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Moisture Management of Below-Grade Construction ‘Basements’
Building on a strong foundation has been a well-recognized principle of construction for thousands of years, but we still continue to violate the concept. One of the most violated areas of below-grade construction is the failure to include moisture management.
Joseph Cubby says, “Learn the past. Decide the present. Build the future.” With apologies to Cubby, let us examine common below-grade construction moisture management errors from the past that are still in practice, and then make some decisions today on how to correct them so that we can build stronger foundations for tomorrow’s residential and commercial properties. Most residential and commercial structures feature below-grade construction that is usually 10- or 12-inch CMU or eight- or 12-inch poured concrete; a much smaller percentage involve ICF and wood walls.
The footings (or caissons) that support these walls can vary in depth and width depending on a variety of soil conditions. The basements may be fully below-grade or partially exposed (a berm) and there may be walkouts; many configurations are possible. However, they all have one thing in common – unique moisture management requirements. (See Image #1.)
The two most common sources of moisture that impact below-grade construction are infiltrating moisture (in liquid form entering through walls or floors) and condensing moisture (in vapor form being deposited on the interior surfaces of the basement walls and floors to create water droplets). Two very different concepts in the realm of the laws of physics, but in many cases, absolutely connected in the realm of wet and dry basements. The factor that joins them is temperature.
Moisture (water in the fill that surrounds below-grade construction) can impact this construction detail 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 this construction detail creating “leaks.” Second, moisture, because of its molecular density, can transmit temperatures 25 times more efficiently than air to the exterior surface of the below-grade construction to potentially create a dew point temperature on the interior surface of the below-grade construction creating the possibility of condensation. (See Image #2.)
It’s back to the old rules of moisture management: Keep moisture away from and off of construction details, and get it out of construction details as quickly as possible. If these two rules are adhered to, there will be less risk of moisture damage to the construction detail.
Learn from the past
A dry basement begins with a building site that can be managed to control moisture. The familiar phrase “high and dry” comes to mind when planning a foundation. In the nautical world, high and dry is a bad thing, but in the world of below-grade construction, it’s the pinnacle of moisture management perfection. When you get to start with a high-quality building site like the one above, a wet basement isn’t very likely! (See Image #3.)
However, do not let looks deceive you. Once you have found an exceptional building site, be sure that the purchase agreement contains a contingency for a soils report generated by a licensed soils engineer. The information used to create this type of report comes from borings – cores of soil taken at varying depths in prescribed locations on the building site. Taking ownership of a building site that can’t be built on is a bad start. Finding out from the excavator that the soil conditions are bad and will require soil reconstruction and/or budget-busting oversized footings or pilings to support your structure, is also a bad start.
The soils report should also contain information such as the water table and the presence of any potential chemical contaminants. If there is any question about adjoining lots that may leech in contaminants, they should be tested, too. The structural engineer will take the information from the soils report and design the appropriate structural footing and walls that form the basement and support the structure. 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. A jobsite that is allowed to turn into a mud hole will have long-term negative impact on the soils that surround the below-grade construction and as a result, have a potentially negative impact on the moisture management of the below-grade construction. Jobsite moisture management, in the form of a jobsite drainage plan and the ongoing implementation of this plan, is critical. Allowing jobsite surface water to run toward and into below-grade construction excavation and details can have a permanent negative impact on project. The inadvertent saturation of soils that surround and support footings and walls is not good. (See Images #4A and #4B.)
The list of potential violators of a jobsite drainage plan is long and includes plumbers, sewer and water utility personnel, and electrical contractors. The standing jobsite rule on this should be that anyone who moves or adds fill to a jobsite should return it to the original jobsite elevation drawings and plans and compact fill as to the specs.
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. (See Image #5.)
Upon completion of 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. Firstly, “Do not fool around!” It is imperative that all parties involved adhere to a strict timeline and complete the project on schedule. The amount of excess time this excavation is left open invites moisture management disasters in the form of excavation cave-ins, flooded excavation, etc. Construct the basement and get it waterproofed, drained, and backfilled ASAP!
Secondly, “Don’t take shortcuts!” Do not skip any of the important construction procedures. 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. Working cohesively as a team is crucial. There are usually several subcontractors involved including plumbers, electricians, waterproofers and excavators. The coordination necessary to keep these subcontractors on schedule is also crucial.
Additionally, there is a real danger at this time that one or more subcontractors will, because of gross negligence, ignorance or other reasons, undo important drainage or waterproofing details. Studies have shown that as much as 85 percent of all labor time conducted on or near waterproofing systems is conducted by labor that has no understanding of how these systems are supposed to function.
Decide What Works: Key Plan Elements
Note: A structural engineer should do the required engineering to facilitate this detail. Connecting the exterior drain tile to the interior drain tile serves two important purposes. 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. Prevention of this phenomenon is a must in maintaining a below-grade moisture management system.
Connection of the vertical drainage planes on the exterior of the perimeter wall systems to the drain tile system is important. Any restriction of moisture flow at this point may cause water pressure build up. This transition from vertical to horizontal is accomplished in a number of ways. The two most common are:
Building professionals have been engaged in a long, ongoing struggle over two concepts: drainage vs. waterproofing and venting vs. sealing. Is it better to seal water out with waterproofing or drain it away? Should contaminated air be vented out or sealed out? It is only by combing waterproofing with drainage and venting with sealing that we truly have an acceptable solution to each question.
For many years manufacturers of waterproofing and sealant materials have controlled the market. They had the marketing dollars to influence the consumer, so they have held sway. There have been few manufacturers with either the money or a reason to dispute the efforts of the waterproofing and sealant manufacturers to push product.
As a result of this imbalance of forces, waterproofing and sealant companies have won the day and instilled a sense of security with many building professionals. In many cases this sense of security is well-founded as many sealants and waterproofing products of today are nothing short of fantastic.
Another negative force that must be addressed is construction design detail conflicts. Sometimes, drainage and venting are in direct design conflicts with waterproofing and sealing. Construction details that have high quality drainage, waterproofing, venting and sealing components perform better and last longer than details that don’t have all four.
So how does this pertain to below-grade construction? One primary example is a construction detail where the basement floor comes in contact with the exterior perimeter wall and the footing that supports it. (See Image #7.)
There are a number of moisture management concerns that need to be addressed at the intersection of the basement wall, structural footing and basement floor.
The temperature transfer concerns start with a large mass of concrete like the structural footing. This cold sink excesses because the footing is located at the lower point of the basement construction and is in constant contact with the surrounding fill temperatures that hover around 54oF. Another factor that affects the temperature of this detail is the fact that is away from any heat source and because the basement wall rests on the footing and is exposed to fill temperature around 54˚F. The bottom of the basement wall and the footing transmit the 54˚F 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 to 54˚F. This cooled surface temperature may represent a dew point temperature and the diminished airflow in the inside corner is a recipe for condensation. (See Image #2.)
The voids that remain between the surfaces of these three construction details also represent entrance paths for both air and water. What seems like the obvious remedy (caulk and seal these voids off) may not be the best long-term moisture management solution. Placing a bead of caulk over the void in the corner where the floor meets the wall is, in fact, a very poor caulk detail; it is a three-sided adhesion issue that has been historically proven to have a short life.
A final issue with this detail is what happens if the interior surface of the basement wall gets wet from condensation or a leak? Where will the water go when it runs down the face of the wall and meets the floor? This is an example of the design conflict we mentioned earlier: sealing and waterproofing versus venting and drainage.
Building professionals and building code tend to lean towards waterproofing and sealing alone. However, venting and drainage play a critical role in a total solution, and they should not be omitted if a long term, low (or no) maintenance outcome is desired. If the drainage material and the moisture management system are of the right design, drainage, venting and temperature can be successfully addressed with one system. The bonus is that little or no maintenance will be required. (See Image #6.)
Build for the future
There is too much evidence that traditional moisture management practices are not working with 21st Century materials and the desired outcomes of sustainability and green building. It is time to embrace new ideas and employ better moisture management practices.
John Koester is CEO of Masonry Technology Inc.
|Last Updated on Friday, 08 July 2011 09:35|