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Reflective Insulation in Unvented Roof

SamGU | Posted in GBA Pro Help on

I’m remodeling with an unvented Cathedral ceiling in Climate Zone 4.  I have asphalt shingles, underlayment the rafter cavities that are 9″ deep which were filled with faced High density R-30.  Adding exit vents is not a feasible approach and I don’t want to use spray foam for a couple reasons.  A contractor today came by and recommended Reflective Insulation (Sold under the Low-E brand) and all kind of reasons why.  Reduces transfer, keeps hot from meeting cold thus eliminates condensation, excellent moisture barrier, cheap, easy to install, etc.  He gave me a sample, reflective on both sides, small 1/8″ of core insulation.  He told me it’s installed under the roof deck (from inside of house) then simply install sheetrock and no need to add any unfaced fiberglass. It has an R-11 value so I think I would add a layer of unfaced R-19 to reach the code requirement of R-30.  Will these reflective barriers really keep the moisture away in an unvented space.  Any comments or inputs are welcome.

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Replies

  1. Expert Member
    Zephyr7 | | #2

    It won't prevent condensation. Condensation occurs when a surface -- ANY surface -- falls below the dew point of the surrounding air. That's all there is to it, there is no magic involved. The only ways to prevent condensation are to either keep the temperature of the first condensing surface above the dew point (normally done with additional R value of insulation), or reduce the moisture content of the air (normally done with a dehumidifier). Radiant barriers (the fancy term for "reflective insulation") don't prevent anything, they just add a SMALL amount of additional R value.

    It isn't R11 regardless of what your contractor says. 1/8" of bubble insulation is maybe R1 if that, pretty close to the 1/4" fanfold type XPS insulation. That's all it counts for in terms of code too. The radiant barrier part is usually considered to add about another R1 to the overall assembly, but that's ONLY if it faces an air gap, and ONLY if it's clean and shiny. If it's dirty, or if it's installed directly against a surface, it doesn't really do anything. The radiant barrier part doesn't count towards code required R values at all.

    Radiant barriers won't do anything in regards to moisture as I described above. What radiant barriers can do is help reduce summertime attic temperatures a little, but that doesn't make them count as "insulation" in the usual sense. If you want to put up a radiant barrier, it's probably not going to hurt anything (although if it's not perforated, then it's a vapor barrier, so you have to be sure not to create a moisture trap). For additional R value in your attic, you're far, far better off just blowing in some loose fill cellulose on top of your existing batts on the attic floor. Blown cellulose is cheap, it's an excellent thermal insulator, it's green, and it can help reduce air leaks a little too. If you have to make a decision as to where to invest limited insulation upgrade project funds, I'd skip the radiant barrier and just add the cellulose.

    Bill

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