What Works Above, Doesn't Below
By Tom Inglesby
The two important aspects of moisture control below grade are proper drainage and a moisture barrier or membrane.
While membranes are a tool in the battle against water intrusion, the war is won with careful attention to drainage. A complete design approach is presented by Dale Kerr, general manager of GRC Building Consultants of Newmarket, Ontario, in a technical paper called "Drain and Dry Your Walls Fast." He speaks of the "4-D" strategy: deflection, drainage, drying and durability.
"Keeping rain off a wall (deflection) is the most obvious way to prevent water penetration — no water, no problem," he said. "Unfortunately, methods to accomplish that are not so obvious. While the use of overhangs and the protection afforded by adjacent buildings will help deflect some rain from the wall, there is no way to keep it all away. Walls must be designed to get wet.
"The control of exterior moisture requires an effective water shedding surface and an effective water-resistive barrier. The term 'water shedding surface' refers to the surface of assemblies, interfaces and details that deflect and/or shed the vast majority of water impacting on the wall.
"The 'water resistive barrier' is the surface furthest into the wall from the exterior that can accommodate some exterior water without causing damage to interior finishes or materials within the assemblies."
As Jim Keene, CEO of Keene Building Products of Pepper Pike, Ohio, explained, "All walls must be drained to function properly. Drainage involves a number of products, but the most important is the use of an accumulation pipe to move the water from the soil away from the foundation. This accumulation pipe is most commonly a perforated pipe placed at the bottom of the footer.
"The entire wall is typically drained with a sheet drain," he continued. "This sheet drain is either a 'dimple drain' type product or an 'entangled net' type product. Both products work well and have their advantages and disadvantages. Dimple drains have greater capacity to drain and entangled net products offer more resistance to the rigors of crushing during backfill. In either case, both materials have many times the capacity required to handle the water flow from soil, and both materials have the strength or compression set to handle the backfill."
Membranes come in a variety of types from bituminous (rubberized asphalt) to clay, and in sheets, sprays, liquids and even cardboard forms. Obviously, each has its advantages and disadvantages.
According to Keene, "In commercial construction, the most common membrane is a bituminous sheet, 60 mils thick. This product is applied to the wall in its entirety. It creates an impermeable barrier to moisture even if a hydrostatic head of water builds. The membrane is installed to the point of soil grade on the top and down over the footer on the bottom. Penetrations are detailed to ensure water tightness around them. This same bituminous material can be applied in a roll or spray-on coating, and many manufacturers provide products such as this, plus a significant number supply urethane-based materials that act in a similar manner and perform equally."
All of these membranes should be implemented with a "protection board" to guard against the rigors of the backfill process. For sheet membranes, it is also typically required to parge the wall, or apply a coat of mortar to the block, to make the surface smooth to easily fasten the sheet. A parging coat would not be required for a liquid membrane applied with a roller or spray.
"In residential applications, the use of a membrane is less common," Keene said. "The typical detail for moisture in residential is to use a vapor barrier coating, such as a thin asphalt material."
Several companies, such as Cosella-Dörken, offer a "dimpled sheet" product. Sandin explained, "The membrane's key job — the first principle of failsafe water management — is to keep water from ever touching the wall on the way through the soil to the footing drainage systems. Our Delta-MS provides two lines of defense: the first is an impermeable plastic sheet that is waterproof; the second is the dimpled structure that offers a free drainage path. In the event that water passes the first line of defense, it can flow freely down the air gap to the footer drain. Unlike a coating, Delta-MS bridges all size cracks, ignores damage points, and deflects water, as well as soil dampness. It also allows for the drainage of construction moisture to the footer drain.
"Delta-MS is a highly effective waterproofing protection system based on a uniquely formed air-gap membrane," he added. "The main component is the plastic (HDPE) membrane, formed in a dimple pattern to create an air gap at the foundation wall. This design allows any water getting past the dimpled plastic sheet to fall freely to the footer drain. This membrane is remarkably impermeable to water and water vapor, and impervious to acids and other agents. The HDPE is tough, resilient and durable with an expected service life of more than 50 years."
Of course, the quantity of moisture doesn't matter if you're using an above-grade product improperly for below-grade protection.
"There are some things that are used on block walls that won't work below grade," Keene said. "For example, penetrating sealers designed to make the surface of block impenetrable by water. Plastic sheeting that is sometimes used as a vapor barrier but won't stand up to the rigors of backfill and doesn't adhere to the foundation wall. Waterproofing products that are cement based and coat the outside of the cement block with a finer grade cementitious product. Hydraulic cement materials that plug the holes or cracks in the mortar joints — the water will just find another hole. And finally, interior coating of block as the only approach to waterproof the wall — there is no way of eliminating the hydrostatic head pressure from the inside of the building."
About the Author
Tom Inglesby is a San Diego-based freelance writer whose work has appeared in numerous online and print publications. He is the winner of the Construction Writers Association's 2002 Boger Award for Special Reports.