Replies

1. Expert Member
   BILL WICHERS | Nov 19, 2019 12:40pm | #1

   Assuming this is a vented attic, you don’t need a vapor barrier in the ceiling (the attic floor).

   I’d do some air sealing work, put in baffles at the eaves to deal with wind washing, and then blow in cellulose until at least getting to R49.

   Bill

1. 
   GuyGF | Nov 19, 2019 06:01pm | #3

   Thanks for the thoughts on no vapor barrier -- Dana's response adds confirmation -- nice to have that decision done. I humidify the house fall/winter/spring (but I abide by the recommendations of very reduced humidity levels as the outside temps drop), so trapped humidity is a real concern for me. I'm now thinking that I'm better off with batts or blown-in cellulose, so as not to have any vapor trapping vs some trapping with foam boards. Batts are easier for to do, but if I go blown-in I'll do the baffles. Thanks....

2. Expert Member
   Dana Dorsett | Nov 19, 2019 02:41pm | #2

   There is no place for a vapor barrier in this stackup. To be effective it would need to go on the conditioned-space side of the assembly, but with low-permeance foam board up top it would create a moisture trap. But as long as there is sufficient R-value to the foam for dew point control at the attic decking layer it will be fine.

   In your climate zone as long as the foam board's R-value exceeds the R-value of the blown insulation there won't be mold issues with the attic decking. Assuming R3.7 /inch for mid-density blown fiberglass or low density cellulose it means it needs more than 4 x 3.7= ~R15 for the foam board layer as dew point control.

   To have any margin would take 3" of polyiso, or 4" of EPS (or XPS, derated to it's warranteed R value). The only "green" foam is reclaimed foam, but the least-green foam (by far) would be virgin stock XPS, which is blown with an HFC soup, the predominant ingredient being HFC134a with an extremely high global warming potential (~1400x CO2 @ 100 years). EPS and polyiso are blown with much lower impact hydrocarbon blowing agents (usually a variant of pentane, at ~7x CO2.)

   https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

   Going with 3" (R17) or more of reclaimed 2lbs density roofing polyiso would make it "walkable" (but not "stompable") surface, though adding a layer of half-inch plywood or OSB atop the foam board would be needed if used for storage.

   For source of used foam, check this search every week or so- it's out there:

   https://minneapolis.craigslist.org/search/sss?query=rigid+insulation

   This 4' x 4' x 3.5" EPS would be a good start if they have enough of it:

   https://minneapolis.craigslist.org/dak/mat/d/dundas-foam-insulation/7020283806.html

   Even though it's advertised by the seller as R14, it's probably Type-VIII (1.25lbs per cubic foot) roofing EPS, which would be labeled R14.5 measured at 75F median temp through the foam, but a hair over R15 at temperatures that matter from a dew point control perspective. It's just enough.

3. 
   GuyGF | Nov 19, 2019 09:15pm | #4

   First -- thanks for taking the time to describe all of this. Lots to digest! The link showing materials is very helpful -- cellulose and batt are far lower in carbon footprint than the XPS I was first checking. If I go with foam boards I'll use EPS. And I'll definitely not put a poly vapor barrier on top of the attic floor/below the foam board.
   I didn't mention in my original post -- I have a whole house humidifier, controlled so it varies the max humidity based on outside temp. The attic currently has limited ventilation (some soffit vents). So humidity in the attic is something I want to avoid.
   My options look like either I go with something that has moderate vapor permeability (such as 6" of EPS) or go with something with no vapor control ( 10" of unfaced fiberglass or blown-in cellulose). Have you any suggestions? Thanks!