A breathable sealer is defined as a sealer that allows brick to breathe - i.e. allows for some moisture to penetrate through the surface. It is important to use a breathable sealer on brick because non-breathable sealers will peel or flake when moisture becomes present.
It is a debate in the concrete and masonry industry as to what should be put on brick. Some manufactures will push a penetrating sealer while others will sell a high gloss acrylic. Here is what you need to know about buying a breathable brick sealer.
Penetrating Sealers: Penetrating sealers are designed to penetrate into the brick and work entirely below the surface. They won't change the look or color of the brick but they will help to keep the brick protected. Under the category of penetrating sealers you have silicate based sealers and silane-siloxane sealers:
1. Silicate Sealers: Silicate based sealers, most commonly sodium or lithium in base, are designed to penetrate and chemically react to form Calcium Silicate Hydrate (CSH) within the pores. The primary purpose of a silicate is to increase the strength and density of the brick. Silicate based sealers do not bead water and they will not help against deterioration caused by water absorption. If you need to densify, consider following with an application of a silane-siloxane based sealer 5-7 days later.
When applying a silicate to brick you must proceed with caution. If a silicate is over applied, a white cyrstalline residue will be left on the surface. This crystalline residue is hard to remove if left on too long. Brick is very porous which makes it easy to over-apply a silicate so if applying a silicate to brick, do so in multiple light coats.
2. Silane-Siloxane Water Repellents:Silane-Siloxane water repellent sealers do just that - repel water. They chemically react below the surface to form a hydrophobic barrier within the pores. Surfaces sealed with a Silane-Siloxane water repellent sealer will reduce the absorption of surface water and other liquids. By reducing the absorption of water you reduce a variety of issues caused by it including cracking, spalling, pitting, staining, ice and salt damage, and the growth of mold and mildew. It is the best type of breathable sealer that can be used for sealing brick.
3. Wet Look and High Gloss Sealers: Paint is not breathable, most epoxies and urethanes aren't breathable, but, most acrylics are breathable. The problem will putting an acrylic, or any type of coating, on brick is that the brick is very porous and allows for large amounts of moisture to pass through. Coatings, which is described as something that leaves a surface film, are very sensitive to moisture. When coatings are applied to brick they will peel within 2 weeks-6 months. The only time it is okay to apply a coating to brick is if a vapor barrier was installed below the brick, and if the brick was properly prepared. Even then, the life of any coating on brick in unpredictable. If you decide to go with a coating, proceed with caution, do your research, and apply it to a test area first.