Below are the 4 most common methods I have seen used to install a vapor barrier. If you talk to different builders, you will generally end up with many different methods of installing a vapor barrier in a crawl space. These are the most common:
1. Pea on top of a plastic vapor barrier – This must be the dumbest thing I’ve ever seen, and yet it’s probably one of the most common. Some builders have told me that the plastic in the soil PREVENTS groundwater and moisture from rising from the ground. Now if there is no moisture or water in the soil, this could be possible; But if that was the case, there would be no need for the barrier in the first place. So here’s the basic idea: ground floor (bottom) – plastic (middle) – 4 “gravel fill (top) – 6” gravel on top of plastic. In the end, what always happens is that the water enters from the walls and the ground floor and ends up on top of the plastic. Therefore, the end result is a pool liner that retains water in gravel for extended periods. Almost all of the water and moisture from the gravel fill has evaporate into the structure. Another example of construction and science practices that have turned a blind eye to driveways for decades.
2. Vapor barrier on top of the ground floor – By far the most common practice for installing a vapor barrier. A 6 mil polyethylene vapor barrier is placed over any ground floor. Here’s the idea: ground floor (bottom) – plastic (top). The ground floor can be river rock, gravel, dirt floor, sand, etc. The seams typically overlap 6 “to 12” and are rarely taped. While this will temporarily stop moisture evaporation, it does not seal in moisture from the inside perimeter wall where most of the water penetration occurs. Moisture can also leak out of the seams and the plastic is not strong enough to drag. It almost always ends up with a lot of punctures and holes.
3. Vapor Barrier at the Bottom of Floor Joists – This is rarely seen and usually only attempted by homeowners. This is probably the method that explains more wood rot than any other method. If you are even thinking about doing this, stop thinking and call a professional to fix your access space. Most driveways are ventilated, and cooler surfaces, such as ducts, pipes, and the floor, will condense in the summer. The plastic will trap the condensation against the floor frame and mold and rot of the wood will develop. Good intentions don’t always produce good results.
4. Vapor barrier attached to sill plate: There is a new industry in driveway repair that encapsulates driveways. The encapsulation process involves installing a heavy plastic liner on the floor and foundation walls. The plastic liner is attached and sealed to the base wall and all overlapping seams are taped. All possible gaps or seams in the liner are meticulously sealed to prevent moisture from evaporating. The vents are then sealed in the encapsulation system to prevent hot, humid air from entering in the summer. There is another system that is sold and installed out there that is represented as an encapsulation system, but it is far from it.
This “other” system is a 6 mil plastic that is passed through the foundation walls and stapled to the sill plate. None of the overlapping seams are glued. It is basically a glorified vapor barrier at the top of the ground floor that is climbed up the walls and stapled to the wood. They seal the vents without properly sealing the moisture from the ground floor or foundation walls. The problem with attaching the plastic to the sill plate is that the moisture will “soak up” the base wall and the moisture will soak into the sill plate and floor joists. They are giving free access to all the moisture under the siding to rot the sill plates and the floor joists. Not to mention, all the moisture will evaporate through all the unglued seams and the plastic liner is only 6 mil and will eventually get punctured and ripped. Be very careful with which contractor you choose to properly encapsulate your access space.