Hello: Note: Labour is not an issue as I will be doing it my self. From outside to inside: Canexel siding Strapping 2" R-10 rigid EPS 2x4 framing (24" oc) with R-12 Fiberglass batts 2" R-10 rigid EPS 2x4 framing (24" oc) with R-12 Fiberglass batts Strapping Gyproc. Thanks, Rob.

The cost & density of R5/inch EPS is high, and the vapor permeance low. (Or did you mean XPS?)

At 24" o.c. 2x4 framing may not meet the minimum structural load requirements unless both studwalls are structural.

Without cut-in bracing you'd need structural sheathing layer (not mentioned in your stackup) to keep the wall from racking under wind load.

Low density fiberglass is tough to install without gaps & compressions. Friction-fitting R15 rock wool or fiberglass are superior products, and can be installed a lot closer to the perfection required to get the tested-rated performance out of them.

Putting all of the foam on the exterior would yield a higher wintertime R-value, since the inner layer of rigid polystyrene would run cooler than if it were inside the outer studwall, and EPS/XPS increases in R value with falling temperatures.

A 2x8 studwall 24" o.c. with 4" of EPS on the exterior would have NO structural load issues, the same number of board-feet of lumber, only about half the number of framing timbers to cut, and would yield the same or slightly higher whole-wall R. It would also put all of the structural wood at a warmer (read "less susceptible to wintertime moisture accumulation") layer of the stackup.

Posted By robert.thompson on 21 Nov 2012 10:57 AM
Hello: Note: Labour is not an issue as I will be doing it my self. From outside to inside: Canexel siding Strapping 2" R-10 rigid EPS 2x4 framing (24" oc) with R-12 Fiberglass batts 2" R-10 rigid EPS 2x4 framing (24" oc) with R-12 Fiberglass batts Strapping Gyproc. Thanks, Rob.

Robert,
2x6 studs are probably minimum @ 24" oc,
but before using a poor insulator like fiberglass, it makes more sense to use high efficiency SIPs

Thanks Dana1. :) I like your idea much better than mine. I was considering double walls with blown-in cellulose (DIY) but my friends are saying it will settle over the years and I'll end up with voids at the top of the walls. I'm trying to achieve R-40 or greater walls with minimal air leakage - all this at the lowest cost. Any additional input would be greatly appreciated. Rob.

the lowest cost for an R40 wall can be achieved with a simple double 2x4 wall with taped ZIP sheathing. (two rows of 2x4's with a 5" space between) My preferred insulation is dense packed cellulose (which will not settle), installed by an expert. Fiberglass might work if the wall is air tight enough, but rock wool would be better - perhaps 3-1/2" batts in the stud cavities and a horizontal layer of 5-1/2" between. The strapping under the siding is a "must", especially with wood siding. 2" EPS would be helpful as an outside layer beyond the sheathing, but not critical.

Posted By Bob I on 23 Nov 2012 11:04 AM
the lowest cost for an R40 wall can be achieved with a simple double 2x4 wall with taped ZIP sheathing. (two rows of 2x4's with a 5" space between)

Therefore, the lowest cost R-50 wall can be achieved by simply increasing the 5" spacing, right? Thanks, Rob.

Posted By Dana1 on 21 Nov 2012 01:55 PM

A 2x8 studwall 24" o.c. with 4" of EPS on the exterior would have NO structural load issues, the same number of board-feet of lumber, only about half the number of framing timbers to cut, and would yield the same or slightly higher whole-wall R. It would also put all of the structural wood at a warmer (read "less susceptible to wintertime moisture accumulation") layer of the stackup.

Hi Dana1:

How do you attach 4" of foam to the 2x8's? Just use long screws?
Then, how do you attach the vertical strapping for the siding? Even longer screws?
Please do not get me wrong, this is a serious question.
Rob.

"Therefore, the lowest cost R-50 wall can be achieved by simply increasing the 5" spacing, right? Thanks, Rob."
yes.
I've found that not all cellulose contractors who do "dense pack" are familiar or comfortable doing thick walls,so find one that is. If you can't, perhaps you can divide the space into two equal spaces or a 5.5" and a 8.5" space. The 2x8 + EPS foam should work, but Building Science Corp recommends an equal R value outside & inside to prevent condensation. this might be mainly using non permeable foam like aluminum covered polyiso; not sure about EPS.

use a few 6" screws & 3" washers to attach the foam to the sheathing, then fasten the strapping on with 7" screws into the studs. Be sure to stagger the joints on the foam and tape the joints. Air sealing is critical here!

Thanks Bob, I appreciate your time. Rob