Kermit,
Thanks for your thoughts. Obviously you have a lot of experience and I appreciate your (and others) willingness to share what you know here. I’m not questioning building choices made by others; clearly I have no standing in this area. I’m just one gal trying to figure the best I can how to get our house built.

I’m having trouble picturing what this means:

and the shoe/ plate thermal break is a new twist we brainstormed yesterday.... using a 3/4" EPS Performguard 5" strip at the shoe & the plate... the mooney there becomes a 1x3 furring strip, while remaining 2x2 in the field.

In the above quote, is the shoe/plate the treated sill plate that sits on top of the foundation? I don’t get where you’re proposing the 3/4" x 5” strip of rigid insulation is supposed to go.

Obviously this my novice POV as I’ve got no hands-on framing experience. I understand if the framing details are just too much of a pain to try to discuss in words on a forum … so it’s not critical that I know this. I was just trying to understand it better.

Thanks

if you have a wooden floor joist system, then all of that is below the sub-floor... the area on top of the foundation...under the subfloor, is commonly referred to as the band-joist area.... that has a different strategy /technique for insulating

the shoe.. refers to the bottom horizontal 2x4 (2x6) ... and the plate is the double 2x4 (2x6) top horizontal part of the wall... these are both areas that have a lot of thermal bridging... which you can choose to ignore, or develop a strategy to defeat....

i don't know how to post drawings/ .jpg's on this forum, or i would be glad to show you the strategy we came up with

naturally, all of these strategies depend on your climate and your goals...where do you want to build ?

My take on Mooney walls is that while they're pretty decent performers, the extra complexity isn't worthwhile with only 2x2 or 2x3 laterals, since you can easily get the same or better performance with a simpler assembly (requiring much less detailing) using exterior rigid foam. This is borne out by the apples-to-apples wall assembly comparisons in the Building Sciences document:

http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

I have nothing against cellulose (I'm a real fan, in fact, and I wouldn't build a Mooney Wall or double-studwall without it for it's hygric buffering capacity) but leaks can go undectected for quite awhile since it's so good at storing & redistributing moisture, particularly in thicker assemblies. There's a reasonable argument to be made that high-density new-school fiberglass has similar air-retardency, but

A: make leaks more immediately obvious and

B:Is able to dry in place once the leaks were fixed.

Still, the difference in cost of cellulose as compared to 1.8lb Optima or 1.8lb Spider tips me toward the cellulose as a good value. At lower density the fiberglass products fall off in air-retardency, whereas minimal density cellulose is still pretty good.

With Birdsong's Larsen truss approach, where the cellulose is right up tight to the exterior siding seems a bit risky to me though, since without a rainscreen cavity to dry into the exterior drying capacity of the assembly is hampered, and exterior rain leakage events can wick in large amounts of moisture undectected. Rainscreen gaps (as required by Canadian code) are very cheap insurance against those types of issues, and highly recommended for just about any stick-built wall assembly.

Dana...
i've done a couple exterior foam homes..... and i've resided about 10 from the '80's

one of the problems is detailing the exterior......
IF... you apply the foam to the studs and sheath over, then you have to block out for openings
and the shear strength of the sheathing is lost

IF you apply the foam to the exterior of the sheathing, then you still have to block out for all of the openings
and , if you have typical exterior trim, you need blocking for the trim too

i've worked on many homes that applied the trim directly to the foam with long nails...
not a pretty sight... lots of movement, lot's of bad joints

and all of the ones we resided had extensive insect infestation.. we sprayed all of the areas with Bora-Care before
closing up... but I don't hold out a lot of hope for the long term.

here's some of the trim i'm talking about:
casing, corner boards, water table, soffit, cornice, light blocks

also.. most of the shear detailing and uplift requirements are facilitated by the judicious use of the plywood sheathing

so i would think that exterior foam will wind up outside the sheating
naturally... anyone who has a favorite energy framing strategy also has evolved a whole system of tricks and field expedients to make it work

but we both know that half of those applications are just ignoring the problem ..not really solving it

my favorite example is exterior foam on foundations:
in RI it is ACCEPTED practise to install foam on the exterior of the foundation..... wall typically 1" Styro-SM,
then they cut it OFF at the finish grade line..... i can show you 100's of examples of this
and our BI is very fastidious... this is allowed under RI code and the lesser of 2 weevils...
it exposes the concrete so the termite inspections can be made
of course it does ZERO for the most exposed part of the foundation in terms of R-value

detailing the exterior.
Thats one of the real advantages of a 1x3 "rain-screen" which you attach through the foam to the studs with long screws - you have a good solid surface to attach trim and possibly windows. I've used the technique on new & old buildings and have decided it is an excellent system. Easy to do, inexpensive and takes care of a what has been an ongoing long term problem - the deterioration of wood (and cement) siding. Allowing the back of the siding to dry out prolongs it's life and that of the paint finish. Vinyl doesn't need it, but I think it is preferable to a zillion nails perforating the foam.

Posted By Bob I on 12 Apr 2011 03:12 PM
detailing the exterior.
Thats one of the real advantages of a 1x3 "rain-screen" which you attach through the foam to the studs with long screws - you have a good solid surface to attach trim and possibly windows. I've used the technique on new & old buildings and have decided it is an excellent system. Easy to do, inexpensive and takes care of a what has been an ongoing long term problem - the deterioration of wood (and cement) siding. Allowing the back of the siding to dry out prolongs it's life and that of the paint finish. Vinyl doesn't need it, but I think it is preferable to a zillion nails perforating the foam.

Long-nailing the siding through foam is a LOUSY approach and something of a straw-man argument for kermit to make. Just because it's possible to do a hack job with exterior foam doesn't mean you HAVE to do a hack job with exterior foam.

IMHO rainscreens should be required by code for stick-built structures in the US  just as they are in Canada (at least anywhere that gets more than 20" or annual rainfall, or more than 6000 heating degree days). It's a relatively low cost cure for a world of moisture loading ills.  Rainscreens would be of advantage in Mooney Wall assemblies too- even more so than with the foam-clad approach. But in the foam-clad case,  furring over foam to support the siding (furring long-screwed to the studs) is absolutely the right way to go.

The reason the lifespan of painted back-primed wood siding with 1x furring for a rainscreen is substantially longer than with wood siding applied directly to the felt/housewrap, or wood siding with no paint on the interior face due to the quicker drying, lower moisture content,  and much reduced moisture cycling of the wood under the paint.  The vapor-pressure of the moisture released from the wood from temperature cycling is orders of magnitude lower, minimizing flaking/bubbles/cracks.  Being lighter than fiber-cement, it requires fewer fasteners for hanging the furring as well, but that's a pretty minor issue.  I'm not the biggest fan of vinyl, but vinyl siding is inherently back-ventilated, and doesn't need much depth to the rainscreen and is light enough to be mounted on the thinnest of furring.

Up to about 3-4" of foam this approach is pretty easy, but for fatter rigid foam the screw lengths required for hanging the furring become a real PITA, and the moment-arm is long enough to require tighter spacing.  Still, 4" of iso on an advanced-framing 2x6 24" o.c. cellulose-filled studwall comes in at ~R40 for a whole-wall R using a 15% framing factor.  You can't quite get there with a Mooney Wall using mere 2x6 studs, even with 2x6 laterals (the rim joists and plates being the thermally bridging Achilles heels of a high-R Mooney Wall.) To hit real-world R values higher than ~R38-40 other approaches would be easier and more robust.

Sounds to be like a nice idea, if i were constructing these, this would have been my plan for sure....