Creating a Thermal Break and Preventing Water Condensation?

[Flynnworx]

I installed 2 layers of 1/2" foam walls and ceiling only and taped all the seams and over the metal ribs with metal foil tape. I drilled holes into the metal ribs every 12" to insert low expansion spray foam to eliminate thermal bridging.

[chiefairwrench]

Ok I am totally confused now I was totally convince you should ideally use 1" insulation on the walls and ceiling. I read alot since I decide to make the lead and notice alot of just inserting the foam and recovering. Being here in the south I have to worry about proper air flow due to high humility and if it does not rain once a week we are in a drought. So if I am reading this I need maybe a portable air conditioner since most have a dehumidifier and some kind of a vapor bearer to help keep the cold and heat out and I take that 6mm plastic may not be a good idea like I have used in my house. If I am misunderstanding this let me know cause I am a noobie and like doing thing one time if i can.

[McDave]

This should help to make everything clear as mud.
http://energy.gov/energysaver/vapor-bar ... -retarders
McDave

[chiefairwrench]

Ok so the plywood siding is acting like a barrier per the article. Thanks for the help I think i have a plan now.. well at least a idea.

[MtnDon]

A potential issue with using an interior vapor barrier on the inside wall of a CT is that the exterior skin is also a vapor barrier of sorts. Aluminum is pretty much impervious to the transmission of water vapor. If there is a second barrier, like polyethylene, on the inside of the wall ribs vapor may be trapped. Not saying it will happen for certain but it definitively can. I took the approach of attempting to seal the foam I installed to the framing, in order to try and keep any vapor from the inside making its way to the inside face of the Al skin. That was my goal with my layered foam approach; keep any interior airborne water vapor away from the interior face of the metal skin. I probably won't know if I was 100% successful unless I rip out a random section or have some serious odor issues or whatever. I'm also lucky to be in a basically dry to very dry climate most of the time.


The use of 6 mil poly in a humid climate was mentioned. If that is Georgia that may have been a mistake. For the past 6 years+ vapor barriers have not been recommended for the southern climates of the USA. (See map; zones 1 thru 4 are best w/o VB). GA map. That is especially true when the buildings interior is air conditioned, cooler than the exterior temperature for months at a time. If a contractor was involved in the installation you might want to talk to him about that and do some google researching. Usually it is only northern climates that have recommendations for VB.