Hilda has a century-old house and is repointing the bricks. She wants to know if it is wise to seal the wall with Silicone to prevent water from getting in.
The answer is -- no.
Brick is designed to shed water, absorbing a bit, but drying out quickly as soon as the rain stops. In addition the air space behind our brick veneer walls is designed as a drainage path to let any water that does get in from cracks or poor flashing, to drain down and out. As the water drains, moisture does get into the back side of the brick and needs to dry out through the front. Even moisture sneaking through the wall from the house during winter needs to move right through the bricks. If you have a barrier on the outside of the brick, especially a solid film like an oil paint or polyurethane, it will trap water in the brick, which will freeze and push off the paint or even break off the surface of the brick. We call this "brick spalling".
So a brick wall works best when there are no cracks to let liquid water into the wall, and no sealant on the brick, allowing for the best possible drying.
Unlike oil paints, silicone and some other sealers do not create a complete vapour barrier on the wall. They actually let water vapour get through, so we call them ?permeable sealers?. They work by making the pores in the brick slippery, causing water to bead up rather than flow into the brick. But they do create more of a barrier than nothing at all -- and when faced with 100-year-old brick, most historical societies across the country say that this brick is too delicate to put anything at all on it. Just do a good job on the joints and the flashings.
If you feel you must put something on your bricks, siloxane is a form of sealant that breathes ever better than silicone and acrylic latex house paints could be acceptable. The sealant we used on the show was Planiseal S75 made by Mapei. But you may have gotten the message that I find little reason for putting anything over brick at all.