As he works out the details for a new house, Jason Huffine has become concerned about the potential for trouble in applying a capillary break between the concrete footing and the foundation walls.
A capillary break would prevent the foundation—in this case steel-reinforced, concrete-filled block —from wicking up moisture. Huffine’s general contractor isn’t familiar with this technique, and the two of them wonder whether the capillary break would weaken the connection between foundation and footing.
“Though I’m as green around the gills as one could be and learning as I go, that sounds like a good question to me,” Huffine writes in a Q&A post.
Huffine might not be considering this approach at all except that the footings will be about 5 feet lower than a road bed at the front of the house. While stormwater drainage appears to be adequate, adding a capillary break might provide some additional insurance and help protect the foundation and footing and prevent “any potential rising damp issue.”
Are Huffine’s concerns well grounded? That’s the topic for this Q&A Spotlight.
A cold joint has only minimal adhesion
Unless the footings and foundation walls are poured at the same time, creating a monolithic concrete structure, there will be a cold joint between the two, and in that case there is only minimal adhesion, says Malcolm Taylor.
To connect poured footings and foundation walls, a shallow groove or keyway can be formed in the top of the footing, Taylor says. When the foundation walls’ concrete is poured, it fills the groove to provide resistance to any lateral pressure. Adding rebar is also a common way to reinforce the joint and could be done with a block wall.
But the use of a capillary break in this location still hasn’t reached the construction mainstream.
“Right now capillary breaks are confined to a small group of high performance builders,” Taylor says. “That isn’t to disparage them in any way, but it means they are a niche technique most of us aren’t familiar with.”
More questions than answers
To Jon R, there are plenty of questions about capillary breaks but not a lot of reliable answers.
He suggests an experiment in which a wet piece of paper towel is separated from a dry sheet by a piece of plastic with a tiny hole in it. Press the layers together for a few minutes, Jon R says, and the dry paper towel becomes quite wet.
“Not sure what to make of it,” he says. “Thick sheets of EPDM (vs. thinner, more delicate alternatives) are better for capillary breaks? Or concrete admixtures? That full gravel surround (under slab, under footings, against walls) is best for dry concrete?”
“I see almost no data pertaining to good, modern concrete and the various techniques,” says Jon R.
Although wet concrete isn’t a structural issue, Huffine says, it can introduce moisture into the house. “If the latter occurs, the fix is expensive and involves someone drilling into your foundation and injecting a sealant that will be absorbed by the concrete and set up to retard the movement,” he writes. “I figure if I’m already aware that moisture will need addressing, might as well get those low-hanging improvements. A few hundred on a barrier now might save a headache’s worth of problems later.”
Membrane or liquid-applied product?
GBA Editor Brian Pontolilo directs Huffine to an article by Mike Guertin on capillary breaks. He notes that a capillary break can come in the form of a membrane or a liquid-applied product, such as the ProtectoWrap LWM200 that Mike Guertin used on the FHB ProHome.
As Guertin explains, membranes can be difficult to install if there is rebar sticking out of the footing, making the liquid-applied product a little easier to use.
Another option would be to use asphalt damp-proofing, adds Taylor. “If asphalt damp-proofing is going to be used to protect the concrete from water intrusion from the outside,” he says, “and can be applied to uncured concrete and backfilled quickly, it might be a good choice to use as a capillary break, too.”
Placing the footing drain properly
Where the footing drain is located is another important detail, says Jason Cole. Most builders place it so the drain is below the top of the footing, giving water an easier escape route than through the footing.
“However,” he adds, “some contractors take shortcuts and put the drain above the top of the footing to save time and money, which just means that the lowest level that the water can sink to is just barely above the cold joint between the CMU and the footing.” To get the drain below the top of the footing would require extra work—so they just skip it.
Even with the drain below the footing, Taylor says, the area is still vulnerable to moisture because the bottom of the basement slab rests directly on the footing. A better way is to separate the two with several inches of fill.
“This both provides a path for moisture to move underneath the slab, and makes the slab less likely to crack at the edges due to differential settlement of fill,” he says.
Getting the building inspector on board
One hangup is Huffine’s building inspector, who has yet to accept nor reject the idea of adding a capillary break.
“He has expressed concern that putting another layer between the foundation and the footing would affect the frictional forces,” Huffine says. “I’ve sent information (like the Fine Homebuilding article) on it. But so far, no one feels convinced it’s the route to go.”
And it’s not just the building inspector. Huffine says he has spoken with representatives for ProtectoWrap and was told that their product is not normally used in this application (the ProtectoWrap LWM200 that Guertin described using is more often applied as a waterproof coating in window and door openings).
“It took quite a bit of conversation for them to even understand the application,” Huffine says. “Although in the end, they said it should work, they definitely acted as though this is not the norm and not in their cross-hairs. Maybe this just hasn’t caught on?”
Maybe having a capillary break isn’t even necessary, Taylor suggests. The “rising damp” issue that Huffine originally mentioned is a phenomenon of British homes, which are continuous concrete or masonry structures extending from footing to roof.
“The situation in North American stick-frame construction seems to me to be a lot different,” Taylor says. “Can someone link to the original research that showed this is a significant problem here? I think it came from [Building Science Corporation] research, but all I can find is their concern that the salts in soil may cause masonry to deteriorate prematurely.”
Our expert’s opinion
Here’s how GBA Technical Director Peter Yost views the issue:
Some great questions have been raised in this Q&A:
First, does adding a capillary break weaken the footing/foundation wall connection? When I was at Building Science Corporation, this issue came up with a building inspector on a Building America project. Joe Lstiburek was adamant: the only circumstances where we really need to worry about resistance to perpendicular shear across this connection is in really high seismic and wind areas, and with expansive soils.
The Residential Structural Design Guide: A State-of-the-Art Engineering Resource for Light-Frame Homes, Apartments, and Townhouses—2nd Edition bears this out [7.4.2 Concrete or Masonry Foundation Wall to Footing, pg. 7-24]:
“Footing connections, if any, are intended to transfer shear loads from the wall to the footing below. The shear loads generally are produced by lateral soil pressure acting on the foundation (see chapter 3) or loads in shear walls from wind or seismic events.
“Footing-to-wall connections for residential construction often are constructed in one of the following three ways (refer to figure 7.5 for illustrations of the connections):
- No vertical reinforcement or key
- Key only
- Dowel only
“Generally, no special connection is needed in non-hurricane-prone or low- to moderate-hazard seismic areas. Friction is sufficient for low, unbalanced backfill heights, whereas the basement slab can resist slippage for higher backfill heights on basement walls. The basement slab abuts the basement wall near its base and thus provides lateral support. If gravel footings are used, the unbalanced backfill height must be sufficiently low (that is, less than 3 feet), or means must be provided to prevent the foundation wall from slipping sideways from lateral soil loads. Again, a basement slab can provide the needed support. Alternatively, a footing key or doweled connection can be used. In spite of the many probable locations across the country where no physical connection might be required between the foundation wall and the footing, the best practice remains to attach these two elements. Without the physical connection, this critical part of the load path arguably is not continuous. Often, problems arise that could have been reduced or eliminated if a physical connection had existed between the foundation wall and footing. This is particularly true when any extreme loading occurs, such as high winds or even a moderate earthquake.”
A second question is whether we really need to worry about wicking between the footing and the foundation wall. Yes, capillarity or wicking occurs in all porous materials, until the pores are just too small or disconnected, such as 7,000 psi concrete. Wood, masonry, concrete, and gypsum wallboard all can wick water.
We need to worry about that more with new construction because we have shifted the hygrothermal balance in basement walls. Again, from Joe Lstiburek, Building Science Digest 103, pg 4:
“Groundwater exists in more than the free-flowing liquid state. Water from wet soil can also wick (capillary flow) and move by diffusion through the soil and the materials used to make basements. Therefore, the basement wall should be damp-proofed and vapor-proofed on the exterior and a capillary break installed over the top of the footing to control “rising damp.” Damp-proofing and vapor-proofing in these locations is often provided by a fluid applied coating of bitumen. In the past, capillary breaks over footings were not common. They were not needed when basement perimeter walls were uninsulated and unfinished on the interior because these conditions permitted inward drying of the migrating moisture. For finished basements they 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.”
Third, how much water comes out of freshly-cast concrete (moisture of construction)? A lot, thousands of pounds, as it turns out. See this resource from the National Ready Mix Concrete Association. Is this one-time load something to worry about? It certainly can be. I was involved in a recent project where, from breaking ground to moving in (right around Thanksgiving in Climate Zone 4A), was only 11 weeks. I calculated the moisture of construction for the first six months of this building (curing concrete, drying wood, plaster, and paint) in excess of 16,000 pounds of water. Definitely a moisture load that needs to be actively managed.
Finally, I decided to check in with building scientist Pat Huelman of the University of Minnesota, who has done a ton of work for our industry related to foundations and basements. On the particular issue of the need for a capillary break between footing and foundation wall, he had this to say:
“In general, I have always used the Building Foundations Design Handbook, 1988 (and subsequent Builder’s Foundations Handbooks) as the source for this requirement. Bear in mind at that time there was considerable push back from the masonry (CMU) folks and they didn’t include a capillary break on those details. In later details that changed as footing pins replaced the keyway. And a spray on material was allowed.
“…When there is an unfinished foundation wall of sufficient height, the water will evaporate to the indoors (or outdoors, if the wall sticks out of the ground) before reaching the sill. However, for most walls today the exterior exposure is limited and may even be covered with a coating, insulation, or both. And when the interior is covered with a low-permeability material the water will continue upwards. Depending on the foundation material and lack of drying potential, the capillary water could go tens of feet. Of course, block is a tricky one as vapor can move upward readily by diffusion and convection.”
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.