There are many today who advocate sealing a crawl space. Even ignoring the very high
cost of such a project, there are serious problems with such a strategy.But the real question
Should You Even Have a Vapor Barrier on The Ground?
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 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, not only covering the ground with a vapor barrier,
but also 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, termite
control chemicals, etc. There occasionally may be dead critters, mice, etc. Any odors, etc.
in this closed 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 there 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
In these crawl spaces that have standing water after a heavy rain, the ground will never dry
out if a vapor barrier is installed over the ground. 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 going 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 sub floor
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 re-attaching the vapor barrier to the walls.
This annual inspection will not be inexpensive.
We believe a better method is to not install a vapor barrier anywhere on the ground or any
wall, 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.
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. Essentially this
means 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.
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
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.