I'm having difficulty sealing the perimeter sealant joint between a window and a masonry wall. The front corner of the window is recessed from the back corner of the masonry at the opening. It is virtually impossible to use a backer rod in this resulting gap. Do you know of any way to seal this joint?
There are a couple of methods of installing sealant joints in this situation. One approach is to use a triangular backer rod, either cut from a standard backer rod or foam sheet, or specially extruded. The edges of the triangular backer rod are attached to the masonry on one side and the window on the other. However, triangular sealant backers are hard to find and expensive to make. In some cases, they are difficult to secure into place because the edges are weak and the backer can be pushed into the joint easily when tooling the sealant. I recently tried another approach, which worked well and was relatively inexpensive. Instead of using a triangular sealant backer, I used a strip of semi-rigid polyethylene. This polyethylene strip can be spot-adhered using small balls of butyl tape at regular intervals along the length. Make sure the butyl does not contaminate the bond surface. The strip of polyethylene should be rigid enough not to be pushed into the opening during tooling, but should be flexible enough to buckle when the joint compresses. To ensure that the joint is uniform in thickness, it is important to tool the surface of the joint with a straight tool, keeping the tool parallel to the sealant backer. An example of this method is shown above.