By Greg Stopher | With all of the fantastic weather we’ve had in Juneau this summer, it’s hard to stay focused on house projects for as long as we probably should. One space you should give attention to at least once each year is your crawlspace. Just because the crawlspace in your home is not something you see every day, it’s not something you should ignore.
Crawlspace moisture and air conditions can and certainly will affect a home. As cold moist air moves toward the warm air of the home, moisture in vapor form can enter the home from the crawlspace. This movement of air is called stack affect. More simply put, a house acts like a chimney for the crawlspace.
For Example: When the dryer is running, the bathroom exhaust is going, and the range top is on, air is exhausting but where is the replacement air coming from? It is likely coming from gaps and cracks in thermal envelope such as around doors, windows and outlet boxes. Some of the replacement air is almost certainly coming from the crawlspace. This crawlspace air could be moisture laden, and potentially hazardous.
How can you be sure your crawlspace is not causing problems in your home? This is going to require you to venture into your crawlspace. No one wants to go into their crawlspace especially when it is musty, moist or full of spider webs. If it is moist and musty then it might be time to do something about it. Even if the dirt is clean and dry it might not always be that way. A dry crawlspace will still have evaporation or moisture vapor coming up from the surface. First, check to see if there is a vapor barrier. A typical vapor barrier is made up of a plastic sheeting (polyethylene 6 mil) on the ground of the crawlspace. You will want to inspect your vapor barrier to see if it is properly sealed so it can be as effective as possible.
How do I go about installing a moisture barrier in my crawlspace? There are a couple of ways to go about sealing a crawlspace. The first is to use the polyethylene plastic sheeting or poly (typically 6 to 10 mil), acoustical sealant, 1×2 lumber (enough lineal footage for your exterior walls), concrete or wood screws, and a vinyl or house wrap tape. The first step is to make sure the crawlspace is clean of any organic material like wood scraps, paper, cardboard boxes, and large chunks of concrete or rocks. Inspect the subfloor and floor joists for any moisture related problems, high moisture content will suggest there is problem in the crawlspace. If there is any type of growth, fungal or mold, this is the time to get rid of it. With these items completed it is time to start laying out the vapor barrier. I like to use 20’ x 100’ poly sheeting and I plan out which direction I should run the sheeting based on the specific crawlspace.
It is recommended you bring the plastic up the wall at least two feet. However, I prefer to bring it all the way up to the sill plate (normally a 2×6 treated lumber plate that sits on top of the foundation wall).
Now it is time to roll out the sheeting across the ground of the crawlspace leaving enough poly to reach the sill plate on all sides. Apply a ¼” bead of acoustical sealant to the sill plate extend the vapor barrier up the wall and fasten it to the sill with the 1×2 lumber and screws. This will keep the vapor barrier in place and provide a continuous seal around the perimeter of the crawlspace. Once the perimeter is secured, it is now time to seal around any posts in the crawlspace. At the posts, I will run the poly up the post like a sleeve, then caulk and seal any joints. Lastly, any joints in the polyethylene should be overlapped at least 6” with a bead of acoustical sealant applied and the joint taped with vinyl or Tyvek tape. I also like to seal any holes between the subfloor and living area using spray foam or caulk. The holes were likely created to run wires, pipes, and ducting. Sealing these holes will all but eliminate airflow from the crawlspace into the living area. It is also good practice to seal all exposed ducting with mastic or a foil tape; do not use duct tape.
Another crawlspace sealing technique includes a variation to the above procedure where polyethylene sheeting is placed only a few inches up the foundation wall. Then using a commercial spray foam applicator, spray foam is applied to the foundation walls of the crawlspace taking extra care to seal the vapor barrier to the foam. The foam will create an airtight seal up the foundation wall. This procedure will also improve the crawlspace by further insulating the space.
Other crawlspace sealing membranes on the market range from 12 to 20 mills in thickness and accomplish a similar airtight vapor barrier as the technique above. These materials sometimes come in rolls of 3 or 4 feet wide and are applied and sealed up the foundation wall. Some are glued together, some are taped, and some have pre-applied tape strips.
Some crawlspaces need to have drainage or a sump pump added to remove unwanted water. If a sump pump is required, it should be placed in a perforated drum at the lowest point of the crawlspace. All surface water should be able to drain into the sump pump drum. The vapor barrier can then be sealed over the top the drum, like sealing around a post.
The key to all techniques is to be sure there is an airtight seal. Make sure all joints are overlapped and sealed. Seal around the perimeter and up posts. If you do the project yourself materials will likely cost around $0.50 per square foot. If you decide to hire a licensed professional to complete the project the price can range $1-5 per square foot depending on the material choice. A properly sealed crawlspace will reduce moisture in your home and can improve air quality.
Greg Stopher has over 16 years of experience in the construction field and earned a degree in Construction Technologies from the University of Alaska – Southeast. His company, Stopher Construction, LLC, is a general contracting company specializing in remodeling, custom finishes, additions and new home construction projects. He can be reached at 907-321-2350.