Vapor barrier in Tennessee

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Author Message Tom Peck Senior Member Username: tom_peck_csi Post Number: 10 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 12:09 pm:    We have a project in Chattanooga, Tennessee where the exterior wall will be constructed with a 16 inch concrete block wall, and furred out on the interior. The exterior of the block will be finished with an acrylic coating (Thorolastic-type coating) and the interior will be furred out, insulated with extruded polystyrene, and finished with painted (latex) gypsum board. Question ... would a vapor barrier or vapor retarder be appropriate for this location? From a review of Heating Degree Days (HDD) and CDD, I would assume that the vapor barrier/retarder would go to the inside (warm side in winter) of the insulation ... correct? Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 116 Registered: 12-2002 Posted on Thursday, September 15, 2005 - 12:15 pm:    What is the building used for? Retail, light industrial, skating rink, etc. Wood furring? Doug Frank FCSI CCS Senior Member Username: doug_frank_ccs Post Number: 121 Registered: 06-2002 Posted on Thursday, September 15, 2005 - 12:19 pm:    Boy Tom, I can't wait to see the (I'm guessing many and varied) answers you get to this question. In Houston we normally place the vapor retarder Outside the insulated envelope. In Upper Michigan I'll bet you'd put it Inside. In the middle of the country like Chattanooga, I've heard that it may be best to not have a vapor retarder at all (but don't go by me, I'm sure no expert). Smarter folks than I will have opinions for you I'm sure. Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 117 Registered: 12-2002 Posted on Thursday, September 15, 2005 - 12:30 pm:    I was kinda hoping this was a cold storage facility but darn it, there's sheetrock. Air conditioned? Hot tub showroom? Tom Peck Senior Member Username: tom_peck_csi Post Number: 11 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 12:52 pm:    Alright, alright ... it will be a movie theater. We are located in (brrr) MN, so know what to do here. Understanding that there is a transition line (if you will) from MN to the south, where the VB/VR moves from interior to exterior, I figure we must be approaching that "line" ... but maybe not quite yet with a higher HDD than CDD. Framing will be steel studs (and, yes, we can go back to a previous thread for galvanizing, etc.). AC - yes. I've heard the same thing about no vapor retarder, too. So that is in the back of my mind. Also, I suppose a non-paper faced gypsum board for the furred gypsum board would be appropriate, too, for mold "control"? Thanks. Lynn Javoroski Senior Member Username: lynn_javoroski Post Number: 241 Registered: 07-2002 Posted on Thursday, September 15, 2005 - 12:56 pm:    or you can use one of the newer mold-resistive gypsum boards Robert E. Woodburn Senior Member Username: bwoodburn Post Number: 75 Registered: 01-2005 Posted on Thursday, September 15, 2005 - 12:57 pm:    Years (a few decades) ago, USG addressed this question, publishing a technical insert in its magazine ("Form & Function"?) with a map of the US with two lines meandering across its middle (approximately just north and south of Kansas, if I recall correctly). The gist of the article: above the north line, VB on inside; below the south line, VB on outside; between the two lines, since it's essentially a toss-up, no vapor barrier may be necessary, or perhaps even desirable. But it all depends on the use; anticipated unusual humidity conditions could override these rules of thumb. In actuality, since conditions change so much throughout the year, any solution short of a vapor barrier that automatically repositions itself (or adjusts its permeability) to suit the conditions may at best be a compromise, but it's usually judged to be an acceptable one. One could always do one or more dew point analyses, to see if there's a clear indication... Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 118 Registered: 12-2002 Posted on Thursday, September 15, 2005 - 01:11 pm:    I vote no VB and here's why: 16" CMU offers a LOT of drying potential to the exterior. I'd focus attention on the exterior finish coating, to make certain it will allow drying to the exterior. Then, I'd minimize the contact between metal studs and the CMU. If you could make the metal studs self-supporting, with an air gap, that'd be good. Why? Because you do have condensation potential December through March. This way, you have two walls, and drying to both the interior and the exterior, and everything's beauty-mous. But you'll have to sell it because nobody's gonna want "two walls". If you can't sell two walls, you might depend on the fact that 16 inches of CMU can be a buffer for a heck of a lot of moisture, and that's very good. John Bunzick, CCS, CCCA Senior Member Username: bunzick Post Number: 406 Registered: 03-2002 Posted on Thursday, September 15, 2005 - 01:15 pm:    I would look at some of the web sites on building sciences. I have read some pretty persuasive materials that suggest that except for very hot humid climates and very cold climates (like, perhaps, MN), vapor retarders should not be used. However, it gets much more complex than that because you need to be looking at pathways through the interstitial spaces of the building which can carry moist air; air barriers at the building envelope; interior climate, HVAC systems and their ability to remove moisture; vapor drive through building systems; and location of insulation. That's not a comprehensive list. There is a "movement", albeit small at the moment, that intends to work to get the IBC changed in its treatment of vapor retarders. Tom Peck Senior Member Username: tom_peck_csi Post Number: 12 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 01:18 pm:    Of interest, I just found this article from Building Science Corporation that basically says no vapor barrier/retarder in this region except for the exterior acrylic coating and the interior latex paint. We would have a wall assembly similar to Assembly No. 3 (on page 5) less the stucco exterior coating. http://www.buildingscience.com/resources/articles/Understanding_Vapor_Barriers_ASHRAE_2004_08.pdf I would anticipate the HVAC system will temper the humidity levels within the building, but that will be a local contractor's design-build thing to deal with. Unfortunately, I don't know what they will provide, so I can only assume the worst ... that it may not do a very good job at pulling out moisture. "A VB/VR that automaticlly adjusts its position" - not heard of one?? Is there such a thing? Or are you just pulling my leg? If there is such a thing, would this be similar to how a bentonite-type waterstop works? But it makes me think ... if this is true, wouldn't we then be better off providing dual VB/VR. This way, in the summer, the exterior VB closes due to moisture/humidity levels), allowing the wall to dry to the interior (thru the "open" VB). And vice versa in the winter. Tom Peck Senior Member Username: tom_peck_csi Post Number: 13 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 01:19 pm:    My second to last paragraph should've qouted " ... adjusts its permeability". Sorry. Richard Howard, AIA CSI CCS Senior Member Username: rick_howard Post Number: 59 Registered: 07-2003 Posted on Thursday, September 15, 2005 - 01:24 pm:    Look at CertainTeed "MemBrain" Doug Brinley AIA CSI CDT CCS Senior Member Username: dbrinley Post Number: 119 Registered: 12-2002 Posted on Thursday, September 15, 2005 - 01:25 pm:    Joe Lstiburek's Mixed Climates: ISBN 1-56158-375-8 Tom, You need Cold Climates: ISBN 1-56158-374-X for Minnesota Wayne Yancey has probably earned a PhD on this stuff by now. Wayne, if you're following this - the Marriott Courtyard next door to our office is falling down and needs Morrison Hershfield 'like yesterday'. Maybe you could dispatch a triage team of moisture mechanics. Colin Gilboy New member Username: colin Post Number: 1 Registered: 09-2005 Posted on Thursday, September 15, 2005 - 01:31 pm:    MemBrain - interesting product: http://www.certainteed.com/CertainTeed/Pro/Builder/Insulation/Prodindex/Residential/MemBrainProdIndex.htm Tom Peck Senior Member Username: tom_peck_csi Post Number: 14 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 01:31 pm:    Understood. But remember, this thread is for a Mixed Climate in TN. Tom Peck Senior Member Username: tom_peck_csi Post Number: 15 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 01:58 pm:    Wow, interesting product, indeed. But, I am confused ... the literature says with increased humidity, the permeability increases ... isn't this a problem? Why increase permeability when we want to prevent permeability in order to keep the moisture out of the insulation? Would this product allow placement of the VR on the inside OR the outside of the insulation without "worry" - since it can dry to either side? I suppose the concern would be to make sure the wall can actually dry to both sides - that we haven't placed something impermeable (eg., VWC) in the interior. Placing the VR on the interior of the insulation will still allow condensation to occur either within the insulation or on the interior face of the insulation in the summer. (In MN, I suppose this is still true since we do AC, but that becomes the lesser of the two evils.) So placing the VR on the interior is just slightly the lesser evil since there are still more HDD than CDD? We haven't yet, but will be checking the local codes for VR requirements, too. Robert E. Woodburn Senior Member Username: bwoodburn Post Number: 76 Registered: 01-2005 Posted on Thursday, September 15, 2005 - 02:28 pm:    MemBrain is exactly what I had in mind when I mentioned "a vapor barrier that automatically... adjusts its permeability." I had heard that such a product was under development, but didn't know it had been announced. Allowing the wall to breathe when necessary is the key. (Dual vapor barriers are the worst of both worlds - water can get in through a leak, but it can't get out either way.) This will likely preclude research on automatically-repositioning VBs... Richard Howard, AIA CSI CCS Senior Member Username: rick_howard Post Number: 60 Registered: 07-2003 Posted on Thursday, September 15, 2005 - 02:36 pm:    I think this presupposes placing the membrane to the inside of the insulation. My understanding is that this product is not intended for the deep south's hot and humid climate. It would serve no purpose there as it would always be in its more permeable mode. For northern or middle climate areas, the intent is to let the moisture out of the wall construction itself. This product tends to be seasonal in its performance to stop moisture. In the summer, we reduce the moisture within the space by air conditioning, but the moisture trapped within a wall cavity has the potential to cause mold growth and dimishes the effectiveness of the insulation. In the winter, we want to stop the moisture drive at the inside of the insulation to prevent condensation within the insulation. During this heating season, the indoor RH may be higher than outdoors, but is still generally much lower than 45 percent. Phil Kabza Senior Member Username: phil_kabza Post Number: 133 Registered: 12-2002 Posted on Thursday, September 15, 2005 - 02:46 pm:    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. Tom Peck Senior Member Username: tom_peck_csi Post Number: 16 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 02:52 pm:    Yes, in our educational processes, dual VB's is wrong (eg., poly + VWC), and I don't necessarily want to go that route ... more so throwing that out there for discussion purposes. But if the VB "breathes", then wouldn't that be ... somewhat ... okay? And I had the same thought about getting moisture in-between if there were two - what happens if the VB "fails" (eg., can the pores get clogged?)? But if the VB "breathes" then won't the wall eventually dry out - although it could take quite a while to dry out. Somehow, I had a feeling you were kidding about a re-positioning VB Tom Peck Senior Member Username: tom_peck_csi Post Number: 17 Registered: 08-2004 Posted on Thursday, September 15, 2005 - 03:08 pm:    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?). J. Peter Jordan Senior Member Username: jpjordan Post Number: 126 Registered: 05-2004 Posted on Thursday, September 15, 2005 - 03:18 pm:    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). Phil Kabza Senior Member Username: phil_kabza Post Number: 134 Registered: 12-2002 Posted on Thursday, September 15, 2005 - 04:39 pm:    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.