I know I have seen the answer to something on this any number of times, but now that I need it, I can't find it.
Things have just switched around and the move to NM is probably put off for 9 months while I help my daughter in Houston reno a 1907 bungalow.  Part of that will be to add on to the back.  Will use stud construction, sealed like it should be, and EPS on the exterior under the siding with airspacer.  But in this humid, cooling dominated climate, do I use a vapor barrier on the inside?

In general vapor barrier goes on warm side. In houston I would say on the outside.

There isn't a simple answer to the question, but in new construction it's usually possible (and preferable) to design stackups such that they don't NEED a vapor barrier, or even a vapor retarder more vapor-retardent than standard latex paint. Air-tightness is always more critical than vapor retardency, and if you're detailing the sheathing as an air barrier, as long as you don't use foil or vinyl wallpapers on the interior the sheathing & studs can dry toward the air-conditioned interior without a problem. With the rainscreened/air-spaced siding the bulk-load moisture on the exterior drops at least an order of magnitude, and even with Houston's high summertime outdoor air dew points, you won't end up with condensation in the stud bays or on the sheathing.

Putting a vapor retarder on the interior would INCREASE rather than decrease the potential for mold & moisture problem in the stud bays, whereas with EPS on the exterior and standard latex paints on the interior the moisture from exterior drives will pass through the wall without collecting in any of the layers to cause a problem. EPS at even 1/2" or is by itself a class-III vapor retarder, and in a Houston (US climate zone 2) climate, that's more than enough enough under most siding types. Only if it's under brick/stone veneer or stucco or similar siding material capable of storing up dew/rain water and release it in intense burst when the sun hits it would you need to adjust the vapor permeance of the exterior layers downward (and even then, not by much.)

If the walls are unshaded there is some cooling season performance advantage to using foil-faced EPS with the foil facing the vented cavity under the siding. Foil is a true vapor barrier, and if you go that route, you should avoid putting any other strong vapor retarders in the assembly.

Also in NM, with enough EPS on the exterior there is similarly no need to put any vapor retarder on the assembly, and it will be far more resiliant if done without vapor retarders. The amount of exterior R required varies by climate, and it's embedded in IRC code prescriptions at this point. NM has multiple climate zones spanning US zones 3-5:

http://www.greenbuildingadvisor.com/sites/default/files/images/H-T%20Zones%20with%20Cities%20ABC+.jpg

The prescriptive minimum exterior insulation R value embedded in the IRC for being able to skip the strong vapor retarders in 2x4 construction varies from R0 for zone 3, to R5 for zone 5:

http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-insulating-sheathing-vapor-retarder-requirements/images/faq_table.jpg

Dana,
As always a well thought out and detailed answer. Interesting about the lack of need for a vapor barrier at all in Houston. Definitely wil detail the new construction wall assemble. However the rest of the house is leaky as hell being 105 years old. At this time trying to add 300 sq ft without putting a whole lot of additional load on the house Hvac.
House in NM, will use KAMA -EBBS, all foam steel embedded walls.

Another question on the Houston reno, Dana. Floor is currently one thickness of 3/4" heart pine over the floor joists which are pier and beam. No insulation no vapor barrier over the damp soil. I was thinking of putting down a layer of EPS followed by1/2" plywood. With hardwood on top of that. For vapor barrier and insulation. Does that make sense? I
Can't too thick on the EPS as I don't want to raise the floor anymore than I have to. Also! Will the assembly be sturdy enough? Or spongy? Make any sense to glue the foam to the existing floor and the plywood to the foam? Making a pseudo SIP?

> was thinking of putting down a layer of EPS followed by 1/2" plywood.

I would use polyiso to get more R value in less space than EPS.

While it's OK to put EPS between the planks and a new sub-floor, it's not likely to achieve the air tightness or R-value you're looking for.  Using iso or type-I EPS doesn't cut it in that application due to insufficient compressive strength- you'd need either XPS or Type-IX EPS as the core of your "floor SIP".

Putting half-inch t & g plywood either directly on the planking or with classic rosin-paper as a slip surface separating the ply from the planking, caulking the seams of plywood with acoustic sealant, then spraying 1-2" of closed cell spray foam on the underside of the planking puts all of the susceptible on the inside of the thermal/pressure/vapor boundary, and would air seal it pretty well.  That would still leave the thermal bridging of the joists, but if you wanted a higher effective floor-R you could then add a few inches of fiber or open cell foam below the cc foam, (even cheap R13 batts, facer side down if the joists are a standard spacing.)

Alternatively you could do an all open-cell foam approach with 4-6" of open cell on the underside of the floor, but with open cell you would then incur some amount of moisture accumulation in the pine planking when outdoor dew points hit the mid-70s.  Even an inch of closed cell foam (~1 perm, give or take) would be somewhat better from a moisture cycling point of view, but open cell wouldn't be a disaster unless you over-cooled the place to under 74F all summer every summer.  The mean summertime outdoor dew point in Houston is ~74F, so as long as the planking stays a degree or two above 74F  during the hot weather it woudn't take on enough summertime moisture to matter.  If the interior is going to kept cooler than that, a flash-inch of closed cell on the planking is better, even if you add more R with open cell foam or fiber insulation.

Type-IX EPS = 25 psi
polyiso = 16, 20, 25 (readily available) all the way up to 140 psi

I agree with going underneath the floor if you want high R values.

Seen any 25psi iso down at the local box stores or lumber stores lately? :-) (Special ordered through distributors catering to the commercial construction, maybe.)

The 2lb density "Grade 2" iso commonly used in commercial roofing applications runs ~20 psi, but the most common goods found at most building supply outlets are lower density, lower psi. "Grade 3" roofing iso is higher density, slightly lower R, and runs 25psi. Ultra-rigid 4lb density hard-board iso runs ~100psi, but it's denser than optimal for insulation at R5/inch- (the same as XPS but more expensive, but also more rigid.) I suppose half-inch hard-board iso instead of half-inch plywood would get you some thermal bridging (R2.5 instead of R0.5), but it's probably not worth the upcharge, even if you can get it in the comparatively small quantity needed.

But I suppose Type IX EPS isn't usually found at box stores either, eh?

Even with 25psi goods you'd have to be careful about staggering the seams of the half-inch ply to avoid compressing the edges over time- it's less than ideal. It the plywood on foam sandwich works better when supported by a rigid concrete slab than it would on 3/4" pine planking on joists.

Who makes 140psi iso (and at what R/inch, density?)

Here is info on Trymer 6000 although I have no idea where anyone would buy it.

http://foamsalesandmarketing.com/pdf/Trymer/trymer.pdf

Posted By jonr on 21 Dec 2012 06:54 PM
Here is info on Trymer 6000 although I have no idea where anyone would buy it.

http://foamsalesandmarketing.com/pdf/Trymer/trymer.pdf

The 6lb polyiso is only supplied by Dow in large bun form- fabricating it into sheets (or pipe shells, tank & vessel coverings, as described in that document) is up to the customer.  I doubt it's stocked by any building-supply distributor- probably a full custom order only for folks who actually need it (and aren't too sensitive about cost.)

It's rated R5/inch, just like most vendors' hard board 4lb sheet goods, which are at least sometimes stocked by or at least can be ordered through distributors handling roofing iso.