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Hello,
I just had insulation installed in my steel building. They put the insulation in, then the WMP vapor barrier on top all the way down to the floor.
I am looking to install some plywood for my walls and was wondering if I should cut the WMP down first or is it ok to leave it up and right against the plywood? I am up in the Pacific Northwest, if that's a factor.
Thanks!
Vapor barrier is installed in 'most' cases toward the conditioned space...so it sounds correct to me. I would not remove it before installing your plywood walls.
I would agree with Jim, I can't think of any difference between your application (plywood against vapor barrier) and the common application of drywall against vapor barrier. In both cases you are preventing moist air from reaching the cold surface which in your case is the metal siding. Did you have a specific concern in mind when you made the post?
I just installed insulation on my pole building and will be installing a vapor barrier under my plywood
If you're in climate zone 4, the barrier should probably be removed before installing any wall covering over it. Interior vapor barriers/retarders are only really necessary for zones 7-8 where it gets significantly colder than PNW. An interior polyethylene non-permable barrier actually prevents wall assemblies from drying inward during the summer, a layer of poly can actually make the wall wetter than it would be without.
If you have WMP-50 or similar, I'm sure Lamtec can provide a recommendation.
Lamtec Corporation
5010 River Rd.
Mt. Bethel, PA 18343-5610 USA
Phone: 800-852-6832
https://www.lamtec.com/architectural-support
Thanks everyone, I'll check into it with Lamtec just to be sure. I feel like there may be some exceptions as Peter mentioned. Just want to do it right the first time!
The Oak Ridge National Laboratory has a building research division. (I did some modeling for one of their experimental buildings.) About 20 years ago I asked about vapor barriers. I was told the previous guidelines to install a vapor barrier on the inside of the wall cavity is good for cold climates. In the south where air conditioning is used to cool for much of the year the problem is the warm outside moisture will work through the outer wall and can condense on the vapor barrier and cause the insulation to get wet, lowering the efficiency and likely causing problems with mold inside the walls. At the time they suggested vapor barriers on both sides of the insulation. I see a different recommendation now:Quote:
Originally Posted by Evan Lentz
From this Dept of Energy document, https://web.ornl.gov/sci/buildings/d...heet-2008.pdf:
"If the outside air is hot and humid, then moisture from outside will try to diffuse through the walls toward the dry, air-conditioned inside air. We used to tell people to install vapor retarders to try and stop this moisture diffusion. But we have learned that if moisture moves both ways for significant parts of the year, you're better off not using a vapor retarder in the walls at all."
It might be helpful to contact ORNL and ask. Their mandate is to help everyone who asks. I don't know how to contact the right person now but you can probably get it from the ORNL web site or call the information number.
JKJ
Thanks for the tip - I will get in touch with them too. So far the consensus with both the insulation contractor and the Lamtec Corp. is to keep the vapor barrier in place and put the plywood right on top since it is not considered a vapor barrier. Although the more opinions the better so I can make the final choice on it!Quote:
Originally Posted by John K Jordan
We went around and around on this another time and I agree with exactly what John posted in italics, no vapor barrier in hot, damp climates. I was arguing to another fellow Georgian, however, where I was confident this was the case. Climate is certainly the ultimate factor. For me personally, if it is borderline, I would err on the side of no vapor barrier. The vapor barrier is going to collect moisture, by design. You are putting two materials that are highly susceptible to mold growth in direct contact with it. If the warmer side is the conditioned side, the HVAC will take care of drying, but if the warmer side is the exterior, unconditioned side containing batt insulation, you'll have mold. just my two cents...
Vapor barriers need to be installed on only one side of the wall "sandwich", that combination of layers that include everything from the outside skin to the face of the inside. The reason is to allow moisture vapor an escape route to the outside of the wall. You need to define each of the layers in order to get sound advice. Adding a vapor barrier to the inside of a wall with another vapor barrier would be a mistake. The recommended location changes depending on your climate zone, but using only one does not.
I don't think there is really an option that deletes a vapor barrier. I live in SC and we don't use sheet plastic under the wallboard. But we paint the walls. That is not as effective as the plastic but will retard moisture movement. Plywood will too. The permiability of 1/4 plywood with exterior glue is about 0.7. Painted drywall is around 4. 6 mil polyetheylene is about .06. So the plastic is much less permiable but the other materials will also retard moisture movement. And if there are seams in the plastic, it won't really be .06. Siding also retards moisture movement. What we are really deciding is how much restriction we want.
Sounds like closed cell foam is the right insulation for climates where moisture will go one way in summer and the other in winter. It will block moisture and it is immune to it anyway. (need to seal the joints)
So how does the required (Tyvek or similar) house wrap fit into the equation? (with other than steel buildings)
Wraps are for wind infiltration primarily. They are permeable, AFAIK, so that moisture can escape, however.Quote:
Originally Posted by Ole Anderson