Moisture in Crawlspace (Obviously written during a dry spell)

One good thing about the recent drought is the chance to check the moisture under your house.

Moisture under the house is a modern problem. Years ago when the first settlers arrived in Cabarrus County, they picked the very best house sites. A place like the library in Concord was an obvious home site. Early settlers looked for a fairly flat knoll convenient to a spring These sites are not that common. In my home county, the number of good home sites averages around four or five per square mile. The early homes were clustered on upland areas. Large areas of hillsides with no good home sites were not chosen at first. The home sites on top of the knoll had good drainage.

As time went on, settlers started digging wells. This expanded the available home sites. With more choices they were still able to build on relatively dry sites.

In addition, early homes were built on pillars of rocks that let air freely circulate under the house. Even modern air conditioning may add to the moisture level by creating condensation under the house.

Now that we have a combination of modern foundations, modern heating and cooling methods, and the side by side house construction on every available lot to create moisture problems. I have seen several moisture problems in new homes. In one situation there was a running spring under the house. In another situation, I think the homeowners might have got running water if they cleaned out the spring. From the road, I have seen other houses build in swampy areas. Although I haven't visited these houses, there is no doubt in my mind that they have moisture problems. These houses are ones that are normally sold to transplanted desert dwellers who don't realize the dangers of moisture. In some houses, the problem is not obvious. Still over a number of years, enough moisture can stay under the house to cause decay.

If the soil under your house isn't dry right now, you have a moisture problem. Another indication of moisture damage is discolored wood. Wood with dark stains or wood that is bleached white indicate problems. White mycelium growth indicates a problem. The way I normally get involved is when the homeowner asks about controlling carpenter ants, silverfish, or cave crickets. These insects indicate a moisture problem. Commercial home inspectors and exterminators generally use moisture testers.

There are three things you can do to solve moisture problems.

The first step is to make sure rainwater from the roof and uphill surface water is not running under the house. Water runs downhill. I really shouldn't have to remind you about that but sometimes people forget it. Just last Thursday I visited a site where a storm grate was installed on high ground. If the house is in the downhill flow, you need to create a new downhill. This can take the form of a diversion or tile drainage. A diversion can take the form of a wide ditch. Try to get it wide enough to mow easily. I don't have any recommendations on the depth but if it is just a few inches, it will fill in too easily. A diversion works best when it falls about 1 foot in 10. Steeper falls may start erosion and less fall may allow water to soak in the ground. On some sites you may not be able to get that much fall. Remember you can't change the location where the water would normally leave your property unless you can get along with your neighbor. I have seen some sites where it would be helpful for several neighbors to go in together to solve a drainage problem.

Once you have stopped the exterior water you have to deal with groundwater seeping up. Groundwater can seep in by gravity or by capillary action. Gravity movement will lead to standing water or a spring. If you have standing water use a sump pump. Running water needs to be ditched or piped outside.

The method that most people will use to block capillary action of water to the surface is covering the soil with plastic. While complete coverage is best, any coverage will help reduce the amount of moisture in the air. Sixty or eighty percent coverage may reduce the moisture load enough for the vents to handle the rest of it.

The third major line of defense is a foundation vent system. Vents don't work as well if they are close to the ground (less than 6 inches) or if they are covered by a bush. Duct work on the inside can also block vents. There are automatic vents on the market. Some of the local exterminators are installing these since dry wood solves several insect problems. These vents will open at 40 degrees. This gives you some air circulation even on winter days. They are a definite improvement over manual vents when manual vents are closed at the beginning of the winter. Some of our written recommendations suggest using automatic vents. A better option is to use manual vents and only close them enough to keep the pipes from freezing. 

There are other sources of moisture you might encounter. Sometimes dryer vents run underneath the house. You need to make sure they don't come loose and pump moisture under the house. Bathroom exhaust fans need to be vented outside. Also air conditioning drains may run water back under the house. Extending the drain away from the house helps prevent this problem. Small leaks from the plumbing system will also keep the ground wet. I don't know of any way to check a buried plumbing system other than digging back through the moist soil.

Some people are conditioning the crawl space with heat and air conditioning now.   That sounds like overkill if there is a simpler method to solve the problem but it sounds a lot smarter and cheaper than replacing floor joist if you can’t solve the problem.  

Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center.  Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/

Contact me at David_Goforth@ncsu.edu.  Reviewed 2007.