Posted By Dana1 on 05 Jul 2018 09:29 PM
The 2x6/R20 walls of a zone 5 IRC code-min house typically performs at about R17 "whole wall" after factoring in the thermal bridging of the framing, adding in the R-values of sheathing/siding/wallboard/air films, etc. . Adding as little as 3" of continuous reclaimed roofing polyiso it doubles the performance, cutting the heat loss through walls in half. From a lifecycle cost-effectiveness point of view that would still work if buying sheets of new 3" x 4' x 8' roofing polyiso @ $60-65/sheet, but when using reclaimed goods at @ $15-25/sheet you don't necessarily have to run NPV math on it to make the financial case.

For a rough idea of where the limits of financial rationality are, look at the zone 5 row in Table 2, p5 of this document:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

Those are all "whole assembly R" numbers, not center-cavity. A 2x6/R20 wall + 3" of continuous roofing polyiso comes in between R30-R35 whole-wall, depending on stud spacing and the material stackup of all layers. Note, the U0.24 (max) windows are better than IRC code min, but don't have to be triple-panes either. Double low-E glass with a typical low-E coating on surface #2 (the interior side of the exterior pane) and a hard coat pyrolytic low-E on surface #4 (the interior side that in direct contact with the room air) can hit those numbers even with air-fill, but it's worth the up-charge for argon fill, which brings it down to the U0.20- U0.22 range (depending on thickness.) A number of insulated glass manufacturers offer that type of glass. Cardinal's Loe-180 + i89 glass with argon fill is found in some Pella or Anderson windows, among others. The Loe-180 has an atypically high SHGC, which makes it great for south facing windows, but perhaps too much gain for west facing windows. (Cardinal has lower SHGC glass with the i89 hard coat on #4 if you want to kill the gain on the east & west facing windows.)

The wall stackup matters from a moisture management point of view. What is the siding/sheathing/framing stackup of the planned building? If you build the main building with 2x6 studs 24" o.c. with back ventilated siding you could then insulate that wall with R20-R23 rock wool where your living quarters are, and install 2-3" of reclaimed foam on the interior side, strapping the foam to the wall with 1x furring through-screwed to the studs with timber screws, mounting the wallboard to the furring. There are other possibilities too, but that's an easy R30+ to build.

What do you intend to use for heating/cooling?

Posted By Dana1 on 06 Jul 2018 04:17 PM

If the foam is going to rest directly on concrete on the it's best to use polystyrene, not polyiso, especially if installing it on the exterior of the wall, directly under the siding. Polyiso can wick & retain moisture over time, polystyrene does not. If going that route, fill your brick-ledge with EPS, and start the polyiso at the bottom plate of the studwall, with an EPDM (membrane roof material) extending over the top of the concrete & EPS all the way to the exterior for a guaranteed capillary break.

I was suggesting installing the rigid foam on the interior side of the exterior wall, not on the exterior under the siding.