R-28 wall....possible with 2x4...?

[AUGER]

hi all....
we are adding on to our house and the existing house is 2x4 construction...
with r-12 batted walls.....
here's where my problem lies....the new code requires an R-28 insulation value for homes heated with forced air electric...
so my existing house is fine as is but the second story we are building over it has to meet the new code.....R-28

my choices so far are .. . building 2x8 walls over 2x4 walls....just dosent seem right...lol

or 2x6 R-20 batt wall with 1.5" foam on the exterior....over the 2x4 wall below


or a 2x4 wall with R-13 batt and 2" exterior foam sheathing and 1" interior foam sheathing (and strapping) fewest thermal bridges....with this method...I hear


or 2x4 wall with sprayed rigid foam R-6 per inch and 1.5" foam sheathing on the exterior.....


I'm open for any other suggestions....to acheive R-28 wall that are cost effective....

initially we only budgeted for building 2x4 walls with R-12 batts like the existing...house

ironically the sloped part of my vaulted ceiling as per new code only requires R22
and the flat area's R31....

thanks in advance
Auger

[David]

This might be of help:
http://www.buildingscience.com/designsthatwork/verycold/DTW_VeryCold.pdf

If you don't run the a/c at all, I believe you could insulate further to the inside. That is, you could have rigid foam insulation applied to the inside side of the wall assembly and apply drywall over that.

If you were to run the a/c, like we do, then the rigid insulation could become a surface for condensation. IIRC.

[rabadger]

Is this a quiz? 2+2=5 :)

2x6 wall 2" foam R-4 per inch, 3.5" R-15 batt. 15+8=R23. Exterior gets 1.5" of R-4 per inch. 15+8+6= R29. OR 2X6 wall with 3.5" R15 + 2" of your R6 per inch foam. 15+12=R27 and 1" of R4 on the outside =R31

Get yourself a structural engineer to determine if the 2X4 wall below will support the 2X6 wall above.

Did I pass? :(