©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
This podcast series is excerpted from a two-day class called "Building Science Fundamentals" taught by Dr. Joe Lstiburek and Dr. John Straube of Building Science Corporation.
For information on attending a live class, go to BuildingScienceSeminars.com 
In our last episode, Dr. Joe Lstiburek compared air barriers and vapor barriers , and explained how airtightness helps keep homes free of mold and rot. This week Dr. Joe explains how water and salt move through masonry by osmosis, often causing serious damage to foundations. He also offers some solutions to this common problem.
Osmosis isn’t a problem everywhere
In new construction, it’s real easy: you coat the top of the footing, you’ve got your stone (capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules.) break, you’ve got your dampproofing. You don’t have to worry about salt, and you don’t have to worry about capillarity — life is good. It’s kind of hard to retrofit this. It’s a wonderful way to do it in new construction, but it’s tough if you’ve got a 100-, 200-, or 300-year-old structure to deal with.
What’s so bad about salt and water?
The physics of the osmosis forces works like this: water takes the salt in solution to a surface, the water evaporates, and the salt is left behind. And as more water evaporates, more salt accumulates, so the concentration of salt goes up. As the concentration of salt goes up, water rushes to the concentration of salt in order to dilute it — because one of the rules of physics is that nature doesn’t like these kinds of concentrations. The action of the water rushing to the surface actually creates hydrostatic forces. This pressure from the water rushing through the pore system causes the material to flake apart, and the explosive flaking is referred to as spalling. Let me summarize this: salt is very bad; water is very bad; salt and water together — whoa!
Osmosis is powerful stuff
The pressures are extraordinary. With diffusion, pressures are 3 to 5 psi — it’s nothing. Water vapor never pushed nothing off of nothing. Capillary pressures are fairly impressive — 300 to 500 psi. It moves water to the top of a 400-foot tree. That’s a pretty impressive force. But it isn’t anywhere close to the league of osmosis pressures, which are 3000 to 5000 psi. The compressive strength of even good concrete is 2000 to 3000 psi — salt and water will beat concrete every time. Osmosis beats capillarity which beats diffusion. Wow. Bridges fall down, life comes to an end, when you have salt and water.
Sacrificial mortars are one solution
Well, old-timers figured stuff out. What these folks noticed was that the mortar was eaten away much faster than the masonry, and certain mortars were eaten away much faster than others. The pore structure of the mortar was very critical to this. And someone said, "Aha! Maybe if I get the pore structure just right, all of the salt will end up in the mortar instead of the brick. And the mortar can sacrifice itself to protect the integrity of the brick." That's when we figured out that softer, weaker mortars are actually the ideal complement to clay brick that's been fired at a specific temperature. And the solution would be to re-point the mortar as it was eaten away. You never want to have a mortar that's stronger than the brick, because then the brick sacrifices itself to protect the mortar.
That's why historic preservationists — the old ones that know stuff because they've been around a long time — go to an enormous amount of trouble in old buildings to match the mortar chemistry precisely. The general rule is: if you don't know what's going on, don't mess with the building. Or if something's been around for two or three hundred years, don't mess with the strategy. If you come up with the right mix, all of the deterioration happens in the joints, and you simply re-point them on a 15 or 20 year basis.
Parging protects the entire surface
Well, why not just coat the whole thing with a sacrificial layer? And instead of doing this on a 10-year basis, why not extend this to a 30- or 40-year basis? The way you think of this sacrificial layer is as a sort of lime-based poultice that sucks the salt poison out of the assembly. So how do you know when you have to replace it? Well, when it falls off. It's the building telling you it's time to put on another sacrificial layer.