Vapor Barrier in Crawel Space (posted by John Reiser)

[ASKNACHI]This question was posted on by John Reiser (from Vancouver, WA). [/ASKNACHI]


Could you please tell me what the requirement is for Washington State, Clark county on the following:

Crawl Space: What is the percentage of dirt that may be left exposed? Does 100% of the dirt need to be covered with the plastic vapor barrier?

Thank you,

No idea if there is a regulation but I would say cover it all and have the plastic turn up the wall a few inches .
Tape all joints and where it is at the wall a small amount of sand works well to help seal the gap ( 2 inches high by 3 inches wide +or -.
I would also keep my vents closed tight all the time especially in the summer as the crawl space will be cooler then the out side air .
Out side air in summer is usually very high in humidity and this is not good in the crawl space .
If you have a smell or think it has a concern a dehumidifier could be a big asset.
I recommend a constant drain be hooked to the dehumidifier as it becomes a nuisance to have to dump the water out of them and usually gets forgotten.
An Automatic humidifier is a great thing you can set the Humidity level at 50% and it will do a great job .

Roy Cooke

yes,100% coverage…vents open within 3 feet of every corner

Unless what I have been readings is wrong the new way is to no longer vent crawl spaces as Warm air is usually high humidity and bringing this into a cooler Crawl space can add dampness to this area.

… Cookie

You are correct Cookie that venting crawl spaces in climates such as Washington are bad building science. However, some jusisdictions are simply not caught with the latest building science and may still require ventilation.

Ventilation very important around here unless the ground is completely sealed off. Then you risk moisture in the insulation and floor system depending on how the house is used, conditioned, height above ground, surrounding terrain, ground water etc. If you seal one up you better have a commercial type dehumidifier in there in most cases.

The Mississippi climate is very similar to S.C. Generally, in the coastal plains area from Virginai to Texas, unless you are going to have a completely sealed and conditioned crawl space, you had better leave those vents open. Closing the vents in a hot and humid climate will not keep water vapor from entering, even with a good vapor barrier. Better to have the air moving. How one handles this problem is dependent to a large degree upon one’s climate. If winters are rather mild but extremely wet, closing off vents may be a good idea. One solution that works very well is to install a fan with a continuous duty motor at the access.

When it comes to crawl space ventilation, you will never find full agreement among home inspectors. But the best solution for most climates is the sealed crawl space. This involves some changes, some expense, and may not be practical in all cases, especially if the home is without central heating or air conditioning. In a sealed crawl space, the vapor barrier is extended over the inside of the curtain wall from the top, and draped over the ground vapor barrier. All seams are taped. A supply duct is installed through the floor, and the return air is whatever squeezes through the cracks. If the crawl space is large, the volume of free air space may dictate a larger air conditioning/heating unit. This is where upgrading a home to a sealed crawl space can become expensive or cost-prohibitive.

I forgot to mention that the band joist in sealed and conditioned crawl spaces are insulated, the best method being spray foam which will also cover the top of the 6 mil plastic covering the curtain wall. Sorry 'bout that!

Hello. And Bye.