Expert: Dan Griffin - 7/8/2008

Question
Hi Dan

I am a contractor in SW Iowa and am in the process of remodeling a commerical concrete block building  We have built 2x4 walls inside the concrete block wall,insulated it with r-13 insulation and a vapor barrier then sheetrock.  We have a problem with condensation forming behind the vapor barrier,so much that it runs out the bottom of the wall.  We took everything back off the wall and the concrete blocks are soaking wet,I assume from outside moisture. From what I have read,this problem needs to be addressed from the outside and the blocks sealed up.  My question is with the vapor barrier,if I need it or not.  My thinking is it is neccessary  to prevent moisture transfer into the sheetrock and develope mold on the sheetrock but the building owner was told he didn't need it. Also the humidity inside this 40,000 sq ft building is running about 85%.  Any ideas on how to get it out quick

Answer
Gary, you have a problem.  The best discussion of vapor barrier location I have found is here:
http://www.buildingscienceconsulting.com/index.html
Some basics:
Heat and humidity will form on the warm side.  Heat and humidity move to the cold.  If you live in a mixed climate with many cooling days, locating the vapor barrier on the "normal" interior side will force the wall cavity  to sweat when the hot humid air hits the cold interior and condense of the vapor barrier.
The Kraft paper surface on insulation is an adequate vapor barrier as is a coat of paint unless there are extreme issues.
MOving the trapped cavity air will relieve your problem.
Contact a local mold or flood remediation company in your area. They will have the equipment to drop the humidity until you get a handle on the problem.  You need a major sized dehumiditier.

Your problem seems excessive and extreme.  Is there an external problem increasing the humidity or water above normal on the exterior?  Are you drawing ground water through the slab?  Seek additional input, but I would think you are taking on far too much water.  If the block is not painted, cosnider treating with a silane or siloxane.  If is has been painted I would think you would want either a good permeable latex paint that is well maintained to allow the wall to dry or an elastomeric paint to stop water infiltration.  Vapor transmission at the floor can be reduced with Xypex products and certain types of finish flooring.