New construction in " Green community" that hasn't built anything that wasn't Fiber cement board over Tyvek over OSB with 2x4 glass batt filled wall. I like the community just stuck with the builder. Also stuck with Hardie plank siding, mandatory. I'm asking for 3/4 xps or eps exterior foam over the Tyvek and OSB and something besides glass bats to fill cavity. Can I use 3 inches closed cell ( manufacturer approved), or fill cavity with all open cell foam, or flash 2 inch open with cellulose, or all cellulose to fill 2x4 cavity? Drywall and latex to finish inside. If I don't get the exterior foam, then im back to Hardie plank over tyvek over osb so what can I do to fill cavity of either a 2x4 wall or a 2 x6 wall( not sure if 16 or 24 oc) for max R value without condensing in my zone 4a not far from zone 5 . I know the hfc issues and cost differences. I meet with builder again this week. Wanted SIP's but they don't get it. I'm near St Louis, MO Thanks, Jim

Actually I meant to say flash with 2 inch closed cell not open cell then fill with cellulose. Additionally, big problem with fibre cement board, Certainteed, cracking in this community. Haven't seen a home yet be resided with exterior foam. I called a James Hardie authorized contractor about using Hardiplank over exterior foam and seemed reluctant to do any more than 1/2 inch if at all. I know Hardie says up to 1 inch thick is ok with the caveats. I would expect a little wave action with installation over foam. Homes here are constructed with no eaves on gable with 1 inch facia board on ridge side, so no real room to put build out ( trying to remedy that with new house) otherwise I'd like to strap a gap between siding and foam. I've learned through this forum about drying to inside and outside so if I get exterior foam I doubt that I can fill cavity of 2x4 with closed cell thereby trapping the OSB and that may rule out flash/fill also. Goals air seal, stop thermal bridging with exterior foam if possible, get best R value and seal without exterior foam if not possible with builder. Any advice appreciated, want to go in armed with knowledge when I meet with builder again.

Thanks, Jim

Closed cell foam in a studwall cavity is generally a waste, since it's performance is severely undercut by the thermal bridging of the framing, and the fact that you can't do a full cavity fill since it's too difficult to trim. In a 2x4 studwall open cell foam with a full cavity fill actual seals slightly tighter than 3" of closed cell foam (compared in this lab setup: http://www.greenbuildingadvisor.com/blogs/dept/musings/air-leakage-degrades-thermal-performance-walls ), and only ~R1 lower in whole-wall R after the thermal bridging of the framing is factored in.

From a dew-point control point of view 3/4" of exterior foam on a 2x4 wall would be on the skimpy side for skipping interior vapor retarders in climate-zone 5, but is fine for climate zone 4. On a 2x6 wall 3/4" of XPS would just barely cut it for zone 4, but not at all for zone 5, on R-value alone. But as long as you have at least 1/4" of air space between the foam and the siding, and the foam has no vinyl/poly/foil facers you would still be fine without interior vapor retarders, since at only 3/4" both EPS and XPS have fairly good drying capacity toward the exterior. If you used foil-faced polyiso you'd have a slightly higher R, but would block drying toward the exterior entirely, so from a long-term moisture performance point of view it's going to be better to use EPS or XPS.

Cellulose would give you about the same R value as open cell foam, and would be able to adsorb most of the moisture without damage during those hours when the sheathing was below the dew point of the conditioned space air, lending a bit of protective factor. With any fiber cavity-fill air-sealing the studs & plates to the sheathign with caulk becomes somewhat more critical than with a foamed-cavity. (With either you still have to caulk doubled-stud plates, and between the bottom plate & subfloor, etc.)

If you can't go with exterior foam, the thermal bridging issue looms large. A 2x4 wall without foam is about R10 whole-wall with thermal bridging factored in, and adding the 3/4" of foam brings it up to about the same performance as 2x6 framing with o.c. foam or cellulose fill (no exterior foam). But with as little as 1/4" of back venting on the siding you can skip the interior vapor retarders in your climate. A 2x6 24" o.c. framing package is about the same board-footage of lumber as a 2x4 wall and fewer boards to cut, so it should be about the same or slightly cheaper, and ripping 3/8" ply wood or OSB furring to put over the housewrap as a means of establishing the back ventilation for the siding is pretty cheap & easy too. It may be a better way to go 2x6 with vented cladding rather than 2x4 with only 3/4" foam, since it's the same performance, cheaper, and a bit less than an inch thicker in overall wall thickness.

see: http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-insulating-sheathing-vapor-retarder-requirements/files/FAQ%20Vapor%20Retarders_2011.pdf

With 3/4" XPS on the exterior a 2x6 24" o.c. wall your whole-wall value would be ~R17-R18. Without the foam it would be R14.

A 2x4 16" o.c. wall with 3/4" of XPS comes in around R13, without the foam it's about R10.

Thanks Dana1, I'll more than likely end up with 2x6 oc filled cavity or cellulose to fill and try to get the siding vented . Would the oc foam be any better at noise transmission? Surprised about the cc foam with stud walls, I guess you don't worry about that so much with a cc SIP. If I do get a chance on exterior foam will try for 1 inch xps or eps and vent it for the 2x4 wall or 1.5 inch for the 2x6 wall. Will have to convince them to extend roof line to build out the wall no room otherwise. Will get the top plates and rim joists covered/caulked too I've read a number of your posts, thanks for everything you do to help educate us all. Thanks!

Cellulose will dampen sound a few STC points better than o.c. polyurethane foam.

The thermal bridging of the framing is the thermal performance killer for stick-built, which is why it's usually better to apply the high-R foam-budget outside the sheathing rather than between studs. In a 2x6 wall with a 25% framing fraction even if the cavity insulation was a magical substance that performed at R-million per inch and filled the cavity completely you max out at R23 for whole-wall performance (which is well-short of R5,500,000 :-) ) If you put the max-practical 5" of closed cell polurethane in a 2x6 studwall it's ~R30 center-cavity, but only ~ R15 after thermal bridging. (You get the same performance out of a 2x4 wall with open cell cavity fill and an inch of exterior closed cell or iso.)

Even SIPs have some thermal bridging at the door & window headers as well as top/bottom plates, but it's typically a sub-10% framing fraction- less than half the thermal bridging of stick-built. On really fat SIPs it's possible to thermally break the window & door structural frames.