When it comes to air-sealing, one of the areas that we rarely see done well is the joint between the foundation and the framing. Most houses in America get foam sill seal laid on top of the foundation. This plastic foam product primarily works as a capillary break between the concrete and the framing. As the weight of the framing crushes the foam slightly, it also (sort of) works as an air-seal, but it is not a perfect seal by any measure, because the top of the concrete is never perfectly smooth.
For more about sill sealer, see my video "Framing Tip: Super Sill Sealer" (above) in which I discuss the reasons for air-sealing at the framing stage, and show a better option than plastic foam.
In addition to the sill sealer, we take several approaches to improving air-sealing at the critical juncture between the framing and the foundation.
Good Interior Options
If you’re working from the inside, the easiest and least-expensive method is to simply lay a fat bead of caulk in the gap created by the foam sill seal between the foundation and the framing. Anyone can do this on any house under construction. I recommend using an exterior silicone formulation for concrete or a high-quality polyurethane sealant. Both will stick well to the wood and the concrete. Acoustical sealant is a better, albeit slightly more expensive, option. Caulk often works best at a slab-wall transition.
An even better, but also more expensive, option is to use a high-quality air-sealing tape. Siga makes Wigluv tape, which sticks well to concrete. When you are applying it to the protected inside, you don’t necessarily need to use primer.
Better Exterior Options
While sealing from the inside will work, sealing from the outside is the better way to go. This way, you are blocking the air before it has a chance to find another path to the inside through the wall cavity. But it is also a little more expensive to do it from the outside.
I’ve found two tape options that work well here: One is Cosella-Dorken’s Multi-Band. This tape in the Delta line comes in two widths—60mm (2.36 inches) and 100mm (3.94 inches)—and I have used it frequently with many Delta waterproofing products. Outside, where we have more exposure to the elements, and therefore more intense changes in temperature and humidity, we need to use a primer. With Multi-Band, I use the Delta HF primer. I run a couple of courses of tape to protect the foundation so I’m only coating the first 2 inches of concrete with primer, and I also run the primer up onto the framing. The primer is an adhesive, so you get sticky on sticky when you apply the Multi-Band tape, and it will stick tenaciously.
Another option for sealing from the outside is Siga’s Wigluv-black. This is a new product that Siga claims will stick even better than its Wigluv tape. I have found the bond to be impressive, but I still want to use a primer outside. The wider Wigluv-black tapes (6-, 9- and 12-inch widths) have a split back so you can peel off half to stick it to the concrete and then roll it up onto the framing.
While using a primer makes for a strong bond on both the concrete and the framing, there are always issues with bonding to concrete. Form oils, wax, concrete additives, and especially moisture can affect the bond, and I always feel reserved about the long-term viability of the adhesive bond. With this in mind, I have begun experimenting with some of the newer fluid-applied air barriers. Recently, we used Prosoco’s R-Guard Joint & Seam Filler, which is a moisture-curing compound, so it bonds directly to damp or dry surfaces (with no primer needed) and cures under a variety of weather conditions and even on green concrete. Low temperatures and dry conditions will slow down the drying time, and high temperatures and high humidity or wet conditions will accelerate curing. But the bond will only improve over time, which appeals to me.
For more on using R-Guard, watch the video "Sealing Framing to Foundation" (above), in which I also poke a little fun at the April 2017 issue of JLC where this article first appeared.
Another fluid-applied option is PolyWall’s Blue Barrier Joint Filler 2200 (see photo at the top of this page). This is a bit thicker than R-Guard and fills gaps up to 3/4 inch.
The fluid-applied options form a “rubber barrier” at the base of the home, doing an excellent job to stop air flow. They also protect the vulnerable bottom edge of the wood sheathing against splash-back and give you something to shingle your weather barrier over later.