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MtnDon
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# Posted: 4 Nov 2011 02:57pm
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Some of the best information on the subject of air barriers and vapor barriers can be found at Building Science. An air barrier can also be a vapor barrier, but not always. Proper use of a vapor barrier is climate dependent. There are climates where it is not advised and actually detrimental to install a vapor barrier on the inside of the walls, such as we would use in North Dakota.
Building Science has a section Vapor Barrier Guidance which makes a good starting point.
Since links on the web do move, if that doesn't come up, start at the Building Science main page and go looking from there.
In particular the following may be of first interest:
Air Barriers vs. Vapor Barriers
Understanding Vapor Barriers
And a short article with a climate map and recommendations
Vapor Control Layer Recommendations
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Erins#1Mom
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# Posted: 4 Nov 2011 07:26pm
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MtnDon,
I know that when I actually start my build, I will be taking many notes from your posts. In doing so, my small cabin should last for many years. How did you get so smart? How many cabins or homes have you built?
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MtnDon
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# Posted: 4 Nov 2011 09:26pm - Edited by: MtnDon
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There's lots I do not know, you're very kind. When I get interested in a subject I take a lot of time to read and study. I have a couple years engineering background but did not get the degree as my interests shifted.
I built my first structure as a kid, a tree house. First remodel when I was about 16 with help from dad. The first 'real' building was about 40 years ago.
For about a decade I owned a wood playground equipment company here in the SW. We designed, manufactured and installed redwood playground equipment, mostly in the well to do neighborhoods. I worked with a custom home builder for a half dozen years. I did all the finish work inside; wood staircases, base/door/window trim, hung doors, installed the cabinetry. I like to help people do their own thing in ways that won't come back to bite them later.
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fthurber
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# Posted: 5 Nov 2011 11:39am
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Can someone explain why a vapor barrier is a good idea? I would think that it would promote rot and mold. I just don't understand people installing vapor barriers and then have to do all sorts of venting. What am I missing; is it done for insulation purposes?
Haven't cabins been built for centuries without vapor barriers? I don't get it.
I can see one use for it in the 10x12 cabin I am currently building. I am putting down a plywood sub-floor and laying some cherry over it that a friend gave me on top of that. I was thinking of putting a vapor barrier over the plywood to prevent the off gassing of formaldehyde into the cabin.
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MtnDon
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# Posted: 7 Nov 2011 10:36pm
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First, the hardwood strip flooring. It is usual to install hardwood strip flooring over a layer of building felt. There is a special #7 variety made special for this purpose. It does act as a barrier, reduces potential squeaks and can make the floor sound less hollow when walked upon.
If its any comfort formaldehyde is heavier than air. And exterior plywood glues have a type of formaldehyde glue that releases slower than interior grade plywood. Also people who smoke or use non vented heating appliances are likely exposed to more formaldehyde than what comes out of the sub floor.
Also OSB uses a phenolic resin glue, no formaldehyde.
Whether or not a vapor barrier is needed or a good idea is dependent on the climate. Used improperly or in the wrong climate a vapor barrier creates more problems than it solves. So whether or not it is a good idea for 'you' depends where 'you' are.
A simple and incomplete explanation... IF the structure is located in a cold climate a vapor barrier on the inside of an insulated wall will prevent water vapor from moving through the insulation and condensing on the inside of the cold outer wall surface. There can be exceptions to this; basements in cold climates are one case that require a different treatment.
Water vapor will always move from the warm side to the cold side. It's almost like there was a pump pushing it to the cold side. That means that usually any water vapor that does make it through into the wall will continue to the exterior. It should evaporate to the outside air. However, we want to keep most of that air borne water from ever entering the interior space of the wall.
Cabins and building of all sizes have been built without vapor barriers for time eternal. That does not make it a good idea. Old houses like the one I grew up in didn't even have wall insulation, and that was in frosty Manitoba! That doesn't mean we should consider that to be good practice today.
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trollbridge
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# Posted: 9 Nov 2011 09:59pm
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Thank you for putting this info out there!
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neb
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# Posted: 9 Nov 2011 10:09pm
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I have seen it with my own eye's where vapor barriers installed on walls and ceiling that the sheet rock actually will fall from the ceiling because of moisture. Homes need to breath and homes can be built too tight and that is not a good thing. Energy efficient is a good thing but still they need to breath.
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MtnDon
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# Posted: 9 Nov 2011 11:47pm
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They need to be built right for the climate. Sometimes that means needing an air to air heat exchanger ventilation unit.
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neb
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# Posted: 11 Nov 2011 08:39am
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Quoting: MtnDon They need to be built right for the climate. Sometimes that means needing an air to air heat exchanger ventilation unit.
This is true
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Rebekah
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# Posted: 29 Mar 2012 12:17pm
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I know this forum's a little old, but I have a question!! 
We are building a hobbit home in the woods, using straw for insulation, and spray on stucco on either side. We know we need a vapor barrier because we live in such a cold, wet climate (northwest washington!). We're trying to figure out what to do for a vapor barrier. It seemed straight forward at first, found rolls of plastic barriers for about $50 and told to attach it along the walls and floors! The more I read though, the more it seems like we really have to be specific. Some sites say INSIDE ONLY! and some say OUTSIDE ONLY! so I'm a bit lost. Any ideas?
Thanks!
Rebekah
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MtnDon
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# Posted: 29 Mar 2012 01:09pm - Edited by: MtnDon
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Depends on the climate zone the project is in.
The first message in this thread has a link to here, a pretty good explanation of climate zone and vapor barrier use/placement. 6 mil plastic is excellent for a non permeable layer.
That all applies to conventional construction. There may be big differences with bales.
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