Things To Consider When Using Water Vapor Control Layer
The first side of your new home’s building enclosure is the most important place to start with a water vapor control layer. The first side of a home consists of a concrete slab or in a crawl space. The reason it is very important is because the ground has moisture in it and water vapor diffusion is mighty. No matter where you live in the United States, there is some source of water beneath your home. This includes the desert where the water table is deep in the sandy soil.
Once a foundation (slab or below-grade) is built, the water vapor will reach 100%. Once it diffuses it can seep into the concrete if it is not vaporproof which will lead to failing floors, mold growth and is bad for the home’s HVAC system. A home can be protected by water vapor diffusion by using a vapor barrier. The barrier should be placed directly underneath the concrete slab. Remember a slab has no way of drying out and if it gets damp, it will stay damp.
More than likely a vapor control layer will be used and can also be required. The barrier will stop water vapor from entering the concrete slab, the crawl space or the interior of the home. It is said that you want 0.01 perms which can be produced by using high-performance and cost-effective engineered polyolefin vapor barrier.
When it comes to the material that the barrier is made of, there are differences. Generic poly sheeting is a lot of times low-quality and does not hold up. In fact, the American Concrete Institute says they will not approve generic poly sheeting to be used for this purpose. Generic poly plastic will transmit around 66.8 cups of water per year while a vapor barrier 15mil will only transmit 6.7 cups per year.
Sometimes a vapor retarder that is made for a wall will not work for a slab. A vapor retarder must, at a minimum, have a water vapor permeance of less than 0.1 perms (less than 0.01 perms is, in many cases, even better). Here are several things to consider when choosing a vapor barrier. No matter where you live, geotechnical data or Climate Zone diffusion from the soil will occur. Water vapor diffusion is highest at the foundation of a home where liquid water is already detoured. Never skip using a vapor control layer to separate the home from the soil, a vapor retarder must have a permeance less than 0.1 perms. That leads to the lower the permeance on the foundation the more effective it will be. A vapor barrier that is used below a slap has to meet ASTM E1745.
Remember understanding the ins and outs of protecting the foundation of your home is beneficial. You can use this knowledge to help design, evaluate and select the correct vapor barrier.
