Caulk Caulking a window on a brick house

Published on May 30th, 2016 | by Henri

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Caulking windows in a brick house

Q.  Which type of caulk should I use on vinyl windows that are surrounded by brick? Different people have suggested silicone, poly or masonry. The interior bricks are getting wet below the window sill when it rains. The caulking outside is not in good shape and I’m assuming that’s why I’m getting the water inside. Thanks.

A.  I am assuming that the exterior of your house is brick and that the windows are set in. It also sounds as if you also have bricks inside, which tells me that your house’s exterior walls are made of solid masonry with no insulation in them. If this is incorrect, please let me know and give me more details and send me indoor and outdoor photos of the problem areas if possible.

Considering all these assumptions, you may have more than one reason for the leakage on the interior bricks.

One frequent cause of leakage under window sills is the fact that the sill bricks are set too flat and/or not set deeply enough under the window to be effectively caulked. Take a good look to see if this may be the cause.

The bricks may also be porous and need to be sealed with a masonry sealer, which you can buy in the same place as the caulking compound listed below.

As to the best caulking, in my experience over 60 years in the construction field, and having tried many caulking products (silicone, latex, butyl, etc.), the best I have ever used is polyurethane. My preferred polyurethane caulking is Sikaflex-1a, which you can find in some construction supply houses, such as A.H. Harris (www.ahharris.com). A.H.Harris will ship if you do not have a store near you. I have also found Sikaflex Construction Adhesive tubes in Home Depot’s masonry supplies department, not in the caulk department. It looks like the same product under a different label.

There are other polyurethane brands, and I have used them, but my favorite is Sikaflex -1a, which I have used since the late ‘50s.

If your windows and bricks were caulked with silicone, all residue will have to be removed using denatured alcohol and a scrub brush. For a proper and lasting caulking job, the joints to be caulked need to be clean and dry.

The caulking compound must not be deeper than its width. So if the cracks to be filled are deeper than wide, you will have to insert a backer rod of the right size (buy in the same store). Backer rods have a very fragile skin, which must not be broken to be effective. Insert it gently with a wooden stick and push it in no deeper than needed, leaving a crack slightly shallower than its width. Once you have applied the caulking, use a wet finger to tool it and ensure tight adhesion to both sides of the cracks.

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