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Will building paper over foil faced polyiso negate the radiant barrier?

dJiSubM3Ct | Posted in Energy Efficiency and Durability on

I’m planning an exterior renovation in S.C. (hot, humid zone 3). I’ll be installing 1″ of foil faced polyiso on the exterior. I plan to seal the seams, but am considering a belt and suspenders approach to the drainage plane. I don’t know that I trust the tapes and sealants to hold up under such extreme conditions, so I’m considering installing a house wrap over the top to allow better flashing integration, especially in the area where a gable roof ties directly into the side of the home. If done this way, does the building paper have any effect on the radiant barrier, since it will not have an air space? I am planning on installing furring strips for a ventillated rain screen as well that will allow that air space.

Thanks in advance!

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  1. GBA Editor
    Martin Holladay | | #1

    If you want any benefit from the radiant barrier, it has to face an air space.

    If you cover the foil with building paper, the radiant barrier won't provide any benefit.

  2. dJiSubM3Ct | | #2

    Thanks so much. That's what I thought.

  3. LucyF | | #3


    I am also in SC. 2 years ago we removed the vinyl siding and installed tyvek, 2" polyiso then furring strips and fiber cement board. We installed it in June and July and you would not believe the heat radiating off the foil-faced polyiso. It was difficult for the guys doing the work, but became obvious that a significant amount of heat was not entering my home. My cooling load in the summer is dramatically reduced. In fact, I can only have my AC on for brief periods of time because it gets so cold.

    I don't really trust the tapes either. I wish I had installed 2 layers of polyiso because I feel confident the lower layer would be secure. Could you install 2 layers of polyiso with the seams staggered?

    Also some of the European tapes are reportedly much more secure and long-lasting than what you would pick up here. That's another consideration.

    But don't cover up the foil-facing. It will really help make your house more comfortable in the summer (March to November, right?)

    Lucy, Upstate SC

  4. GBA Editor
    Martin Holladay | | #4

    Before you put up the siding, the foil facing on the polyiso acts like a mirror and reflects sunlight. That's what you were feeling. Once the siding is installed, the polyiso is in the shade, and there is no reflected sunlight.

    In short, the fact that foil reflects sunlight and makes workers feel hot has nothing to do with the way the product works as a radiant barrier.

  5. LucyF | | #5

    Martin, that makes sense. Thank you for clarifying that point.

    So does that mean that there is no advantage to having the foil-faced polyiso as a radiant barrier? Even if it doesn't directly see sunlight and it does have an airspace, it still reflects heat, doesn't it?


  6. GBA Editor
    Martin Holladay | | #6

    A radiant barrier facing an air space has the effect of raising the R-value of the air space from about R-1 to R-2 or R-3. The way it works is that the surface of a radiant barrier has a low emissivity. To learn more about how radiant barriers work, see Radiant Barriers: A Solution in Search of a Problem.

  7. dJiSubM3Ct | | #7

    Thanks for that info. That's encouraging news and what I was hoping to hear. I'll take that extra R-3 all day! Out of curiousity, do you have any problems with high humidity indoors since your A/C doesn't run as much? I will be installing a new unit to manual J specs to size it properly in my case.

    Thanks for additional input. I knew about the R-1 air space, but wasn't aware it grows even more with radiant barrier. Very helpful.

  8. LucyF | | #8


    Yes, I do have a problem with the humidity because my heat pump is over-sized. It would have been perfect before we did this renovation because my house was so leaky, but now it gets it cold before the humidity has been removed. It's not a major problem. I just turn it on for a few minutes when it gets sticky (as we say in the south) and then turn it off again.

    We are going to start building a house for my brother next year and we will definitely downsize the heat pump because his house will be so much more efficient (and comfortable) than mine.

    After reading GBA for the past couple of years (only started posting in the past couple of months), I do think it makes sense to to a Manual J calculation.

  9. BobHr | | #9

    Side walls do not see the same solar drive as a roof. It doesnt pay to detail for a radiant barrier in the wall. You can find some studies on walls at the Florida Solar Energy Center's website.

  10. dJiSubM3Ct | | #10


    Thanks for your post. The main thing for me is the exterior insulation. I'm putting it on anyway, so why not buy the insulation board with the foil face? There is no reason not to that I can see.

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