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Radiant barrier sheathing: how does it work?

user-72921 | Posted in Green Products and Materials on

From the literature it seems that it doesn’t matter which side of the foil the radiation is on, as long as there’s at least 3/4″ air space on one side of the foil or the other. I always thought the air space (which is why you’re supposed to staple foil faced batts to the side of the studs, not the face) had to be between the foil and the radiation you want to reflect. With TechShield, they advertise that +90% of solar energy will be reflected away from your attic with the foil facing down, the air space being on the opposite side from the radiation being deflected. Totally counter intuitive, right? I don’t get it.

thanks to anyone who can educate me about the physics of this.

Bruce Cole

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  1. GBA Editor
    Martin Holladay | | #1

    For TechShield (or any other similar radiant barrier roof sheathing) to work, it needs to be installed shiny-side-down, with the shiny side facing an air space. The usual application is to use the sheathing on a peaked roof over a vented, unconditioned attic. If you climb into the attic, you should have insulation on the attic floor, and you should be able to see the shiny foil by looking up between the rafters.

    It makes no sense to use TechShield for an unvented roof assembly with insulation installed tight to the roof sheathing.

  2. user-72921 | | #2

    Thanks for the reply, Martin. My question is how does it work, ie the physics of it. I can understand how a reflective surface with an air space in front of it works when it's facing the radiation (in this case, the sun), but how does it work when the foil and air space is facing the opposite direction, as is the case with TechShield? I always assumed that the air space would need to be facing the radiative source.

    To put it another way, what is the function of the air space, ie,, how does it make the reflective surface work even when it's on the opposite side of the sheathing from the sun? Conversely, why won't it work when there's no air space on either side? Just curious as to the science.



  3. GBA Editor
    Martin Holladay | | #3

    The aluminum foil is a low-e surface. When the sun heats up the roofing and the roof sheathing, the underside of the roof sheathing will radiate heat into the attic. This radiant heat will warm up any objects within range of the radiation. Ordinary plywood will radiate a lot of heat, while a low-e surface like aluminum foil will radiate less radiant heat. So the radiant barrier reduces heat transfer by radiation from the underside of the plywood to the objects on the attic floor.

    More information here: Radiant Barriers: A Solution in Search of a Problem.

  4. user-72921 | | #4

    Thanks Martin, will read the article. Much obliged.


  5. user-72921 | | #5

    I just read the article and it was very informative. It's what I expected, that the claims for radiant barrier sheathing are overblown, especially since, as you put it, "Once the radiant barrier gets dusty, it’s no longer a low-e surface. Radiant barriers have to stay shiny to work."

    The situation I'm dealing with is a new garage on Oahu at about 300 ft elevation (summer 9 months of the year, basically) with a sloped roof and no insulation planned. Not having ever used the product before (I'm from ME) and there being no "this side up" instructions" on the panels I purchased, I installed it with the foil up. Very embarrassing, to say the least. So now I'm going to correct the mistake and I was planning to install perforated aluminum sheets between the truss bays to the underside of the sheathing as a fix. The two architects we've asked about this say that's a good fix but after reading your article I'm wondering if the owner would be 10 times better off just having me put up some R-13 batts between the trusses instead. This is all about heat gain from the sun, no space heating or AC in the equation at all.

    thanks again for your input


  6. user-939142 | | #6

    Foil faced insulation seems like a good fit for your project; it is especially cost effective and quick and easy to put up. You don't need the fancy bubble stuff, any foil sheeting product with a low e value works.
    Your best bet is to put on a proper roof if you want to control heat so it doesn't absorb it to begin with, this site has info on that.

    Foil won't work magic though, and dust can be an issue.
    You probably won't be able to tell if you use R13 batts or not; it would be a waste in my opinion.

  7. Expert Member
    Dana Dorsett | | #7

    It takes quite a lot of dust to affect the performance of radiant barriers, but it's a "who cares?" sort of thing. With a high-slope roof it'll take decades to accumulate enough to matter.

    You'll usually get more performance/$ out of adding more insulation than more radiant barriers. Low density fiberglass batts wouldn't be the best choice though, since they are highly air-permeable and translucent to infra-red radiation- you'll do better with blown cellulose or rock wool. For RB vs. insulation benefit estimates, parse this very carefully:

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