Q: Should the top of a concrete footing be treated to prevent moisture from wicking up from the soil into the foundation walls? Aren’t the perimeter drains enough?
The practice of adding a capillary break to the top of a concrete footing before forming and placing the foundation walls is apparently not universal and is not required by code. But building scientists, researchers, and well-informed builders all recommend it.
Capillary action, a more elegant way of saying wicking, is the movement of a liquid through gaps in a solid material. Water can, in fact, rise to some amazing heights as it moves through materials such as concrete and wood. In an article titled “Capillarity—Small Sacrifices,” building scientist Joe Lstiburek says the theoretical limit of capillary rise in concrete is 10 km., or about 6 mi. Capillary action explains how water gets to the top of very tall trees and keeps them alive.
Footings often sit in wet soil, even when perimeter drains have been installed. The perforated pipe that drains water away from the base of a foundation wall is higher than the bottom of the footing. As a result, even when the drains are working perfectly, the base of the footings could be soaking wet.
A capillary break is typically applied to the outside of the foundation wall in the form of damp-proofing or water-proofing (either in membrane or liquid-applied forms). Underneath the basement slab, assuming there is one, a layer of crushed rock and a polyethylene vapor barrier provides a capillary break. And at the top of the foundation wall, sill seal prevents moisture from wicking from the top of the foundation wall into the mud sill.
That leaves the intersection of the footing and the foundation wall vulnerable. Without some kind of barrier there, moisture certainly will have no trouble making it up a few feet and into the basement.
In a paper for the Building Science Corporation, Lstiburek says that capillary breaks over footings were not common at one time—and they were not all that important when basement walls were uninsulated and unfinished on the inside. Water that migrated upward could dry to the inside. The lack of a capillary break did no harm. “For finished basements,” he adds, “[capillary breaks] are an important control mechanism. Without them, moisture constantly migrates through the foundation, and then into the interior insulation layer and interior gypsum board lining.” That has got trouble written all over it.
For these reasons, Lstiburek and others recommend a capillary break be installed over the top of the footing to control what the British call “rising damp,” (also the title of a British sitcom), as shown in the illustration below.
Worries about a weakened foundation
Some builders and building inspectors are apparently concerned that applying a moisture barrier between the footing and the foundation wall might weaken that connection, and lower the shear resistance between wall and footing—that is, make it easier for the foundation to be pushed off the footing.
In a Q&A Spotlight devoted to this topic, Peter Yost recalled a building inspector making this argument on a Building America project. At the time, Yost was working at Lstiburek’s company, the Building Science Corporation, and says Lstiburek was adamant that this concern was unfounded. The only time when there was reason to be concerned about shear forces across this connection was where high winds and earthquakes were likely, or where expansive soils are found.
Otherwise, it’s not a problem.
Footings are often connected to foundation walls with vertical reinforcement, such as steel rebar, or by forming a keyway in the top of the footing while the concrete is still wet. When the walls are poured, concrete fills the depression and the interlocking connection resists inward pressure.
Yost cites this reference from the Department of Housing and Urban Development. It says, in part, “Friction is sufficient for low, unbalanced backfill heights, whereas the basement slab can resist slippage for higher backfill heights on basement walls.” While there are many locations across the country where no physical connection might be required, the document goes on to say, it’s always a good idea to provide one.
So, the best practice is to use either rebar or a key at this connection, and provide a capillary break between footing and wall.
What products to use
A capillary break can either be a membrane, such as Vycor or EPDM, or a coating that’s applied as a liquid and allowed to dry. Choosing between the two approaches may depend on whether or not vertical rebar has been placed in the footing. In cases where there is no rebar, a membrane capillary break is relatively simple to install. When there is rebar, a liquid-applied barrier is probably less of a hassle.
There are many such products on the market. In this article from Fine Homebuilding magazine, Mike Guertin discusses his product of choice from Protecto Wrap called LWM200. Although intended for use as a waterproofing membrane in window and door openings in concrete-block walls, Guertin found it worked well in this application. He and a coworker treated 130 ft. of 18-in.-wide footing in about an hour.
There are many more liquid-applied products from which to choose. In this article posted at GBA in 2016, Martin Holladay lists seven possibilities, including LWM200. Prices vary considerably, and so do cure times. Further, some of the products are to be applied only when the concrete has reached 80% of its design strength, which takes 28 days, according to one of the manufacturers. Waiting a couple of weeks for the coating to cure, or waiting a month for the concrete to cure, is not very practical, so some of these instructions are no doubt ignored.
There also are products that don’t have these drawbacks, as Holladay discovered. Some of them, such as the LWM200 that Guertin used, can be applied to fresh concrete and cure in 24 hr. or less. Several products fall into this category.
Adding a capillary break at the top of the footing is neither expensive nor time consuming, and there are lots of products that will do the job. If this is not part of your routine now, it should be.
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.