Exterior wall help on new home construction

Very good assembly as far as I'm concerned. Don't forget the moisture management details.

It's an acceptable assembly, but barely IRC min for exterior R for 2x6 framing in zone 5. There is not a lot of dew-point margin at the sheathign with only 2" of EPS about R8-ish on the exterior, and R20-ish center cavity.

And it's not R30 whole-wall either. A 2x6 studwall at 24" o.c. is typically about a 20% framing fraction (at about R5.5) and with the rest of the stackup of siding/sheathing/gypsum makes the rest comes in around R14 average after thermal bridging. Add in theR8 EPS, and you're around R22 whole-wall (R30 at center-cavity only.)

To hit R30 whole-wall: A 2x4 16"o.c. studwall wall at a typical 25% framing fraction would be about R9 after thermal bridging, with siding/sheathing/gypsum added in. Add 3.5" of rigid polyiso (R21) you'd hit your nominal R30, at about a half-inch thinner wall assembly than the one described.

And note, with a center cavity ratio R21 outside the sheathing, R13 inside, you have pretty huge margin on dew point control at the sheathing.

Tip: polyiso roofing foam is often scavenged during commercial building re-roofing & demolition, and can be had for 1/4-1/3 the cost of virgin stock. It comes in many standard thicknesses, but 2" , 2.5", and 3" are pretty common. A 2.5" or 3" of reclaimed roofing polyiso and a 1/2-1" layer of 1.5lb foil-faced virgin stock on the exterior would cost less than two 1" layers of EPS. I have multiple vendors in my neighborhood, but insulationdepot.com ships nationwide (for a price.) But simply calling local commercial roofing contractors will often dig up a local source.

There is nothing greener than re-use of existing building materials, and in high-R foam-overs it can save thousands in up-front cash too- spend the difference on granite counters if you like (the thermal mass of the granite lowers energy-use too! :-) )

What's the Tyvek doing in your assembly? Is it a necessary and effective air barrier? What do folks think of the spray/roll applied WRBs like envirodri (http://www.guaranteeddrybasements.com/products/enviroDri.asp)? Thanks for the post and also the tip on Insulation Depot.

Thanks everyone for their responses.

I echoed what was stated above to the builder and his responses are below.

1. 2x6 24.o.c may not be right for the house and the given snow and other loads that may need to be factored into the decision.

- my thought process was to implement some advance framing concepts -
Rafter/Trusses align over studs
Energy Heel trusses
Single Top Plate
Eliminating jack studs
Ladder "t-wall" intersections
Two stud corners
Insulated Headers

2. He thinks that installing the foam will result in extra moisture and water damage. His wall structure he prefers is (outside in)
Cement boad siding
(didn't mention rain screen or furring)
Tyvek housewrap
7/16" OSB
Spray foam the interior stud bays (along the gaps, not the whole bay)
R23 BIBS
1/2" drywall

Any thoughts on this?