Which side to put the vapor barrier?

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Old 08-25-05, 04:22 AM
WFO
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Which side to put the vapor barrier?

I live in southeast Texas (hot summers, mild winters, humid ALL the time).

I'm building a shop/kitchen/carport that will have tin walls and ceiling. The area above the shop/kitchen will be open to the carport area (which has no ceiling) so there should be little trapped heat (carport is open on three sides).

Should the vapor barrier go on the inside against the tin, or on the outside next to the siding? Since we're cooling 4 months out of the year and heating maybe one, I'm thinking in this environment there is much more moisture on the outside than inside.

If it matters, the outside will be tin also and has 2x4 framing with foil faced rigid insulation on the outside. Will this foil faced insulation act as a vapor barrier?
 
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Old 08-25-05, 07:41 AM
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http://www.eere.energy.gov/consumeri...heets/bd4.html

This web page should help you with your decision. I realize that this subject is very confusing. This is because people misinterpret heat transported moisture with air transported moisture. Heat transported moisture has to do with an heated object losing temperature. This object can be wood, insulation or air. People in this profession argue that the mass of the object is the determining factor when it comes to moisture related problems.

For example, wood has a relatively high mass in comparison to air. This characteristic of air allows air to absorb moisture quickly and has the potential to expel it easily. Whereas wood would absorb the moisture much more slowly and expel it even more slowly. This leads to the argument concerning vapor barriers among professionals.

This web page recommends that areas that experience less than 2,000 degree days, which your area does, no vapor barrier is recommended. In hot humid climates like yours, they recommend an Air/Vapor Barrier applied under your siding.
 
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Old 08-31-05, 04:08 AM
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Sorry for the delay.
Excellent web site.....Thanks!
 
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Old 09-05-05, 10:34 PM
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I'm also confused about vapor barriers and wraps. My Coarsegold, CA climate is hot and dry in summer. Winters will bring only 15 inches of rain give or take 5. Last winter and spring I'm told we got about 23 but that was a strange year. Humidity this summer probably averaged 25%. I cool with AC not evaporative cooling(EC). Still I assume humidity is a bit higher inside the home than out. My understanding is that vapor travels from high concentration to low concentration. And if that is the case the vapor barrier in my climate would need go over the studs of the interior side to keep vapor generated inside the home from condensing on the insulation. And a wrap on the outside to keep air from penetrating the walls. Risking overkill maybe even an exterior vapor barrier incase the insulated area between the walls is drier than the outside air. Would this line of thinking be wrong? Would vapor barrier planning change if EC was installed? Comments?
 
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Old 09-20-05, 10:58 PM
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Equilibrium Relative Humidity (ErH%)

Your thinking is correct, however, Dew Point, Saturation Point or 100% Saturation of objects does not necessarily mean condensation forms. This is because all objects must maintain some moisture, including air. What those terms do mean is, at that point or temperature the object can no longer absorb any more moisture. With objects of higher mass, like wood, it takes a considerable amount of moisture within the wood before the moisture inside wood would form condensation.

In your area, the website mentioned earlier in this Thread, a vapor barrier is not recommended. An Air/Vapor Barrier for your area is also not recommended. A House Wrap would be recommended, not so much as an Air Barrier but because it performs better than builder's felt against wind driven rain.

Evaporative Cooling or Swamp Coolers as they are sometimes called are based on the GAS LAW. This states for a liquid to change into vapor, the liquid must be absorbing heat. Example, for you to create steam, the water must be heating. When you are boiling water in a pot, the moment you turn off the heating element, the boiling stops. The GAS LAW also states that for a vapor to change into a liquid, that vapor must give off heat.

What makes Evaporative Cooling so unique is the Latent Energy of Vaporization. This states that if you raise the temperature of a certain volume of liquid one degree Fahrenheit, it requires one British Thermal Unit (BTU). However, if the raising of the liquid that one degree causes evaporation, it requires 970 BTU's. Since we can neither create nor destroy energy, when this same vapor condenses, it gives off 970 BTU's.

The heat is extracted from the house because of the rule high to low concerning ErH%. Because of the dry climate you have during the summer, all the objects within the structure will have a low humidity level. So the moisture vapor created from the Evaporative Cooling, never condenses. It gets absorbed by the objects of lower humidity in the house and eventually absorbed by the objects of lower humidity outside the house. Since the moisture was absorbed and did not condense (GAS LAW), the heat inside the house is extracted. Or if you prefer, the inside temperature is lowered.

You need to fully understand this aspect because of your confusion with vapor barriers and intent to use Evaporative Cooling. What vapor barriers actually do is retard, or if you prefer, impede moisture flow. If you were to install a vapor barrier either to the warm side or exterior of your outside walls, it may have an adverse effect concerning your Evaporative Cooling. By adverse I mean it may not cool the house as well and is not considered a moisture problem.

I debated with myself if I should reply to your Post, BlueSkyGuy. With that said, it has come to my attention that some of my Posts have been found on Search Engines. I do realize that this is an open forum and as such I do not find any offense. What dismays me is that the originator of the threads are omitted on these Search Engines. In other words, without including BlueSkyGuy's post, this reply would be misleading and a misinterpretation of my position concerning vapor barriers.
 

Last edited by resercon; 09-21-05 at 04:52 AM.
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