What Do You Use to Seal the Bricks on a Fireplace Chimney?

Water Damage to Brick

• Water damage happens when bricks absorb moisture and then swell and contract. Even though bricks appear to be solid, they are a highly porous. A variety of damage is caused by water, including efflorescence (white staining), spalling (bricks breaking due to years of freeze/thaw cycles) and erosion (bricks simply deteriorating).

Sealants

• Because brick is porous and readily absorbs water, the last thing it needs is to be sealed. To function best, bricks must be able to breathe. Using a sealant on brick traps water inside the bricks and creates a risk of additional damage. You can easily recognize a sealant by the chemicals on the label. Acrylic, stearate, paraffin, urethane and silicone resin are common ingredients in sealant products. Paint is also a sealant and should not be applied to a chimney.

Repellants

• Water repellants are recommended for sealing chimneys by construction professionals. These products are made using silane or siloxane; these are not the same as silicone, which is a sealant. The molecular structure of silane and siloxane is very small, allowing these chemicals to penetrate up to 3/8 inch into the brickwork and bond with the bricks' silica. Repellants allow water to escape from the brick but do not allow it to return.

Application Tips

• Plan to apply the water repellant when the weather is dry and warm enough to discourage dew formation. It will cure best if allowed to dry for five days. Clean the chimney thoroughly before applying a water repellant. This allows the repellant to penetrate the bricks instead of the dirt. Check all mortar joints for damage; these should be repaired or replaced if they are in poor shape. Application with a sprayer is recommended if the brick surface is irregular due to wear. Otherwise, you can apply with a brush.