The Real Way to Manage Basement Moisture
Basements were never intended to be lived in. In the old days, we called them cellars. We stored vegetables and coal and wood in them. They smelled damp… they were damp. Now we put bedrooms, entertainment centers, and nurseries in them. We began to live in every cubic volume of space rather than just on the main levels of houses. We expanded the conditioned space and basements were the first priority of making that conversion (Figure 1).
Building above grade is tough enough, but below grade? It is the most difficult space to get right. Space in basements is cheap, right? Nope. The basement has to hold the building up. That alone is a big deal. The basement has to keep the groundwater, soil gas, and water vapor out, and keep the heat in during the winter and the heat out during the summer.
We are going to avoid the structural part…mostly. Instead, we are going to focus on the other parts. Basement foundations need to be water managed. Water-managed basement foundations rely on two fundamental principles: keep the rainwater away from the foundation wall perimeter and drain the groundwater away from the foundation wall.
Figure 2 illustrates how we want to drain the rain away from the building on the top of the ground before it can become groundwater… send it to your neighbor’s property, give them the problem.
Check out Figure 3. The classic way of doing things since the time of the Romans has been to provide exterior drainage, directing groundwater downward to an exterior perimeter drainage system. It does not get better than this. Or does it? Well, we could use a modern high-tech way of doing the Roman thing – a drainboard.
Now would be a good time to point out that there are only two kinds of concrete in the world. Concrete that cracks and concrete that will crack (Photograph 2). My first structural engineering professor pointed out: “Young Joseph, if you don’t put in a control joint, the building will put in its own.” Ah, Professor Burnett, your wisdom endures. Put in straight cracks and call them control joints (Figure 4). You can sell a straight crack, but the random ones irritate clients. Now, we have to keep the groundwater out of the control joint.
Drainboard and control joints are a marriage made in heaven. One addresses the concrete crackage problem, and the other addresses the problem caused by the solution to the concrete crackage problem.
We done yet? Nope. Concrete “sucks.” Not in the colloquial way, but in the real way. It wicks water by capillarity due to the tiny pores in concrete. Concrete can wick water hundreds of feet. If you don’t believe me, check out Lord Kelvin. Capillarity in basements is a big deal (Figure 5).
Concrete needs to be “dampproofed” by filling the tiny pores (Figure 6)—or, wait for it—made “real big.” A drainboard is a fabulous capillary break as it makes the tiny pore “real big.”
What else? Ah, you need to insulate the foundation wall. I think the best place is on the inside, and it should be done with water-insensitive insulation against the concrete wall first, and fluffy stuff to the inside of that.
Why on the inside? Does thermal insulation not work better on the outside? Thermal mass? Nope. The thermal mass is not very well coupled to the main part of the building. And when you insulate on the exterior, the surface area of heat loss is about 10% larger than the surface area of heat loss from insulating in the interior. Do a perimeter calculation… the linear length of the exterior surface of the exterior insulation on the exterior of the concrete foundation is greater than the linear length of the interior surface of the interior insulation on the interior of the concrete foundation. Ah, isn’t arithmetic grand?
It gets better. You ever try to protect exterior foundation during the construction process? And exterior insulation becomes an insect and animal “interstate” right into your building? Termites will thank you for their new home.
Put the drainage on the outside and the insulation on the inside, and don’t forget the capillarity.
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.