Phil, personally, in any kind of occupied structure like offices, theaters, etc., I dislike single wythe construction … because all you have for a moisture barrier is the “pooky”. And, even with correct flashing, integral water repellants, etc., the moisture - at some time in the future - will find it’s way to the interior and leak out on the floor, or somewhere else - whether thru the wall itself or thru cracks, etc., regardless of the owner’s maintenance program. Luck is on our side if it doesn’t. Unfortunately, the owner prefers this for construction type for sound purposes and for economics (but apparently not, if I dare say, for sound building economics?).
The HVAC’s system to condition the space is what really causes the problem in the first place. You should get the information from your MEP about the design temperature and humidity inside the building and then get some climatic information (maybe from the MEP).
You want to see the direction of vapor pressure from a higher water vapor content to a lower vapor content (a function of the difference between the content of water vapor in the air between inside (conditioned space) and outside (the natural climate). Warmer air generally holds more water vapor than cooler air so generally the direction of higher vapor pressure to lower vapor pressure is from warmer air to cooler air. At some point in a wall assembly there will be a temperature at which water vapor (a gas) will condense into water (a liquid); the dew point. I have learned that water will actually begin to condense on the warm side of the dew point, but the dew point is the point at which all water vapor will condense. The design goal is to prevent water from condensing within the wall assembly, and this is achieved by placing the vapor barier in a position to block the transmission of water vapor BEFORE it condenses into water. This generally leads to locating the vapor barrier on the “warm side” of the insulation.
You need to understand the basic physics, but your MEP should understand enough more to fill in the gaps and assist you to make an intelligent decision. Although a good MEP will be invaluable in helping out, he/she will insist that this is the Architect’s responsibility.
In Houston, we have a few days where the vapor pressure moves water vapor from inside the building toward the outside, but the predominate direction is from outside in. From this we conclude the the best location for a vapor barrier is toward the outside of the wall assembly (the warmer side of the insulation). Note that this solution is not always appropriate at every time on every day of the year, but it is generally appropriate.
On the other hand, folks in Minnesota would generally experience the opposite climatic conditions. They would locate the vapor barrier toward the inside of the wall assembly (the warmer side of the insulation). Again, please note that this solution is not always appropriate at every time on every day of the year, but it is generally appropriate.
As some have noted, there is a point between North and South where a change in location of the vapor barrier becomes appropriate. This has more to do with outside temperature and humidity vs inside design temperature and humidity than it does with latitude (which is why the line meanders around somewhat).
Tom,
Over the mountains in North Carolina we flip a coin on the vapor barrier question. Charlotte and parts north and west may or may not get one inside; Fayetteville and toward the coast will likely get one on the outside, subject to the type of design issues discussed above.
The XPS insulation you’re putting in the wall has a vapor permeance of 1.1 per Dow - darn close to a vapor barrier by itself. I wouldn’t bother with more. I’m more concerned about the single-wythe CMU construction - you’ve got a coat of pooky between you and the great outdoors. Hopefully it’s a good coat of good stuff. I’d look into bottom of CMU wall through-wall flashing just to be safe. And I’d do 1 inch of XPS with Z-channels, then a second layer of shiplap XPS continuously over the top of the Z-channels to isolate the gyp bd from any moisture finding its way through the walls at the furring.