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Foil-Faced EPS as Radiant Barrier

Antonrx | Posted in General Questions on

In a 2×6,  vented cathedral ceiling, I will be using foil-faced 1″ EPS boards for my vent baffels. This will provide a 1″ air gap. Would it act as a radiant barrier if the foil side is facing the roof/sheeting? Under this EPS, I will put Rock wool, then tongue and groove ceiling. I live in climate 2A, central florida (hot & humid).

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  1. Expert Member
    Michael Maines | | #1

    Yes, the direction and location of a radiant barrier makes no difference, so you'll get an extra R-1 or so out of your assembly. Will you have an air control layer as part of your assembly?

  2. Antonrx | | #2

    Yes. If the radiant barrier was going to be that significant, I was going to create the space as follows from roof sheeting down:
    -1” air gap
    -1” Foil faced EPS
    -3.5” rockwool
    -T&G ceiling

    I realize this is under insulated. Technically, this is a “shed” turned into a “movie room” now. Do you think it would be better to use 2” EPS attached directly to the roof decking and not have a vent channel? That would eliminate the radiant barrier benefit but add another Layer of foam.

    1. Expert Member
      Michael Maines | | #3

      I almost always prefer venting roofs when possible. Your proposed assembly is about R-18, including the radiative effect, vs. about R-17 without the gap. That's only R-1, but it's almost 6% of your assembly.

      The 2021 IRC requires R-49 in zone 2 ceilings, while previous iterations required R-38. In a renovation or other situations you may not be required to fully meet the code requirements and a small, minimally occupied space will probably be ok with skimping, but aiming for 35% to 45% of recommended values seems pretty low to me.

      I avoid foam and other materials with high levels of embodied carbon so I would be inclined to find a way to use more insulation and more environmentally-friendly insulation, furring the rafters down if necessary to do so. But to answer your question, an extra inch of foam is R-4, which would increase your roof's R-value by 24%, vs. 6% for the air space. I'm not sure how long it would take to recover the embodied carbon of the extra foam with the savings in operating carbon emissions, or how the financial costs would play out, but using EPS my guess is that it would be better to use the extra foam.

  3. Antonrx | | #4

    I understand the nominal R value attained with the 1" airgap, however, if I understand this correctly, for a radiant barrier to be effective, an airgap is required. In order to have the air gap, i would be sacrificing 1" of foam space. Are the radiant barrier effects & vented space more beneficial than an increased R-value w/sealed setup here in Florida? Due to the Tongue & groove ceiling I plan to install, I will need to seal the foam at the eaves as well as along the parameters of each board, with either setup. Thank you for responding too. I really appreciate your time!

    1. Expert Member
      Michael Maines | | #5

      Correct, the 1" space can either add R-1 as part of the radiant barrier, or R-4 as foam insulation. Despite claims by radiant barrier advocates, they don't do anything magical; R-value is R-value and is the measure of how quickly heat flows through an assembly.

      Sealing rigid foam as insulation baffles is possible but it's hard to do perfectly, and unlikely to remain perfect over time, so it's not a good place for the primary air control layer. I recommend using a variable permeance membrane on the interior side of the rafters, mainly to control air movement.

    2. Expert Member
      BILL WICHERS | | #6

      I would think of this as "if I need to have a vent channel anyway, the foil facer gives me a raidant barrier for free, so I might as well install it that way". If you don't need the vent channel, then it makes more sense to fill the gap with "real" insulation.

      In your particular case, that vent channel is going to make your assembly more robust, so I'd built the vented assembly with the "free" radiant barrier using the foil faced rigid foam. Vented roofs are much less prone to moisture problems. If you need additional R value, you could put rigid foam on the interior under the rafters, and using polyiso you can get about R6 per inch of insulation you add this way. You can then tape the seams of that polyiso and that becomes your air barrier behind the T+G ceiling boards. Even 1" polyiso installed this way gives you around R28 (including R1 for the radiant barrier), which is a lot better than just the polyiso in the vent baffle and the mineral wool alone. You could add 2" of interior polyiso for about R35 for the assembly.


  4. Antonrx | | #7

    Great explanations guys! Much appreciated. I will be going the vented route. I'm assuming it is required to seal the vents at the ends, creating the envelope/chambers that I will fill with rockwool, rather than just having the vent channels and rockwool exposed underneath at the eaves? If I sealed the ends of under the vents, this would create an airtight seal too right? I plan to have 6 low profile can lights as well as 2, 6-8" speakers in the ceiling, so, depth is of concern (mainly with the speakers). I've seen schematics showing a few options and one route is sealing the ends where the vent channel meets the topplate with a piece of rigid and spray foam seams.

    1. Expert Member
      Michael Maines | | #8

      It's a good idea to seal the vent baffles and insulation because any fluffy insulation performs best when in a fully airtight chamber, but it's not as critically important as a lot of things in the building science world. It is important that you have at least one fully sealed, continuous air control layer somewhere in the assembly.

      Recessed lights and speakers in ceilings are not a good idea unless you make sure they are airtight. That's more important in heating-dominated climates where we want to keep warm, moist indoor air from getting into the roof assembly, but in a cooling-dominated climate you could get moisture accumulation (and its good buddy, mold) at the penetrations if hot, humid outdoor air can reach the penetration.

      More info in these articles:

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