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What is the effectiveness of a radiant barrier on ducting in unconditioned space?

kelchm | Posted in Mechanicals on

Has anyone come across a study which looked at the effectiveness of a radiant barrier on ducting which runs through unconditioned space?

Specifically I’d like to better understand the impact of using a product like the Hart and Cooley F218 versus the Hart and Cooley F118. Both of these have a rated R value of 8.0, but this number would obviously not account for the benefit provided by the radiant barrier under specific conditions.

F218: http://www.hartandcooley.com/products/f214-fb214-f216-fb216-f218-fb218
F118: http://www.hartandcooley.com/products/f118-fb118

I’m currently having a 4 zone Mitsubishi system installed. Part of the system uses an air handler and ducting located in what is currently unfinished attic space. I plan to eventually insulate the roof with closed cell spray foam to create an unvented roof assembly and to bring the air handler and ducting into the thermal envelope. At that point I expect it wouldn’t make much of a performance difference, but that work could still be several years away.

Attic Space Photos: https://photos.app.goo.gl/kHqaqTm6HZEuELkA8

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    In terms of average performance the low-E exteriors adds at best R1 to the R8. Even if with the right instrumentation it might be measurable, it's not a major factor.

    >"I plan to eventually insulate the roof with closed cell spray foam to create an unvented roof assembly and to bring the air handler and ducting into the thermal envelope."

    Closed cell foam is one of least green insulation materials in common use, even the In a zone 4 location as little as R15 of closed cell foam is sufficient for dew point control on up to R34 of fiber insulation or open cell foam. Those look like ~9.25" deep 2x10 rafters built up with 1x10 planks (?). If so, 2" of HFO blown closed cell foam delivers about R14, and filling the remaining 7.25" of rafter bay depth with 1.8lb dense packed fiberglass would be R30, filling it with cellulose or half-pound open cell foam would be R27. The total wouldn't quite hit the R49 code minimum but it would perform comparably to 7" (R49) of HFO blown closed cell due to the marginally lower thermal bridging from the 2.25" longer path through the rafter.

    When it's time to re-roof , adding a continuous 1-1.5" polyiso layer above the roof deck could get you the full R49, thermally breaking the rafters for better than code performance.

    1. kelchm | | #2

      Thanks Dana -- The rafters are (partially) sistered 2x6 16" on center, so I only have 5.5" of depth to work with. I don't have much headroom to work with on the shorter rear kneewall (roughly 60"), so I'm trying to avoid adding additional depth to the rafter bays at all costs.

      Currently the finished side of this same roof is a cathedral ceiling with only two sheets of 1" polyiso in each rafter bay. Fortunately this is just the main entryway to the house and not regularly occupied as it gets ridiculously warm in the summer. Whatever solution I decide on for the attic will also be used for the vaulted entryway as well. I've added a photo to the photo album that may help you visualize things: https://photos.app.goo.gl/kHqaqTm6HZEuELkA8

      My hope was that I could make an argument to the HVAC contractor that the polyethylene vapor barrier flex duct they are using is not equivalent in performance to the metalized vapor barrier under certain conditions. Even an additional R1 under specific conditions may be worth arguing for given the minimal difference in material costs (less than $5 per box). The other argument is durability, as the polyethylene variant is nowhere near as durable and I'm not confident that it will hold up as well long term.

  2. Peter Yost | | #3

    Hi Matthew -

    and important consideration for radiant barriers: any type or level of dust/dirt on the radiant barrier surface pretty much eliminates its impact; the emissivity is a surface phenomenon and any change to the surface can radically change emissivity. So, if you want that extra R1, need to keep the duct surface clean!

    Peter

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