Negative-side waterproofing (NSW) is a tough topic that I have frankly been dancing around for quite some time. Manufacturer claims and homeowner anecdotes of successful interior waterproof solutions for basement walls and slabs did not completely add up. But I did not think that I understood the topic or the physics well enough to challenge the claims or explain my skepticism.
But I finally got the adult supervision that I needed, mostly from (2nd edition, 2016) by Justin Henshell.
Key takeaways from the Henshell manual
- Reduce first. Always start by assessing and identifying surface water problems and by implementing management approaches that ease or eliminate the load you are trying to address from the interior of existing below-grade structures.
- “Membrane waterproofing is unsuitable for negative-side waterproofing [emphasis added] because the external water pressure can easily disbond the membrane from the wall.”
- There are three generic types of negative-side waterproofing but the most common today are the crystalline coating materials such as Koster NB1, Tremco PQ200, and Xypex. (See Image #2, below.)
- The standardized test used for NSW is the U.S. Army Corps of Engineers CRD C48-73, Permeability of Concrete. Six-inch diameter, six-inch long cylinders are coated to ½-inch thickness and subjected to a hydrostatic pressure of 200 psi (about 460 feet of head).
- Crystalline coatings penetrate the concrete and build crystal lattices within the concrete such that they can resist water pressure from the opposite side.
- “Crystalline coatings can close hairline cracks – i.e. cracks up to .012 inches wide. Moisture reactivation of the sealing process gives the coatings the ability to self-seal, according to manufacturers. But no negative-side waterproofing system can bridge dynamic cracks and reseal ruptures [emphasis added].”
- “Although it is effective as a waterproofing coating, the higher vapor permeability of negative-side waterproofing is a drawback in underground structures with occupancies and materials sensitive to high humidity [emphasis added].”
- Henshell cites and reprints a useful waterproofing selection procedure from (see Image #3).
Key takeaways from other sources
A best-practice primary waterproofing solution NOT relying on NSW
Hammer and Hand (a builder with headquarters in Portland, Oregon) generously offers and thoroughly details their existing basement approach (See also Image #6.)
“Curtain wall” injection
There is an approach you can take for primary waterproofing from the interior of existing basements or other below-grade structures: chemical injection or “curtain wall” injection.
Injection holes or ports are drilled into an interstitial space or right through to the exterior of foundation concrete, typically spaced 18 to 24 inches on center. A slurry about the consistency of water is then injected so that it can set up and form a continuous water barrier. There are at least five types of injection materials:
- Bentonite injection grout
- Sodium bentonite clay
- Acrylate polymers
Perhaps most famously, Koster’s injection system was used on two different below-grade walls in the Empire State Building, one a 36-wythe brick wall (see Image #7) and the other concrete block (see Image #8).
Henshell cites as the reference that guided these systems. As you can imagine, the injection approach is not inexpensive, but when you have little to no tolerance for moisture intrusion in a below-grade space, it’s good to know that the space can be waterproofed.
- Topical coatings are subject to blistering and failure if you try to use them to repair basement water leaks.
- Penetrating crystalline systems must pass the rigorous USACE CRD C48-73 waterproofing test.
- If you bury your waterproofing treatment behind finished floor and walls such that they cannot be inspected for new cracks, there is no guarantee that your waterproofing system will stand the test of time.
- True interior waterproofing systems exist — for example, the Hammer & Hand approach and the injection approach described above — but the systems are invasive and expensive.
In addition to acting as GBA’s technical director, Peter Yost is the Vice President for Technical Services at in Brattleboro, Vermont. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years. An experienced trainer and consultant, he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? . You can also to get a free report on avoiding toxic insulation, as well as regular posts from Peter.