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New Addition Crawl Space Moisture - How to get rid of it

New Addition Crawl Space Moisture - How to get rid of it

Old 09-27-18, 12:35 PM
wilspeer1's Avatar
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New Addition Crawl Space Moisture - How to get rid of it

I have an existing house that was built in 1953 in the southeastern U.S., Tennessee to be exact where the humidity gets very high in the summer months. The foundation has automatic vents in it with no moisture barrier at all. The ground underneath the house is dry as a bone and hard as concrete. This leads me to the problem, though. In 2007; we added a small addition to the house, 12x24. There is one small opening from the old foundation to the new foundation; just big enough to crawl through to work on a water line if needed. The new foundation for the 12x24 addition does not have any vents in it. The only air that gets to it is from the small opening from the old foundation. Also, there was no moisture barrier put down anywhere in the new addition. In 11 years, the floor joists have rotted due to the excess moisture. We are in the process of repairing the damage. Here are the questions I would like to ask as far a stopping the moisture:

1. I know I need to put down a moisture barrier; what thickness should I use?
2. Should I install vents in the new foundation or should I seal the vents in the old foundation?
3. The new foundation is only about two feet off rhe ground, the drainage is good, should I seal this concrete foundation from the outside in any way?

Any other suggestions will be greatly appreciated.
Old 09-27-18, 01:21 PM
Join Date: Sep 2009
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Any thickness would work as a vapor barrier. However, thin ones may tear when crawled on. Typical vapor barrier sold or installed by contractors are 10 to 20 mil thick. I suggest you first install the vapor barrier, then monitor the humidity with a wireless hygrometer, or a wired one with long cord. If humidity is lower than 50% then you don't have a problem. If it is higher than 60% you may want to ventilate it with a fan (assuming the other part of the crawlspace is dryer), or install a dehumidifier, or you may get away doing nothing. But if it gets higher than 80% then you definitely need to lower it somehow.

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