Basement Moisture: Everything Homeowners Need to Know
Part 2: Where Is This Water Coming From?
Welcome back to our series on everything you need to know to feel confident in your home buying and home owning decisions when it comes to basement moisture. If you missed Part 1: Understanding the Science, check it out for a deep dive on the physics at play in keeping your basement dry (or not). Here, we look into sources of moisture.
Internal Moisture Sources
The main goal is to keep environmental water out of our homes, but don’t forget that we purposefully move water all around the inside of our structures. Ideally, this water never diverts from its intended course, but stuff happens. It’s always a good idea to keep in mind internal sources, could it be leaking pipes, a water heater, or condensation from HVAC operation, ventilation practices, or poor insulation.
Combined Sewer Systems
A rare occurrence for older homes in some areas is flooding from an overwhelmed sewer, storm, or combined system. This can occur if the combined sewer system becomes overwhelmed by heavy rains or the sanitary sewer system experiences an issue. Some older homes may even have a tie into the storm drains through home drainage. This type of drainage is not permitted anymore. However, if you find yourself in the rare situation of dealing with it, backflow preventers can be installed so that sewer water cannot be pushed back into the home.Basement Ventilation
Ideas about basement and crawlspace ventilation have shifted a lot over the last 20 years. In the past, it wasn’t uncommon to ventilate crawls and some unfinished basements with external air in the summer. That influx of hot humid air can lead to increased moisture intrusion and collection. Today, it’s recommended to condition the space using the home’s HVAC system. Not only does this keep air moving but also it manages the internal temperatures to prevent high humidity and condensation.
Vapor Diffusion and Air Leakage
In a home without liquid water problems, vapor diffusion and air leakage is far and away how most moisture enters a home. A typical 32 sq.ft. piece of drywall allows 1/3 quart of water to diffuse through it monthly. A 1-inch hole will pass 30 quarts of water into the home over the course of a month. This increases indoor humidity, which leads to condensation and mold growth. In severe scenarios, it can increase the moisture content of the wood in the home to a level that supports wood rot and deterioration. To keep wood from degrading, it needs to have a moisture content below 20%. Keeping the humidity in your home less than 60% will keep the wood at a low moisture content and prevent surface molds and mildews. If you would add all of the air gaps in home together: cracks, unsealed cable and pipe penetrations, poor door and window sealing, etc. it adds up to much more than a 1-inch hole.
A 1-inch hole will pass 30 quarts of water into the home over the course of a month.
If you have a crawlspace, then you have yet another source of moisture diffusion. A crawlspace floor without a vapor barrier can evaporate gallons of water into a home daily. Crawlspaces should be sealed with a vapor barrier at minimum, but an even better option to control vapor from a crawlspace is to seal and condition the space as if it is a finished part of the home, if your crawlspace is attached and open to your basement you should definitely do this to minimize moisture.