Below are the 4 most common methods I have seen used to install a vapor barrier. If you talk to different builders, you’ll usually end up with many different methods of installing a vapor barrier in a crawl space. Here are the most common:
1. Pea gravel over a plastic vapor barrier: This has to be the dumbest thing I’ve ever seen, and yet it’s probably one of the most common. I have been told by some builders that the plastic in the flooring PREVENTS water and soil moisture from coming up out of the ground. Now, if there is no moisture or water in the soil, this could be possible; but if that were 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). Once the skid pad is built, builders install a 4 – 6 mil plastic on the ground and they dump about 4″ – 6″ of pea gravel onto the plastic. Eventually what always happens is water gets in through the walls and downstairs and ends up on top of the plastic. So what you end up with is a liner of pool that holds water in the gravel for extended periods of time. Almost all of the water and moisture in the gravel fill has to evaporate into the structure. Another example of construction practices and science turning a blind eye to spaces access for decades.
2. Vapor barrier at the 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 is the idea – ground floor (below) – plastic (above). The ground floor can be made of river rock, gravel, dirt floor, sand, etc. Seams typically overlap 6 to 12 inches and are almost never taped. While this will temporarily stop moisture evaporation, it does not seal the moisture from the inside perimeter wall where most water penetration occurs. Also, moisture can leak out of the seams, and the plastic is not strong enough to crawl on. It almost always ends with lots of punctures and holes.
3. Vapor barrier at bottom of floor joists – This is rarely seen and usually only attempted by homeowners. This is probably the method that causes more wood rot than any other method. If you’re even thinking about doing this, stop thinking and call a professional to fix your crawl space. Most crawl spaces are ventilated, and cooler surfaces like ducts, pipes, and the floor will condense in the summer. The plastic will trap condensation against the floor structure and mold and wood rot will develop. Good intentions do not always produce good results.
4. Vapor Barrier Attached to Sill Plate – There is a new industry in crawl space repair that encapsulates crawl spaces. The encapsulation process involves installing a heavy plastic liner on the floor and foundation walls. The plastic liner is fastened and sealed to the foundation wall and all lap joints are taped. Every potential gap or seam in the liner is meticulously sealed to prevent moisture from evaporating. The vents are then sealed in the encapsulation system to keep out hot, humid air in the summer. There is another system that is sold and installed and is represented as an encapsulation system, but it is far from it.
This “other” system is a 6 mil plastic that is run down the foundation walls and stapled to the sill plate. None of the overlapping seams are taped. It’s basically a glorified vapor barrier on top of the ground floor that goes up the walls and is stapled to the wood. They seal vents without adequately sealing moisture from the ground floor or foundation walls. The problem with attaching plastic to the sill plate is that the moisture will “suck” into the foundation wall and the moisture will wick 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 floor joists. Not to mention all the moisture will evaporate through all the seams that aren’t taped and the plastic liner is only 6 mil and will eventually puncture and tear. Be very careful which contractor you choose to properly encapsulate your crawl space.