Cary Grant Does Foundations*
The fundamental principles of groundwater control are to keep rainwater away from the foundation wall perimeter and to drain groundwater with sub-grade drainage and perimeter drains before it gets to the foundation wall. Nothing has changed in several millennia. Let’s look at the groundwater part.
For groundwater, there are two things that need to happen – you need to control hydrostatic pressure and you need to control capillarity. There is no better way of controlling hydrostatic pressure than by providing drainage. There is no better way of controlling capillarity than by providing a gap.
Wouldn’t it be neat (done with or demonstrating skill or efficiency) if you could do both with one approach?
Hydrostatic pressure can be controlled by the placement of free-draining material immediately adjacent to foundation assemblies connected to a perimeter sub-grade drain. Historically, this free-draining material has been sand or gravel. Today, it can be a drainage membrane or a drainage board. This free-draining layer allows for the free flow of water downwards towards a sub-grade drainage system under the influence of gravity. This prevents the development of hydrostatic pressure. In the absence of hydrostatic pressure, water running down the exterior of the basement wall will not be forced in through cracks.
Capillarity acts primarily to move moisture into porous materials. For example, a paper towel with one end in contact with liquid water draws water into itself against the force of gravity as a result of capillary suction. Capillarity is a function of – among other things – pore size and available moisture. If pore size in a material is large, such as clear gravel and coarse sand, then capillarity will not exist. If pore size in a material is small, such as in concrete, silty clay, and paper, then capillarity is possible.
Capillarity can be controlled by getting rid of small pores or by making them big. One of the best capillary breaks is an air space. And if the air space also provides drainage…it would really be neat.
That’s how a dimple sheet such as DELTA®-MS works. The dimples provide both drainage and a capillary break.
Now, provision for removing the water at the base of the free-draining material – the dimple sheet – must be made, typically a sub-grade drainage system.
The sub-grade drainage system is usually a perforated drainage pipe located at the perimeter of the foundation below the basement floor level to the exterior of the wall. The perforated drainage pipe is encased in crushed stone (free from fines) and surrounded by a filter medium. As groundwater rises, it rises into the drainage pipe and is carried away.
Free-draining materials such as dimple sheets have proven so effective that foundation assemblies remain dry even if numerous holes, openings, or cracks exist. Free-draining materials such as dimple sheets rely on the provision of air spaces or openings of sufficient size, which allow water to drain through them. The dimples have to be big…Cary Grant big.
*Cary Grant was the quintessential Hollywood leading man with the world’s most famous “dimples.” Pretty sure his dimples inspired DELTA®-MS. He was debonair, stylish, sophisticated and charming – much like DELTA®-MS.
About Joe Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, Principal, Building Science Corporation:
Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects, to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies.
Dr. Lstiburek is also an acclaimed educator who has taught thousands of professionals over the past three decades and written countless papers as well as the best-selling Builder Guides. His commitment to education earned him the hailing “the dean of North American building science” by the Wall Street Journal. You can find additional details on Dr. Lstiburek on our About the Blog page.
