What is Capillary Rise, and How Does Capillary Action Work?

Just like a dry sponge soaking up water, concrete walls wick moisture up from the footings. Dr. Joe Lstiburek from Building Science Corporation explains what capillary rise is and how capillary action works in this video.


Capillary rise and capillary action: Key takeaways

Capillarity is a big deal when it comes to porous materials. How high water wicks is determined by the size of the material’s pores. The smaller the pores, the higher the water wicks. 

For example, if you placed the end of a column of concrete into a bucket of water, the water would wick up to 10 KM or six miles. (That’s some serious sucking.)

example of capillary rise

What the industry eventually learned is that if the pores are filled, we’re not going to experience capillary rise. Or if they’re made very large, there won’t be capillary rise. So in foundation systems, large holes are put under concrete slabs, stones as the capillary break, and then the perimeter walls are covered with black bitumen or, as Dr. Joe likes to call it, ‘dead dinosaur juice’. 

But what’s done at the footings? Well, nothing. At least back in the old days. That is because we didn’t care about water getting into a footing and evaporating to the inside and into the basement because they weren’t used as living spaces. But now that we make use of them, interior conditions matter, and is why we now put a capillary break on the foundation to manage capillary rise. 

Essential reading…How to Properly Manage Moisture in a Basement

Fluid capillary breaks have been used, and nothing irritates concrete people more. The fluid needs to dry and can’t stay sticky (and they always stay sticky), otherwise it ruins the builder’s work. As Dr. Joe says, you don’t want that, as you’ll get stoned. 

The other method is using a membrane. A fully-adhered one, to be precise, and it has to be pore-free.

The only caveat here is with structural engineers, and their concerns about connecting a foundation wall to the footing. Keyways were a solution in the past, but penetrating rebars or reinforcing rods are now preferred to connect the foundation wall and footing. It’s not a structural concern in Dr. Joe’s opinion anymore, as he believes the slab and dirt on either side is more than enough support. After all, he has yet to see a building fall off a foundation wall. 

At the end of the day, capillarity and capillary action are relatively simple concepts to grasp, but in practice they can be anything but simple. So, remember: 

  1. Make the pores big or make the pores small;
  2. If you’re making small pores, it requires stuff that sticks to other stuff really well, and;
  3. Never irritate a structural engineer.

Now that you’re a capillarity expert, here’s how you can leverage your knowledge of capillary breaks to design and build better, dry basements.

About Dr. Joe Lstiburek 

Dr. 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. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. Dr. Lstiburek holds a Bachelor of Applied Science in Mechanical Engineering, a Master of Engineering in Civil Engineering, and a Doctor of Philosophy (Ph.D.) in Building Science. Get the full scope of Dr. Lstiburek’s work, accolades, and contributions to the industry over at Building Science Corporation.