriers to work. That's true. I don't know about the "foil handles heat better" part though, because a lot of the "foil" radiant barriers are actually aluminized mylar (or similar), so they still have a plastic film backer that could potentially be an issue in terms of heat resistance of the material.

    Note that you can also use foil-faced polyiso here, using the foil facer as a radiant barrier. This gets you some actual insulation instead of just a vapor barrier, and may allow for more flexibility in how it's installed as well.


  2. tundracycle | | #2

    Thanks Bill. I'd assume (maybe bad assumption) that given how they work that any of these products would be able to withstand some significant heat? If stapled to the bottom of rafters in a pole barn it's going to get pretty hot.

    Another question. If you don't have the air gap will it pull more heat out of the room or just loose the radiant barrier benefit? IOW, interior wood to stud and wood to insulation/air will have X thermal bridging. If instead it is wood to foil to stud and wood to foil to insulation/air will thermal bridging be greater than X? Foil is a good conductor so presumably it will increase conduction between the wood walls and the studs but it's a bad emitter so it will not emit/radiat heat in to the insulation/air cavity?

    1. Expert Member
      BILL WICHERS | | #3

      "Pretty hot" for people isn't "pretty hot" as far as most building materials are concerned. Typical insulating products are fine with typical attic temperatures of 120F or even more. I wouldn't worry too much about that. If you were worried about flame spraid or some other aspect of fire safety, that's when you start to want to be concerned with the temperatures the materials can handle before they fail.

      Don't get too hung up on radiant barrier stuff. Without an air gap, a radiant barrier is just a film that does pretty much nothing. The radiant barrier won't "suck heat" from anything, and it won't make thermal bridging worse (or better, for that matter) where it's tacked into place. People sometimes have similar concerns about thermal bridging of fasteners passing through rigid foam. I actually ran the numbers on that on these forums before, and posted the results" it's not worth worrying about.

      BTW, you seem to be very concerned about the radiant barrier aspects of your project. I will warn that the benefits of radiant barriers are often exaggerated, sometimes very exaggerated. Don't worry about it too much. Be more concerned with the performance of your "real" insulation, whether that is batts, polyiso, etc.


      1. tundracycle | | #4

        Thanks Bill, good useful info. Especially that the foil will not enhance thermal bridging.

        Fire safety is not an issue here. 1) First priority is preventing fires. 2) If there is a fire in the sauna then the focus is on getting people out safely. 3) The sauna is going to burn down.

        On the radiant barrier... My sauna and most others do generally have good real insulation. The foil is there primarily as a vapor barrier. However, its benefits for radiant could still be worthwhile. If the temp inside the sauna is 210°f and outside is -30°f it may be of some use purely for heat retention and energy savings?

        Possibly the bigger benefit in a sauna though is if it does reflect some bit of heat back towards the wall boards and keeps them warmer so that they radiate more heat towards bathers then that makes sauna much more comfortable.

        1. Expert Member
          BILL WICHERS | | #5

          In the building science world, radiant barriers are commonly said to add about R1 worth of insulating value if they're installed correctly (air gap, kept clean and shiny, etc.). I don't know if there is any big difference in a sauna application, but I wouldn't expect anything significantly different there. The vapor barrier IS something you get with a foil facer though.

          Basically a radiant barrier won't hurt you, and might help you, but it's not likely to make for any massive advantage over conventional insulation.


  3. tundracycle | | #6

    Thanks Bill. Yep, that seems to sum it up pretty well.

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