EPS seems exemplary. Air barrier, vapor retarder, environmentally less damaging, maintains performance due to the absence of offgassing. Much cheaper at least when bought at Menards.

The only downside is a few extra inches to reach the same r value, which to a non builder seems like an easy adjustment.

http://phius.org/Tools-Resources/Protocols-Calculators/Insulation_GWP_Tool_v1.3.xls

As for what was in the brown paper bag, I’m not telling! ;~)

– the GWP calculator can be unlocked (password: “unlock”) so that the CO2 emissions for electricity can be adjusted for a specific location.

– The tool unfortunately does not model assemblies with more than one type of insulation. However you can fury rig as follows: 1) model with one of the types included in the “reference R value” calculation, then track the other type in the graph. This gives you embodied GWP of the 2nd type plus heating energy GWP. 2) set HDD to zero and track the 1st type in the chart. This gives you embodied GWP of the 1st type only. 3) add the two together. Email me with questions.

– Regarding the total GWP of XPS/ccSPF as the only insulation in an assembly, I want to make clear that even at low R-values it’s hard to get XPS/ccSPF to come close to the rest. E.g. in my NYC 5000 HDD climate at a whole-assembly R-15, you’ll see about 50% higher total GWP (embodied GWP plus heating energy) using XPS as you would using dense pack cellulose in a wood frame. It’s not that it’s fine at low R values; it just gets worse at high R values. By the way, in my opinion it clears up a lot of this to jettison the concept of payback in favor of total GWP, as used in the calculator.

– Regarding accounting for the benefits of air tightness, in my work most air barriers are composed of tape, caulk, and/or sealant applied to materials that are already in the construction for other reasons. I don’t expect the embodied GWP of the tape/caulk/sealant will add up to much.

I request that any responses to the two points above use real calculations. I am allergic to public speculation. I worked hard on that GWP calculator so that you all could download it for free, and use it, thereby sparing me a rash.

It’s painful, because it exposes previous thinking disconnects, but it also takes you on the path to being a great designer who doesn’t fixate on the unimportant.

We need homes that are controllable so occupants experience long term comfort, health, safety, which means getting things as tight as possible so you can manage air quality, temperature, humidity, and surface dew points all within a very narrow band all while driving EUI to 30–50% below average.

Keep going!!