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Exterior foil-faced polyiso question

randygiles | Posted in Green Building Techniques on

I live in the San Francisco Bay Area and my application is as follows:

Plywood – sealing all plywood seams with Signa Wigluv – 2 layers of ¾” foil faced polyiso (all seams taped) – WRB – Rainscreen – Fiber cement siding:

The plywood is serving as my primary air barrier. I understand that any moisture that gets behind the first layer of polyiso (between the ployiso and plywood ) will dry to the inside and any moisture that gets on the outer layer of polyiso will dry to the outside.

My question relates to the area between the two layers of foil faced polyiso.
If water gets between the two layers of foil faced polyiso, the only escape route would be gravity – i.e. to drain to the bottom of the wall assembly. So that would mean that I should not put any adhesive / tape that could block the water between the two layers. Am I thinking correctly here – or do most people put adhesive / tape between the two layers of the foil faced polyiso?

Thanks for any comments?

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  1. GBA Editor
    Martin Holladay | | #1

    As long as your wall assembly does not include gross defects -- as long as you use normal levels of attention when assembling the wall -- there is no way that you will ever get water between the two levels of rigid foam.

    You'd have to really go out of your way to design flashing that would direct water there for this failure mechanism to occur.

    Here's some advice: Don't install the polyiso on a very rainy day. As long as you do that, you'll be fine.

  2. GDARBY01 | | #2

    Why would you use 2 layers of such an inferior product with such a low R-Value?
    Insulation4less is 3/64" thick and costs less than $.30 per sq. ft.
    Energy Star Qualified : ICC-ES Recognized : R 16 unaffected by humidity : Prevents 97% of radiant heat transfer : Vapor barrier : Core sealed on both 100' sides with flange : Elastic : 19dba noise reduction : 90 degree celsius (194 fahrenheit) contact temperature rating : UV resistance : Does not promote mold or mildew : Does not provide for nesting of rodents, bugs or birds : Seals around nails (no leak) : Reflective aluminum foil on each side of 5mm (13/64) closed-cell polyethylene foam center : Keeps its shape over time (doesn't collapse) : Member of US Green Building Council - Made with 100% recyclable virgin raw materials : Over 3 billion square feet sold worldwide.

  3. iLikeDirt | | #3

    Does it do my taxes too? Shine my shoes? Make me morning coffee?

  4. GBA Editor
    Martin Holladay | | #4

    Greg Darby,
    Insulation4Less is a notorious distributor of radiant barriers and foil-faced bubble wrap. Through omission of important information or outright fraud, distributors of these products have been misleading customers for years.

    If you want your insulation to have R-value, then buy real insulation. There is no miracle way to make thin products that come in a roll have a high R-value per inch. (Unless you are talking about vacuum-insulated panels, a very expensive, experimental approach to insulation that is not suitable for most walls or ceilings).

    For more information, see Stay Away from Foil-Faced Bubble Wrap.

  5. Expert Member
    Dana Dorsett | | #5

    According the fine print at the bottom ( ) the conditions under which the Prodex radiant barrier product performs at R16 is, according to the manufacturer are:

    "*Parameters of test: 24-inch on center 2" x 6" wood assembly. Roof application. Test method ASTM 1116. Airspace of 2.64 inch on each side of product. Heat-flow direction down."

    That's exactly how you would build a wall- with horizontal studs 24" o.c., and always with the warm side of the wall up (flip 'em all over in winter!) with 2.64" on each side of a thin mid-cavity mounted roll insulation product, RIGHT !?!

    Maybe in some alternative universe with some extra dimensions it's possible to build a house like but I'd love to see the math on that! :-)

    Also, since the R16 performance requires 2.64" + 2.64" + 3/64", the insulation layer is actually at least 5.35" thick, (nominally 5.5" for a real 2 x 6 assembly) so that R16 is AT BEST only R3/inch in the conditions under which R16 is achieved, and only R2.9/inch at the nominal 5.5" lumber dimension. That's half the performance of rigid polyiso, and almost but not quite as good as the absolute crappiest grades of low density fiber insulation such as low density R11 batts designed for 2x4 framing (which performs at R3.1/inch.)

    That product is not ENTIRELY worthless (it has better thermal performance than say, nano-sphere paint additives), but it's among the very worst performance per dollar values as insulation goes.

  6. lwd3 | | #6

    In California the Energy Code requires that the insulation be listed with the Bureau of Home Furnishings and that it be advertised as approved by the Bureau. The products mentioned above are not listed and can not be used as a building product in California.

  7. user-4524083 | | #7

    Hey, Gregg - Nice try, but I believe that you have wandered onto the wrong site for your sell.

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