I have only come across a couple of encapsulated crawl spaces, but both were performing as advertized. I used to have my worries about them and the building codes do not have a provision for allowing them as some of them are installed.
The building codes allow for having crawl spaces that are unvented but the spaces are required to communicate with the indoor environment—in other words they are heated spaces—conditioned spaces.
This last one I inspected specifically recommends closing the crawl space vents. My knee-jerk reaction was that in time this is going to be an issue. However this one was ten years old, un-vented, not communicating with the indoor environment and had wood moisture levels WAY below normal for our area for even properly ventilated crawl spaces. Moisture levels were actually quite similar to conditioned spaces I have tested.
How can this be? The answer lies in the fact that if the crawl space is truly sealed—I mean REALLY sealed, how can moisture build up in the space? Over time the sealed space can only make balance with moisture levels in the rest of the structure—albeit very slowly. Even very minor gaps in the barrier will not affect the installation appreciably. The only gaps I could find were around the pipe that ran into the sump pit. In this next picture one can see where two of the original crawl space vent locations are covered and sealed.
For moisture to move—there has to be a pathway of “air movement.” Stop the air movement and you will succeed at stopping or minimizing the moisture movement as well.
One of the inherent problems with vented crawl spaces is that the stack effect (upward pull of air created in any house) as well as negative pressures created by exhaust equipment, will try to pull air from the crawl space into the indoor environment. Thus moisture and anything else in the crawl space air can be drawn into the home. Sealing the crawl space will help minimize this issue as well.
I think in times the building codes will make adjustments to allow this type of installation, but for now the installers of the system will continue to provide letters attesting to its being OK to close the vents.
At this point I have to agree with them.
Charles Buell, Real Estate Inspections in Seattle.
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WA State, Home Inspector Advisory Licensing Board