Replies

1. Expert Member
   Dana Dorsett | May 22, 2019 03:25pm | #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 | May 23, 2019 02:57pm | #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 | May 24, 2019 12:22pm | #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