All,

I need help deciding on a basement/wall and roof insulation for our to-be-built home. We don't have any goals in terms of R-value - the only goal is to be super-efficient in terms of insulation properties (and not go over board with the cost)

We live in Zone 4 Marine - very cold winters, hot and humid summers.
Initially we thought about having on the exterior of the basement and walls 4"XPS insulation to provide a continuous thermal envelope.
The plan for the walls was: fiber cement board - furring strips -  4" xps - osb - 2x6 walls 16" oc with roxul mineral wool - drywall. I am not sure if a vapour barrier is needed, and where exactly in this wall structure. I believe 4" xps may be an overkill. Should we go with cellulose in the cavity? What about outside?
Seems 4" xps on the outside may be a little too much for some builders as they raised concerns about installing hardie siding and the windows.

I am worried about termites as well - may not be necessarily common around here, but on the map seems there is some activity.

And then I've started reading about XPS and off-gassing on the forum. Seems it's not very environmentally friendly.


Thanks

I'm in Zone5 Chicago and will likely do either 2" EPS or 2" XPS - leaning toward EPS - cheaper but the R is a bit lower. However, it is below grade so the ground is theoretically at 50deg (aside from the higher grades which reach freezing in winter once you reach the top foot or so). Above grade I am looking at 1.5" XPS exterior and I will be using 2x6 walls with Applegate insulation in the cavity.

XPS is blown with a mix of environmentally damaging HFCs (the predominant of which is HFC134a, which has a global warming potential 1400x CO2, among it's less desirable properties) whereas EPS and polyiso are blown with a low-impact HC (pentane at about 7X CO2). Worse, as the HFCs leak out over 4-5 decades the material loses performance, eventually hitting the same R value as EPS blown at the same density and thickness.  At 4" exterior foam in a zone 4 climate you'll get better lifecycle performance out of 2" of EPS on the exterior, with 2" of polyiso between the EPS and structural sheathing, despite nominally the same day-1 R-value as 4" of XPS.  In most markets EPS and polyiso are cheaper per unit R too. (About 10 cents per R per square foot compared to XPS at about 13 cents/R-foot.)

Cellulose in the cavities is cheap performance, and the borate fire retardents are anti-termite.  The difference in whole-wall R (after thermal bridging of the framing is factored in) between R23 Roxul and a cellulose solution is less than 1, a difference that can be made up with another 1/4" of EPS.

The whole-wall R of either the 2x6 16" o.c. Roxul + 4" XPS or 2x6 16" o.c. cellulose +2" polyiso + 2" EPS is about R35 after thermal bridging, which is on the fat side of being fully economic for a zone 4 climate.  As a starting point, take a look at Table 2, p10 of this document, which recommends R25 for a whole as the point where you need to sharpen your cost-accounting pencil.  A pretty-good R25 wall would be backing off to 2x4 16" o.c. framing and 4" of EPS-only  (or 3" of polyiso only, or  2" of exterior iso + 1" of exterior EPS.) You could also get there with 2x6 16" o.c./cellulose and 2" of exterior foam (or 2" Roxul Comfortboard IS but that's usually more expensive.)

None of these stackups would require an interior side vapor barrier in a marine zone 4 climate, or any interior layers more vapor retardent than standard latex paint (about 3-5 perms), and would be made less resilient to moisture by inserting an interior side vapor barrier.  An inch of exterior foam or rigid rock wool would be enough, even if you WEREN'T  giving the siding furred out ventilation gap/rainscreen.

Building the foundation walls with insulated concrete forms (ICF) with the exterior foam of the ICF co-planar or a half-inch shy of your wall foam would make it easier to get a continuous thermal envelope for all 6 sides of the cube (including your sub-slab foam- 2" EPS recommended there).  With an exterior-side only approach to basement insulation you would have to insulate under the footing too, not just the slab.  With an interior side only approach you'd be fine with 1" EPS that extends down to the footing, thermally breaking the slab edge from the foundation wall, and building out a non-structural 2x4 studwall with R15 Roxul (safer below grade than cellulose, since it can't wick water in the event of flooding), then insulating the band joist and foundation sill with an inch of closed cell spray foam  sealing it to the basement wall foam and stud plate, plus R23 Roxul to fatten out the R to reduce the thermal bridge between the foundation sill and exposed exterior concrete.

I would take a good look at 2x6 @ 24" walls filled with cellulose and covered with EPS (either inside, outside or both). Think carefully about air barriers.