Friday, April 17, 2009
Crawlspace Vapor Barrier Mythbusters
I am going to outline 4 methods I have seen used in installing a vapor barrier. If you talk to 10 different builders, you usually will end up with many different methods of installing a vapor barrier in a crawlspace. Here are the most common:
1. Pea Gravel on top of plastic vapor barrier – This has to be the all time dumbest thing I have ever seen, and yet it is probably one of the most common. I have had builders tell me that the plastic on the ground KEEPS ground water and moisture from coming up from the ground. In never never land, this might actually happen but not in any crawlspace I have seen. So here is the idea – ground floor (bottom) – plastic (middle) – 4” pea gravel fill (top). Once the crawl foundation is built, builders install a 4 – 6 mil plastic on the ground and dump about 4” – 6” of pea gravel on top of the plastic. Eventually, what always happens is that water comes in from the walls and the ground floor and ends up on top of the plastic. So what you end up with is a swimming pool liner that holds water in the gravel for prolonged periods of time. Nearly all the water and moisture in the gravel backfill has to evaporate into the structure. Another example of building practices and science turning a blind eye to crawlspaces for decades.
2. Vapor barrier on top of 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 (bottom) – plastic (top). The ground floor could be river rock, pea gravel, dirt floor, sand, etc. The seams are typically overlapped 6” – 12” and almost never taped. While this will temporarily stop some moisture evaporation, it does not seal out moisture from the internal perimeter wall where most water penetration occurs. Also moisture can come up from the seams, and the plastic is not durable enough to crawl on. It nearly always ends up with many punctures and holes in it.
3. Vapor barrier on bottom of floor joists – This is rarely seen, and usually only home owners attempt this. This is probably the method that accounts for more wood rot than any other method. If you are even thinking of doing this, quit thinking and call a professional to fix your crawlspace. Most crawlspaces are vented and the cooler surfaces such as duct work, pipes, and the floor will condensate in the summer. The plastic will trap the condensation up against the floor structure and mold and wood rot will occur. Good intentions do not always produce good results.
4. Vapor barrier fastened to sill plate – There is a new industry in crawlspace repair that encapsulates crawl spaces. The process of encapsulation is to install a heavy think plastic liner on the floor and up the foundation walls. The plastic liner is fastened and sealed to the foundation wall and all the overlapped seams are taped. Every potential gap or seam in the liner is meticulously sealed to prevent any moisture from evaporating. Then the vents are sealed in the encapsulation system to prevent hot humid air from entering in the summer. There is another system being sold and installed out there being represented as an encapsulation system, but is far from it. This “other” system is a 6 mil plastic that is ran up the foundation walls and stapled to the sill plate. None of the overlapped seams are taped. It is basically a glorified vapor barrier on top of the ground floor being ran 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 fastening plastic to the sill plate is that moisture will “wick” up the foundation wall, and moisture will absorb into the sill plate and floor joists. They are giving free access to all the moisture under the liner to rot the sill plates and floor joists. Not to mention that all the moisture will evaporate up through all of the seams that aren’t taped and the plastic liner is only 6 mil and eventually will puncture and tear. Be very careful in the contractor you choose to properly encapsulate your crawlspace.
If you are in need of a new crawlspace vapor barrier or encapsulation system, give us a call @ 1-877-409-2837 or SIGN UP ONLINE for a FREE CONSULTATION.
Also, visit our website to see more about our crawlspace vapor barriers.
Thanks for reading the rambling thoughts of a crawlspace inspector,
Larry Ralph Jr.