Chimney Sealer: How to Waterproof a Brick Chimney (and Which Sealer to Use)
No Sales Tax in any state on every chimney sealer we carry. Brick and mortar soak up rain like a sponge. Each winter that water freezes, expands and pops the faces off the bricks (spalling) and washes the mortar out of the joints. A good chimney sealer stops the water getting in while still letting moisture inside the chimney get out. This guide covers which kind of sealer to use, how much to buy, and how to apply it yourself.
Quick answer: the best chimney sealer for a brick chimney
- Use a breathable, water-based siloxane water repellent, not paint, acrylic or silicone caulk. ChimneySaver is made for exactly this.
- Cheapest way to buy it: the 1 lb ChimneySaver powder mixes with water to make 5 gallons, enough for most chimneys, for $78.99.
- Ready to use: ChimneySaver Water-Base, 2.5 gallon.
- Cracked crown on top? Seal it first with CrownCoat. A water repellent won't bridge cracks.
All ship with No Sales Tax.
Is sealing a chimney a good idea?
Yes, with the right product. A masonry chimney takes more weather than any other part of the house: rain on all four sides, snow sitting on top, and freeze-thaw cycles all winter. Sealing the brick cuts the water it absorbs, which slows spalling and mortar loss and keeps water out of the attic and the fireplace.
The catch is the type of sealer. Brick has to breathe. Moisture from inside the chimney (from the flue gases and from the house) moves out through the masonry. A film-forming coating such as paint, acrylic sealer or a rubbery "waterproofing" coat traps that moisture behind it. It then freezes inside the brick, and the face of the brick pops off with the coating attached. That's why chimney professionals use a vapor-permeable water repellent on the brick, not a film.
Types of chimney sealer
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| Sealer type | Breathes? | Use it on | What we carry |
|---|---|---|---|
| Siloxane / silane water repellent | Yes | Brick, block, stone and mortar joints | ChimneySaver |
| Elastomeric crown coating | Flexible membrane | The chimney crown only: bridges hairline cracks | CrownCoat |
| Paint, acrylic or "waterproofing" film | No | Not recommended on chimney brick. Traps moisture and causes spalling | – |
| Silicone caulk | No | Small gaps and joints only, not the brick face | – |
Find the leak before you seal
A water repellent fixes water soaking through the brick. If the water is getting in somewhere else, sealing the brick won't stop it.
- Through the brick: damp patches or white chalky deposits (efflorescence) on the chimney, spalling brick faces, eroded mortar. Fix: ChimneySaver.
- Through the crown: cracks across the concrete top or around the flue tile. Fix: CrownCoat. See our chimney crown repair guide.
- Down the flue: water on the damper or in the firebox after rain. Fix: a stainless rain cap on a lined flue, or an Energy Top top-sealing damper on a fireplace flue.
- Through the flashing: leaks on the ceiling or in the attic right where the chimney meets the roof. That's a roofing repair.
How much chimney sealer do you need?
Work out the brick area: add up the width of all four sides and multiply by the height of the chimney above the roof. A 2 ft x 3 ft chimney standing 10 ft above the roof is (2 + 3 + 2 + 3) x 10 = 100 sq ft.
A water-based siloxane covers roughly 100 to 125 sq ft per gallon on brick. Soft, porous or old brick drinks more, so round up. Check the label for your product's coverage.
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| Chimney | Brick area | Buy |
|---|---|---|
| Small: 2 ft x 2 ft, 6 ft above the roof | about 50 sq ft | 1 lb powder (makes 5 gal) |
| Typical: 2 ft x 3 ft, 10 ft above the roof | about 100 sq ft | 1 lb powder (makes 5 gal) or 2.5 gal Water-Base |
| Large or two-flue: 3 ft x 5 ft, 15 ft tall | about 240 sq ft | 1 lb powder (makes 5 gal) or 5 gal Water-Base |
| Several chimneys, or a contractor | 500+ sq ft | 5 gal Water-Base, Multi-Purpose or Advanced |
The powder is the best value for a homeowner: you mix it with water in a bucket or sprayer and get 5 gallons for $78.99, against $181.99 for 2.5 gallons ready-mixed.
How to seal a brick chimney
- Fix the masonry first. Repoint loose or missing mortar and replace badly spalled bricks. Let new mortar cure fully before sealing; the label gives the wait.
- Seal the crown. If the crown is cracked, coat it with CrownCoat. The water repellent goes on the brick, not the crown.
- Clean the brick. Brush off moss, loose dirt and efflorescence. Let it dry.
- Pick the weather. Dry masonry, above freezing, and no rain due for the rest of the day. Follow the label for exact temperature and dry-time limits.
- Protect what you don't want coated. Cover windows, siding and roofing below the chimney, and nearby plants.
- Apply it. Use a low-pressure garden pump sprayer and saturate the brick and mortar joints until they're wet but not puddling, working one side at a time. Follow the label on how many coats.
- Check it. Once dry, splash water on the brick. It should bead and run off instead of darkening the brick.
A breathable sealer doesn't change the look of the brick and doesn't leave a shine.
Common questions
What is the best sealant for a brick chimney? A breathable, water-based siloxane water repellent made for chimneys, such as ChimneySaver. Avoid paint, acrylic and film-forming sealers on the brick.
What's the best sealant for a chimney crown or cap? For a concrete crown, an elastomeric crown coating like CrownCoat. A metal chimney cap doesn't need sealing; if it's rusted or missing, replace it.
Can I seal the chimney from the outside only? Yes. Water repellent goes on the outside face of the brick above the roof, which is where the weather hits. Never coat the inside of the flue.
How long does a chimney sealer last? It depends on the product and the exposure. Check the warranty on the label, and re-test with the splash test every few years.
Will sealing stop a chimney leak? Only if the water is coming through the brick. Check the crown, flue and flashing first (see "Find the leak" above).
Related guides
- Chimney crown repair: seal the cracks yourself or rebuild?
- Chimney caps and liners: what you need, what size, and why
- How often should you clean a chimney?
Not sure what's causing your leak? Email a photo of the chimney to info@stovesource.com or call 1-888-914-0022. Every chimney sealer ships with No Sales Tax in any state.