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

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.

Asked by Eric Stear
Posted Jul 31, 2014 1:52 PM ET


10 Answers

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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.

Answered by Dana Dorsett
Posted Jul 31, 2014 3:10 PM ET


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

Answered by Malcolm Taylor
Posted Jul 31, 2014 11:15 PM ET


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?

Answered by Eric Stear
Posted Aug 1, 2014 9:08 AM ET


"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:

http://www.mnshi.umn.edu/kb/scale/basement.html (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.

Answered by Dana Dorsett
Posted Aug 1, 2014 11:13 AM ET


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.

Answered by Malcolm Taylor
Posted Aug 1, 2014 10:26 PM ET
Edited Aug 1, 2014 10:27 PM ET.


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.

Answered by Martin Holladay, GBA Advisor
Posted Aug 2, 2014 5:57 AM ET


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.

Answered by Roger Anthony
Posted Aug 2, 2014 3:31 PM ET


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."

Answered by Martin Holladay, GBA Advisor
Posted Aug 3, 2014 6:51 AM ET
Edited Aug 3, 2014 6:52 AM ET.


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.


Answered by William Geary
Posted Aug 7, 2014 3:27 PM ET


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.

Answered by Roger Anthony
Posted Aug 8, 2014 7:16 AM ET

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