We hear it all the time, “my basement is wet/damp/musty in the spring and summer – but that’s alright, it’s supposed to be like that. We just run the dehumidifier for 6 months straight and empty it once or twice a day.” When a basement/crawlspace has high moisture, so does the rest of the house. The natural path for air to move throughout a home is up, which means that too is the natural path for moisture. This can lead to condensation on windows, mold/milder on surfaces, mold/mildew in the attic, and overall sub par indoor air quality.
Even if your basement/crawlspace is bone dry and you have an exposed dirt floor, you should still install ground cover. Why? although moisture may not be a concern, other gases could be. In the northeast Radon is an issue - and it's ever changing, if your radon levels are good this year, they may be elevated in the future. Poly ground cover can help keep those gasses at bay by covering up exposed dirt floor.
Many home already have a vapor barrier/poly laid out over exposed dirt; HOWEVER, it is not sealed at the seams or the edges near the foundation wall. Vapor is like air and water, it will find a way out unless sealed tightly!
The Solution: At the least, we use 12 mill reinforced poly as it will last the test of time. If your floor has exposed stone or rock you may want to first install an anti microbial felt layer to further protect the poly from ever tearing. Cut sections to cover the exposed area(s) and leave enough excess to run up the foundation wall a bit. If you have to overlap sections of poly we like to have ~2' overlap. Lay out your first section of poly on the ground and run up the foundation walls approximately 8-12″. Depending on your foundation wall (poured concrete, cinder, field stone, brick), you can adhere the poly to the wall with strapping/concrete screws/washers OR you can spray foam it in place with Closed Cell Spray Foam.
If you have to overlap sections of poly, make absolutely sure that you thoroughly tape the seams with high quality poly tape that is at least 3" wide. Many different brands make these tape, they can cost up to $100 per roll or more.
If done properly, you can usually feel the improved air quality within 15-30 minutes of the poly being sealed. The air will feel dryer and the smell will be cleaner.