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Community and Q&A

Is a capillary break between the footing and foundation wall really necessary?

Eric Stear | Posted in Green Building Techniques on

We are meeting a lot of resistance from local concrete contractors & structural engineers when we tell them that we want a capillary break between the footing and the foundation wall. Are there any documented cases of this capillary action being a source of moisture/ mold / finish material problems? Are there any scientific studies that compare the moisture levels in foundations walls with and without a capillary break? The capillary break makes sense to me intuitively, but nobody wants to pay for it or change their ways unless we can show them proof that it is necessary.

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  1. User avatar
    Dana Dorsett | | #1

    The proof is everywhere, appearing most commonly as efflorescence on both near the bottom of the interior side & near grade on the above-grade exterior sides of the foundation. This isn't a new problem, and not rocket science (even the Romans had well evolved methods for managing it without capillary breaks.) In the UK they call it "rising damp". Do a web images search on "rising damp" and you'll find literally thousands of not-so-pretty pictures.

    With well-drained footings the problem may not always show up as mold, but it is measurable as higher interior air humidity.

  2. Malcolm Taylor | | #2

    If you are more comfortable not including a break, you can stop the capillary action with concrete admixtures like this one:

  3. Eric Stear | | #3

    Dana, I appreciate your reply. However I'm afraid if I show your evidence to the contractors in my area they are going to tell me this is only a problem in the UK (where it is considerably wetter than it is in the Midwest, correct?) A "rising damp" image search returns a lot if photos of brick masonry foundations in old buildings that probably have no waterproofing on the exterior face of the wall, and maybe no footing drainage system. Does anybody know of a well documented case of "rising damp" damage on a house in the US with cast-in-place footings and foundation walls, waterproofing on the exterior face, a good footing drainage system, and a proper under slab vapor barrier? I also acknowledge your comment that the problem may show itself as higher interior humidity levels. Has anybody ever done a study that supports this assumption?

  4. User avatar
    Dana Dorsett | | #4

    "I'm afraid if I show your evidence to the contractors in my area they are going to tell me this is only a problem in the UK (where it is considerably wetter than it is in the Midwest, correct?)"

    Annual rainfall in Cardiff Wales is about 44".

    Annual rainfall in Manchester England is 32".

    Annual rainfall in Rome Italy is about 33"

    Annual rainfall in Peoria IL s 36".

    Annual rainfall in Chicago is 39"

    Annual rainfall in Duluth MN is 32".

    There is no special dispensation that gives the midwest the ability to defy the laws of physics regarding the movement of moisture. The midwest even has higher humidity air to deal with than most of Europe.

    There is no mystery about why basement air humidity is higher than above grade rooms. The proof is in what happens when you DO put down ground vapor retarders and capillary breaks. I can't point to some specific study, but this stuff has been fairly well understood for centuries (millenia?), but perhaps better quantified in the pass century than prior. If you want references to scientific authority (rather than a formal controlled peer-reviewed study with the full data & write-up), try the Oak Ridge National Laboratory prescriptives:

    Or figure 2-3 and 2-4 from the EPA's moisture control handbook:

    Or the Minnesota Sustainable Housing Initiative's prescriptives: (between 1/2-2/3 of the way down, under the "Durability" section.)

    See the section on moisture transport mechanisms beginning on p13, and the comments on p23 of this document:

    Putting the capillary break under/around the footings is pretty straightforward prior to pouring, and arguably better that putting it between the footing & wall.

  5. Malcolm Taylor | | #5

    Dana, Not to downplay your point about the importance of some form of capillary break, but I'm not sure how useful annual precipitation levels are in determining the effect of climate on buildings.
    What the statistics don't tell you is how that precipitation occurs. The cities you cited are all in the same range as Victoria B.C. and Seattle, but as a past resident of the PNW you will no doubt remember the climate where, even though there may be no measurable precipitation, there are weeks of damp overcast or foggy conditions where moss grows on metal and a cloth left outside doesn't dry. The ground is saturated from October until May and foundations subject to those conditions are, as you say, prone to dampness. A city like Montreal, with a higher annual precip. rate, with much of its precipitation coming in the form of snow and the rest as infrequent but intense deluges, doesn't experience anything like the same problems and I doubt buildings there would see much benefit from capillary breaks.

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Here's my take on capillary breaks between the footing and the foundation wall:

    1. Rising damp is one factor affecting the rate of moisture transfer from the soil to the basement. It may be hard to quantify, but it is real.

    2. Installing the capillary break at the time of construction is incredibly cheap and easy. Retrofitting a capillary break later is either incredibly expensive or close to impossible. So including a capillary break during new construction is what we in the building science community call a no-brainer.

  7. Roger Anthony | | #7

    The type and frequency of precipitation is irrelevant, what counts is, is the soil damp/wet can the walls/foundations absorb water. If they can then water will rise up the wall by capillary attraction to a height of four feet above the water source. Having a capillary break more than nine inches above ground level, will ensure the wall above stays dry.

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Even more than four feet.

    Here's what I wrote about capillarity in a ruminative piece published in the January 2004 issue of Energy Design Update.

    "Historians report that the first person to describe capillarity was Leonardo da Vinci, who recorded his observations of capillary action in 1409. Many centuries later, in 1901, Albert Einstein found the phenomenon of capillarity intriguing enough to make it the topic of his first published scientific paper, Folgerungen aus den Capillaritätserscheinungen. The paper examines capillarity from the perspective of the laws of thermodynamics and Einstein’s theory of molecular forces. Some Einstein biographers consider it likely that Einstein’s brilliant Serbian girlfriend, Mileva Maric, provided important contributions to the paper.

    "Capillary rise occurs when the forces of adhesion (the attraction between the molecules of a liquid and those of a solid) are stronger than the forces of cohesion (the attraction of the molecules of the liquid for each other). In some circumstances, equilibrium between these two forces is not achieved until the liquid has risen many feet.

    "Contrary to popular belief, capillarity is not responsible for the rise of maple sap from underground roots into sap buckets. In many houses, however, it is responsible for the rise of water from damp soil beneath concrete footings into foundation walls, contributing up to 15 gallons of water a day to a house’s interior moisture load. Capillarity can also cause water to rise up behind courses of lap siding.

    "Capillary action in soils and masonry—called “rising damp” in Britain—is a function of pore size. Small-particle clay soils allow water to rise as much as 20 feet by capillarity, while crushed stone permits water to rise only a few inches. That’s why a 4-inch layer of crushed stone under a concrete slab is an effective capillary break."

  9. William Geary | | #9

    Here you go:

    See page 11, Figures 9 and 10:

    Ask them why they would waterproof the outside of a foundation wall while "leaving the door open" to moisture being sucked up through the footer? As Joe says "concrete sucks" (moisture). Then be quiet while they try to explain their "logic." Hint: "we've always done it this way" is not logic.


  10. Roger Anthony | | #10

    From my experience studying solid concrete walls – the problem is due to the amount of air entrained into the concrete mix. Where you use good quality concrete and a vibrator the air can be shaken out of the mix and water may not rise at all.
    With block and brick walls, the mortar is the problem, with water rising through the air bubbles added to the mortar when mixing.
    From a building perspective I have never know water to rise above four feet in a traditional wall, unless the wall has a waterproof render, probably meaning the rise is to an extent subject to the speed of evaporation from both surfaces.
    Interesting point, a hundred years ago and more, before the onset of modern water proof materials, it was normal for expensive homes to be built with the living area more than four feet above the ground, to avoid rising damp, others used two of three rows of staggered slates to provide a capillary break.

  11. Jimmy Nguyen | | #11

    We just finished pouring the concrete for our footers and basement walls. Before the walls were poured, we used Super Thoroseal to serve as a capillary break. Our footers were 18" wide and totaled 204 linear feet. The basement walls were 10" wide so we didn't need to coat the entire 18" footer with the thoroseal, but we did anyways because it wasn't the much extra work. Using 2 - 35 pound buckets of the product, we were able to do 2 coats with some leftover to do a third coat over the keyway. Each 35 pound bucket cost just $25.00. On another GAB forum about capillary breaks, a product that was mentioned for use as a capillary break was Tremproof 250GC. I called around to check on the price of that product and it was around $250 for a 5 gallon bucket. I probably would have needed 2 buckets to cover my footers and so the cost would have been expensive. Also, I kind of liked the fact that the thoroseal is cement-based and might be tougher than some of the thinner waterproofing paints, especially considering the amount of stomping and scraping it will have to withstand when the workers form the basement walls. Most importantly, I called the makers of the thoroseal and they said I could apply it after 24 hours of the footers being poured. Being cautious, I waited two days to allow the concrete to cure a little bit more before I applied it. After the first coat, you have to wait 12 hours to apply the second coat. The application is done with a thick block brush. The product is supposed to be applied with a pancake batter consistency so it's not exactly like you are painting with thin latex and it will take some more time to brush on. Wetting the footer will help.

    For homeowners actively involved in the building process, I think this is a good product because if your foundation contractor gives you a hard time about capillary breaks, you can just go to your local Big Box store and buy this stuff and apply it yourself. Just tell your foundation contractor to hold off pouring the basement walls for a few days, while you put this stuff on. Maybe you can do it over a weekend.

    While I had the footer all to myself for a couple of days, I decided to apply Earth Shield Type 20 waterstop to prevent water from running into my basement through the footer/wall joint. I understand that the spray-on exterior basement wall waterproofing that will be applied is going to down onto my footer to cover that joint, but I wanted extra protection. I have lived in a house that had a basement that flooded and most of the water came through that footer/wall joint. The waterstop product came in 16'-8" strips and cost $21 each. So to cover 204 linear feet, it cost me almost $300. I applied the waterstop on the side of the keyway closest to the exterior and not directly in the keyway like the website shows because I didn't want to have to go around the rebar that was placed right in the middle of the keyway. I think it will still do its job. I didn't use the primer that the website recommends because I talked to the company’s technical support person and he said the clay-based waterstop strips are already sticky enough and if you have a relatively clean footer, it will adhere to it with some foot pressure. And this was true. As an alternative to the primer, you could use the 3M adhesive spray that come in cans. It might save you some money.

    I have attached some pics for your viewing pleasure. Notice the paper backing still on the waterstop strip. You are supposed to remove it before the walls are poured. I kept it on because there was a slight drizzle the morning of the pour and I didn't want the clay-based waterstop to expand. That's another thing, do not apply this stuff when it is raining or it will compromise its effectiveness.

    I'll be documenting my build on my website: Check me out.

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