Moisture and vapor barriers

Started by jraabe, November 16, 2004, 02:36:01 PM

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jraabe

There has always been lots of discussion and questions about vapor barriers, where to put them and when they can cause problems. If you seach the old forum you will find 10 pages of discussion on the term "vapor barrier" — http://countryplans.master.com/texis/master/search/?q=vapor+barrier&xsubmit=Search%3A&s=SS.

Here is a good article on some of the myths of vapor barriers. http://oikos.com/news/2004/03.html#Anchor-Moisture-3800.

To add even more confustion to this issue -  Joseph Lstiburek (Building Science Corp) appears to be advocating tight EXTERIORS.

Check out his site: http://www.buildingscience.com/resources/resources.htm

Builders: Let us know what works well in your climate.

glenn-k

Hi John--
Me again working off a friendly unsecured wireless in Santa Maria.

From my pilots weather training, I remember that when warm air reaches its dew point by contacting a cool mass, it releases the water it can't hold in the form of visable moisture, so no matter what anyone writes, the law is that if its cold on one side of the wall and warm on the other, water will condense on the cold side of the wall if it reaches it's dew point-- hope that's all right- been a few years since I studied it.

This law means that your moisture barrier should be swapped when you turn on the air conditioner inside during the summer.  OK-- so that is at least inconvenient-- but keep it in mind as it can explain some moisture problems.  

Glenn


jdew

Glenn - you're right, but the only way condensation can for for is if the water vapor is there in the first place.  That's the theory of a vapor barrier - to not allow the vapor to get there.

This also why depending on where you live the location of the vapor barrier is different.  In climates that are predominately cold, where heating is the main priority, then the vapor barrier (if you agree that a vapor barrier should be there in the first place), should be on the inside of the exterior walls.  In climates that are hot where you are usually to cool the house - like Florida, then it should be on the outside of the wall.

The problem is I think that most of us live somewhere where we are both heating and cooling for a significant part of the year.

jraabe

One thing that simplifies the VB issue is that most moisture and condensation problems ride on currents of air.

If you block the airflow from the warm moist side to the cold dry side you have stopped some 85-95% of the potential problem. It doesn't really matter whether this air barrier is on the inside, outside or in the middle. Vapor diffusion through an airtight wall, floor or ceiling system is a much smaller issue. One that probably doesn't even come up except on the extremes of the spectrum.

As long as the air can't flow freely, and the surfaces are insulated so that it stays warm on the warm side or cool on the cool side, not much condensation will happen.

Amanda_931

Nice links.  I'm learning a lot.

And having to right now because of allergies--mold among them.


glenn-k

With allergies you may want to look toward getting away from manufactured materials.  Many people with allergies have problems with the glues in plywood or OSB, carpets etc (formaldehyde - offgassing of other chemicals, dust mites-ugly little creatures).

Real wood and earth based structures won't bother you.  At the cost of real wood commercially though, you need to find a friend with a sawmill.

Amanda_931

Oh, yes.  

Carpets are a no-no.  Rugs, maybe.  water leaks? avoid like the plague.  Dust mites--encase bedding, (try to) change sheets twice a week.

There's at least one local sawmill that will cut flooring with kiln-dried wood.

Still working on--do I pressurize or just keep air moving, both can work, although there needs to be no pockets of stagnant air--in closets or stacks of books, for instance--in the moving air strategy.  With pressurization one just about has to have on-grid electricity, as far as I currently see.

Breatheable walls?  Earthen floors?  I'd love to have PAHS--passive annual heat storage--or its descendant AGS (whatever that one stands for), and I'm talking to people with allergies who are doing it, but it's a lot of engineering and planning.

So I may put up one of John's houses to get me out of the travel trailer, use as a guest house later.

This travel trailer has been pretty good for four years now--Gulfstream and Jayco at the time did pay some attention to outgassing, some of the others still didn't.  Don't know about now.