Do Chimney Sealers Work?

Two very different kinds of product

The word "sealer" covers products that behave in opposite ways. The Brick Industry Association separates them into two groups:

• Penetrating repellents soak into the brick, up to about three-eighths of an inch, and do not form a surface film. Silanes and siloxanes are the main types. They reduce liquid water absorption while still allowing water vapor to move out of the masonry.
• Film-forming coatings sit on the surface like a clear paint. They can block water well, but they also limit vapor movement. BIA states that film-forming products are not recommended for brick masonry in freeze-thaw environments.

New Jersey winters bring repeated freezing and thawing, so this distinction matters here.

Why breathability matters on a chimney

Water gets into chimney masonry from several directions: rain on the outside, moisture from the house, and water vapor in flue gases. If a coating stops that water from drying back out through the brick face, it stays inside. When it freezes, it expands. BIA warns that coatings with low vapor permeability can lead to clouding, spalling, and damage from salts crystallizing behind the coating.

The National Park Service makes a related point about historic masonry: water-repellent coatings and waterproof coatings are not the same. Even a breathable repellent stops liquid water from passing through, so water that gets behind it can still be trapped. The NPS also notes that a repellent can form a surface layer that, if it fails, may pull off even more of the brick face.

Repairs come first

Every authoritative source on this topic says the same thing: fix the cause before applying anything. BIA's guidance on colorless coatings says to correct conditions contributing to water penetration before applying a coating, including failed sealants, deteriorated mortar, and flashing defects. It adds that water repellents cannot compensate for poor construction or design.

On a chimney, that usually means checking and repairing these first:

• The crown and the joint around the flue. See crown coating vs. crown rebuild.
• A cap over the flue opening.
• Flashing where the chimney meets the roof.
• Open or crumbling mortar joints. See repointing vs. tuckpointing.
• Cracked or spalled bricks.

When a sealer can make sense

BIA lists chimneys among the elements where colorless coatings may be appropriate, along with parapets and walls in high-rainfall exposures. A penetrating repellent can help when:

• The brick is sound and the joints are in good condition.
• The crown, cap and flashing have been checked and repaired.
• The chimney is exposed to heavy wetting, for example a tall stack on the windward side.
• The masonry is dry when the product is applied.

On historic masonry, the NPS is more cautious. It says a water-repellent coating is not a recommended treatment unless there is a specific problem, and that most historic buildings have survived for decades without one.

When to skip it

• The chimney is leaking and the source has not been found. A sealer will not stop a leak at the flashing or crown.
• The brick is already spalling. Coating damaged brick can speed up the damage.
• The product is a paint, a thick clear coat, or anything that leaves a glossy film.
• The masonry is wet or efflorescing. Salts need to be dealt with first. See white staining on brick.

Questions to ask about any sealer

• Is it penetrating or film-forming? The product data sheet should say. For brick in a freeze-thaw climate, BIA favors penetrating products.
• What is its vapor permeability? BIA notes that coatings with high vapor permeability allow natural evaporation and reduce the risk of problems.
• What repairs are being done first? If the answer is none, ask why the sealer is expected to solve the problem.
• Will it change the look of the brick? Some products darken the surface or leave a sheen. A small test area shows the result before the whole chimney is treated.
• Is the masonry dry? Applying any repellent over damp brick seals moisture in.

Crown coatings are a separate category

Products made for chimney crowns are usually flexible, waterproof membranes rather than penetrating repellents. They are designed for a horizontal surface that sheds water, not for the vertical brick face. Applying a crown membrane to the brick sides of a chimney would create exactly the kind of vapor barrier BIA warns against. Keep the two uses separate.

How long repellents last

BIA notes that most coatings need reapplication every 5 to 15 years, with penetrating products typically lasting longer than film-formers. It also cautions that repeated applications can reduce vapor permeability over time. That is another reason to apply only what is needed, and only after repairs.

How we approach waterproofing

We start by finding where water is actually getting in. That means looking at the crown, cap, flashing, mortar and brick, and checking the attic side when it is safe and accessible. When the source is not clear, we use controlled water testing to isolate it. You get the findings with photos before any work is scheduled, and a repellent is recommended only after the repairs it depends on.

Next step

If you are seeing water stains, wet brick, or flaking, request an assessment. Learn more about our chimney waterproofing and chimney leak repair services.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 6A: Colorless Coatings for Brick Masonry
2. NPS Preservation Brief 1: Cleaning and Water-Repellent Treatments for Historic Masonry Buildings
3. Brick Industry Association, Technical Note 46: Maintenance of Brick Masonry

This guide is general information. It can't diagnose a specific chimney; an inspection can.