Hello. First time post here from a forum lurker over the past month or so. I am planning a house design some 2 years from now that will have some passive solar elements, and a bit of passive house features. We live outside Ruidoso, NM, in the southern rockies, with HDD in the 5500 area, 7100 ft. altitude. High percent of days with sun. Low humidity area. No need for AC. My question is whether or not to use a vapor barrier. As of now, my thoughts on a wall design for 2x6, 16" OC from outside to inside: stucco, 1" XPS (like Dow Blue board), 1/2" cdx plywood, 5.5" insulation (leaning to blown-in cellulose), 8 mil vapor barrier, sheetrock. Plans include an HRV. My reasons for including the vapor barrier involve what I read in James Kachadorian's book, "The Passive Solar House," plus some other internet research. What do you think about the vapor barrier? Is it worth it to include it on the warm side of our wall? Thanks!

At 5500 HDD you're essentially in climate zone 5, and can get away with air-tight methods on the interior side wall surface (glued/caulked gypsum, caulked sealed electrical boxes & plumbing/electrical penetrations on exterior walls) along with vapor-retardent paints to bring the permeance down to class-II vapor retardency.  If you actively humidify the interior to 40% or higher in winter, a poly sheet under the gypsum would be better (class-I vapor retarder). See:

http://www.buildingscience.com/documents/information-sheets/3-water-management-and-vapor-control/info-sheet-310-vapor-control-layer-recommendations

http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers?full_view=1

and

http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

The fact that it's a dry climate is consequence when determining whether a vapor retarder is needed. It's all about the average coldest-week outdoor temperature, and the INTERIOR  temp & RH.   If interior water vapor can enter the wall and condense on the colder part of the wood/insulation, you can develop issues.  Cellulose is a good choice, as it can buffer that humidity quite a bit and prevent mold growth in your dry climate (it dries reliably & quickly at the low pressures of 7100', with big diurnal temperature swings), but repeated condensing/drying cycles will cause it to sag/settle a bit over time- without a vapor retarder you'll be topping it off in a decade or so (unless you dense-pack with wet-spray which glues it in place.) The significant thermal mass of cellulose compared to other fiber alternatives gives it a performance boost with the overnight temperature plummets you see in the mountains.  Definitely dense-pack (wet or dry blown) to maximize this effect, maximize the hygric buffering capacity, and minimize infiltration & intra-cavity convection, and minimize/eliminate settling over time.
 

Dana1: Thanks very much for taking the time to post a most informative reply. I had read over the 3 documents that you referenced in your post, and the last one caused me to question whether or not I need the vapor barrier. As I see things right now, I am kind of on the bubble. I am leaning to the high performance 8 mil vapor barrier offered by these folks: http://www.conservationtechnology.com/building_films.html In our current house, we often have indoor humidity greater than the outside on the very cold days. We installed a very nice wall fountain with copper and slate near our front door. My wife has stated she would like to have one in a new house as well. Again, I appreciate your counsel! Regards, Steve

Of course indoor RH will always be higher than outdoor RH, that's not the issue- outdoor RH is completely irrelevent.  The real issue is what temperature is the dew-point of the interior room air, will that temperature occur repeatedly or at long duration in susceptible materials inside the wall structure.

When it's 68F/30% RH inside, the dew point is about 36F/2C. (Read laterally to the left from the 20C/30% intersection to find the temperature at which intersects the 100% curve.)  The average point within the insulation during the coldest weeks of the year where that temperature occurs is what we're trying to keep interior air & water vapor away from. And thats a function of the average coldest-week temperature vs. the interior air temp. 

For the sake of argument assume that the coldest week averages 32F/1C)  for the daily temperature (the average daily highs might be 42F, and the lows 22F but the average is 32F.)  Let's assume you're keeping interior at 68F/30% RH.  With R5 of sheathing and a R20 of studwall for a total of R25,  the point in the structure where dew point (36F, from the previous paragraph) occurs at R25 x(68F-36F)/(68-32F)= R22 out from the interior, which is inside the foam, so you wouldn't need any vapor retarder at all (the foam doesn't absorb release humidity quickly, and it's performance & longevity is impervious to condensation.)

But if the average temp of the cold week is 10F lower, or the interior RH 10% higher, the dew point would occur within the outer portion of the cellulose/studs, and a vapor retarder IS required.

In your location the outward drying capacity is good, and unless you keep it downright tropical indoors on the coldest weeks of the year (72F, 65% RH) you don't need a class-I vapor retarder.

Also, your altitude affects the dew point- it'll be lower at lower air pressure, which gives you more margin.  Without researching your weather history in detail, I'm still fairly certain you don't need a poly vapor retarder with that stackup & cellulose insulation- vapor-retardent paint should be more than enough.  Cellulose fibers are hollow so it rapidly wicks away & redistributes minor micro-condensation- it only becomes an issue if the condensation zone is deep & persistent allowing it to saturate (or frost).

Wow! Again, thanks so much for sharing your knowledge Dana1! I love the graph and the analysis that you provided. Regards, Steve