2020 Boise Idaho Real Estate Blog
Vapor barrier-whats the big deal Main The Durable Green Home
Why do we have a vapor barrier in the crawl space? How is it supposed to be installed? But it is wet under my vapor barrier-what should I do? Do I need to dry the dirt out? How come the builder did not install the vapor barrier correctly? what is a vapor barrier? Should I clean it off? I have water under my vapor barrier-what do I do? should I tape it? These are some of the questions I have had on Vapor barriers in crawl spaces in the last year. Here is the skinny.

A crawl  space should be designed and constructed to be dry and pest free. A dry crawl space is good for building durability and it's inhabitants.

A vapor barrier in the crawl space is required by code, why do we need it? because literally hundreds of pounds of water comes out of the dirt (dry dirt) every day, every minute, every hour. the 6 mill plastic vapor barrier is installed over the dirt to keep the moisture in the ground and to not allow it into the building assembly.The  dirt will be wet, this is good, keep the moisture in the ground where it belongs. You can not dry out the dirt, don't let anyone tell you they can, moisture will always be in the ground and that is where it should stay. I have  not seen a tape that sticks to most vapor barriers (not hat there are not other systems out there) but the vapor barrier should cover all dirt, over concrete piers or footing if possible, and attach to the stem wall above the cold joint. The poly should be lapped a minimum 12 inches in both directions, and be clean. This will sufficiently keep the moisture in under the poly where you want it. If you have standing water under the poly then you need to find the source and correct the water intrusion, get the water out, and lay the poly back down. If you have standing water on the poly, then you need to get the water out, lay the poly down or replace it and then dry the crawl space out with lots of air. Doug Ness of Ness water management is a good source for this type of procedure.

In testing existing homes we rarely see a vapor barrier that is installed correct or has not been compromised. what happens when it is not covering the dirt? Well I have seen some home this year that are raining on the exterior walls inside the home, floor joists that are no longer structurally sound, rim joists you could eat with a fork, ice cycles hanging on the outside of siding, frost frozen up the walls on the exterior of the home by foundation vents, etc. These are all signs of moisture coming out of the ground and into the building assembly, and then condensing inside the wall and floor assembly. Now we have a problem, that could have been prevented with some understanding of the necessity of a vapor barrier for building durability.  So  what do you do? Inspect the crawl space yourself, or have a home inspector do it for you. Then correct the problem, follow the recommendations they make.We all want a building to last.

Posted by Tad Duby at 2/11/2007 2:49:00 PM
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Michelle Penick, Build Idaho Client Real Estate Services

Michelle Penick

Client Care Team
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