Nearly every home with a crawl space has a vapor barrier on the ground, but is this really the best methodology? Some moisture inevitably comes from the ground, but probably most of the moisture is airborne. To help with this moisture problem the building code has specified a certain number of passive vents or mechanical (fans) vents for crawl spaces, but there are significant shortcomings with passive crawl space ventilation. Because of this many now recommend no ventilation of any kind, covering the ground with a vapor barrier, and sealing the walls of the crawl space.
But there are several problems with sealing the crawl space and having no ventilation.
The problem is that this sealed crawl space now becomes a closed, stagnant area that is never cleaned, so it likely will develop odors and other gases from insect killers, termiticides, etc. There occasionally may be dead critters, mice, etc. Any odors, etc. in a closed crawl space will migrate into the home’s living space, and, of course, the home’s living space is by far the single most important factor.
With a vapor barrier on the ground moisture is trapped under it so that the ground can never dry out, keeping the relative humidity under the vapor barrier at 100%, 100% of the time. So, mold and bacteria is going to grow. Sometimes, after heavy rains, in many crawl spaces, there is some standing water most of which is usually contained under the vapor barrier on the ground.
In these crawl spaces that sometimes have standing water after a heavy rain, if a vapor barrier is installed over the ground, the ground can never dry out, and in such an environment the water levels may increase over time, requiring the homeowner to install a sump pump. Keep in mind the sump pump will remove some of the standing water, but cannot reduce the relative humidity under the vapor barrier.
Because of termites, when vapor barriers are installed up the crawl space walls, a gap of several inches is usually left open so you are not creating a hidden path for termites. Some installers will run the vapor barrier up to the wood rim joist and subfloor of the home, but that requires pulling the vapor barrier loose every year to be able to inspect for termites, ants, and other insects, then reattaching the vapor barrier to the walls. This annual inspection will not be inexpensive.
It seems to be forgotten that years ago homes in the south were built on pilings, and their crawl spaces were completely open around the sides. These homes had no vapor barrier on the ground, and none of these homes had mold or wood degradation because of moisture.
We believe a better method is to not install a vapor barrier anywhere on the ground or walls, to not install any sump pump unless standing water is present all the time, but to install an intelligent crawl space ventilator instead of the usual passive vents. We do recommend insulating the sub floor between the floor joists with insulation that has a vapor barrier on one side, such as asphalt paper, and that the asphalt paper side is tight up against the sub flooring. This is a matter of comfort (not walking on cold floors) as well as being helpful to flooring.
What is an intelligent ventilator?
An intelligent crawl space ventilator is a ventilator that compares inside and outdoor absolute humidity, not relative humidity. Absolute humidity is the actual amount of water vapor in the air. Relative humidity is humidity relative to temperature. If one used relative humidity for comparison, this would mean, for instance, you might bring in warm outdoor air at 50% relative humidity, and when that air cools down to crawl space temperature, the humidity now rises above 90%. Engineeringtoolbox.com defines absolute humidity as the actual mass of water vapor present in the air water vapor mixture. In addition, the intelligent ventilator has to have a fan capacity high enough to do the job well.
So, with a crawl space that has
a. intelligent ventilation
b. no vapor barrier on the ground
c. standing water in it after heavy rains
d. one Smartvent intelligent ventilator for each 800-900 sqft of crawl space
we see that in a few days the standing water is gone, and the ground is dry, and no mold has ever been seen. The benefits of construction over a crawl space without a vapor barrier but with recommended ventilation are quite significant.
1. The home's living space air quality is improved.
2. If radon is an issue you need to ventilate.
3. The home's elevated appearance may increase value.
4. As opposed to a slab the home's comfort is raised.
5. The foundation's posts, piers, and support pilings can now be dry. Whereas, when they were enclosed with a vapor barrier they were constantly kept in a wet environment, making them subject to settling or degradation, which would require expensive repair.
6. The reduced installation costs using this methodology is quite significant, no vapor barrier on the ground or walls, plus annual inspection costs.
7. There are many good reasons why a home ought to be built over a crawl space. The elevation may make the home more attractive, plus electrical, plumbing, air conditioning modifications and repairs can be done. In addition, almost all structures built on slabs will eventually experience some cracking of slab. It's the nature of concrete.