Building on a strong foundation has been a well-recognized principle of construction for thousands of years, but we still continue to violate it. One of the most violated areas of below-grade construction is the failure to include moisture management. Let’s examine common below-grade construction moisture management errors that are still in practice, and then make some decisions on how to correct them so that we can build stronger foundations for residential and commercial properties.
Most residential and commercial structures feature below-grade construction that is usually 10- or 12-inch CMUs or 8- or 12-inch poured concrete. The footings that support these walls can vary in depth and width depending on the soil conditions. The basements may be fully below-grade or partially exposed and there may be walkouts. However, they all have one thing in common--unique moisture management requirements.
The two most common sources of moisture that impact below-grade construction are infiltrating moisture and condensing moisture. These are two very different concepts, but the factor that joins them is temperature.
Moisture can impact construction in many ways. There are two that are most relevant to this discussion. First, moisture can pressurize against the surfaces of below-grade construction and force its way through voids in the construction, creating leaks. Second, moisture can transmit heat 25 times more efficiently than air to the exterior surface of the below-grade construction. This creates the possibility of condensation.
It’s back to the old rules of moisture management: Keep moisture away from and off of construction, and get it out of construction as quickly as possible. Follow these two rules and there will be less risk of moisture damage to the construction.
Learn from the past
Sometimes, the best way to avoid a wet basement is to not build one. If the soil conditions of the lot are not appropriate for below-grade construction and the budget is tight, it is wise to choose another foundation option for your structure.
A dry basement begins with a building site that can be managed to control moisture. Taking ownership of a building site on which you can’t build is a bad start. Learning from the excavator that the soil conditions are bad and will require soil reconstruction or budget-busting oversized footings or pilings to support your structure is also a bad start.
Once the site has met the critical tests, the next step in the process is jobsite moisture management during construction. The below-grade construction phase of many building sites is negatively impacted by poor or nonexistent jobsite moisture management plans. Jobsite moisture management, in the form of a jobsite drainage plan and the ongoing implementation of this plan, is critical.
Construction specifications should clearly state excavation dimensions required to safely and efficiently construct the below-grade structure and minimize the size of open excavation, therefore, minimizing the amount of water entering the excavation.
Upon completing the excavation, form and pour the footings and walls, or form and pour the footings and lay the block. There are two critical principals that must be followed at this stage of the construction process. First, do not fool around. It is imperative that all parties adhere to a strict timeline and complete the project on schedule. The amount of excess time this excavation is left open invites disasters in the form of excavation cave-ins and flooded excavations. Construct the basement and get it waterproofed, drained, and backfilled. Second, don’t take shortcuts. The foundation of the entire project is literally being fabricated. If any part of it is either hurried or omitted, the entire project is at risk.
Decide what works
Establishing a moisture management plan results in a positive outcome for a below-grade construction project. Following these critical steps leads to an optimal end product and good site preservation practices.
Select a building site with good soil and good drainage. A location that is too low and soil that doesn’t drain invites moisture problems. Drainage systems should be located at the lowest elevation of the basement construction. First, it provides a drainage pathway for water in an exterior drain tile of a full in-ground basement to have access to the sump pump and sump basket located in the interior of the basement. This facilitates it being pumped out and away from the basement detail. Second, it provides immediate elimination of potential water pressure build up.
Connection of the vertical drainage planes on the exterior of the perimeter wall systems to the drain tile system is very important. Any restriction of moisture flow at this point may cause water pressure build up.
There are a number of moisture management concerns that must be addressed at the intersection of the basement wall, structural footing, and basement floor: temperature transfer, condensation, water infiltration, and soil gas/contaminated air infiltration.
Temperature transfer concerns start with a large mass of concrete like the structural footing. Another factor that affects the temperature of this detail is the fact that it is away from any heat source and because the basement wall rests on the footing and is exposed to fill temperature around 54º F. The bottom of the basement wall and the footing transmit the temperature to the edge of the concrete floor. This results in the inside corner where the basement floor meets the basement wall being constantly cooled. This cooled surface temperature and the diminished airflow in the inside corner is a recipe for condensation.
If the drainage material and the moisture management system are of the right design, then drainage, venting, and temperature can be successfully addressed with one system. The bonus is that little or no maintenance will be required.
Build for the future
Those are the key elements of our 21st century moisture management plan. Most of us know change is needed. Too many of us have witnessed too many below-grade construction failures due to moisture issues. It is time to embrace new ideas and employ better moisture management practices.