How to Add a Chimney Damper or Balloon Seal (4 Options)

TL;DR: An unsealed chimney flue acts like a 10-inch hole in your roof, losing $200–300 per year in heated or cooled air. Installing a throat damper ($150–350 pro install), top-sealing damper ($400–800), inflatable chimney balloon ($50 DIY), or silicone flue blocker ($60 DIY) stops 90% of that draft. Choose based on whether you still burn fires and how much access you have to the flue.

_Last reviewed: July 2026 · 6 min read_

If you stand under your fireplace opening on a cold day and feel a draft, your chimney is pulling expensive conditioned air out of your home 24/7. Even a closed throat damper warps over time, and many older fireplaces never had one. Adding or upgrading your chimney seal is one of the highest-return weatherization moves you can make—especially if the fireplace is unused.

Okoniq Property Hub logs the install date, warranty period, and next inspection reminder so you never forget to deflate a balloon seal before lighting a fire or to check a top damper's cable mechanism.

Which chimney seal is right for a fireplace you no longer use?

If you've burned zero fires in the past three years, an inflatable chimney balloon (ChimneyBalloon® or similar, $50–70) or a silicone flue blocker (FlueBlocker®, $60) are the fastest, cheapest, and most reversible options.

• Chimney balloon: A 3-layer plastic pillow you inflate by hand inside the flue throat. Grips the walls with friction tabs. Deflate and remove before any fire.
• Silicone blocker: A wool-wrapped silicone disk on a handle. Sits in the throat, wedged by spring tension. Remove before lighting.

Both block 95%+ of airflow, work in masonry and metal flues, and install in 5–10 minutes with no tools. The balloon has a small "weep hole" that deflates it slowly if you forget and light a fire (a safety feature, though you'll smell burning plastic). The silicone option has no weep hole—if you light a fire with it in place, it will melt and you'll need a sweep to clean the residue.

If you think you might burn one or two fires per year, these are still viable—you just need a system to remember. Stick a bright reminder tag on the fireplace screen or log it in Okoniq's maintenance tracker with a recurring pre-winter check.

What if I still burn wood regularly?

For active fireplaces, install a top-sealing damper ($400–800 installed, $200–300 DIY + parts) or replace a broken throat damper ($150–350 pro install, $50–100 DIY if you can reach it).

Throat damper (traditional cast-iron or stainless plate inside the firebox, just above the smoke shelf):

• Cheaper to install if your fireplace was built with a mounting frame.
• Easy to open and close from inside the room (handle or chain).
• Still leaks 10–20% when closed because metal warps from heat cycles.
• Not an option if your chimney has no throat or the existing frame is corroded.

Top-sealing damper (silicone-gasketed cap mounted at the chimney crown, operated by a cable that runs down the flue):

• Seals 95%+ when closed—much tighter than throat dampers.
• Acts as a rain cap, keeping water out year-round (a $300 value on its own).
• Requires climbing on the roof to install (or hiring a chimney sweep).
• Cable can freeze or corrode; inspect and lubricate the mechanism every other year.

For a fireplace you burn monthly in winter, the top-sealing damper pays for itself in 3–4 years through energy savings and eliminated rain damage. For occasional use (two fires per year), a throat damper or removable seal is simpler.

How do you install a chimney balloon or flue blocker yourself?

Chimney balloon (10 minutes, no tools):

1. Measure your flue width and depth with a tape measure (reach up into the throat with a flashlight). Most rectangular flues are 9×9 to 13×13 inches; round flues 8–12 inches diameter.
2. Order the next size up—balloons over-inflate to grip irregular surfaces.
3. Clean soot off the throat walls with a shop vacuum and damp rag so the grip pads stick.
4. Fold the balloon, push it up into the throat past the smoke shelf, then inflate with the included tube until snug (40–60 breaths, or use a tire pump with a needle adapter). The writing on the balloon should face down so you can read the deflation reminder.
5. Tuck the inflation tube behind the balloon or tape it to the firebox wall. Do not cut it off—you'll need it to deflate before the next fire.

Silicone flue blocker (5 minutes, no tools):

1. Measure the flue throat opening (usually 8–12 inches wide).
2. Order a blocker that matches your flue shape (rectangular, round, or oval). The spring-loaded handle compresses the disk; when you release it inside the throat, it expands to wedge in place.
3. Clean the throat, insert the blocker, pull the handle to test that it's seated firmly.
4. Hang a bright tag from the handle that says "REMOVE BEFORE FIRE."

Both products come with a warning sticker to place on the fireplace surround. If you rent the unit, photograph the sticker in place and note the install date in your maintenance log—a future tenant lighting a fire without removing the seal is a liability you can avoid with one photo.

How do you install or replace a throat damper?

Replacing a warped throat damper is straightforward if:

• The existing damper frame is still bolted or mortared in place (cast iron, rectangular slot just above the smoke shelf).
• You can kneel inside the firebox and reach the frame with a socket wrench or masonry chisel.

Step-by-step (30–60 minutes, $50–100 in parts):

1. Measure the frame opening (width, depth, and the spacing of mounting holes or slots). Take a photo.
2. Order a replacement damper assembly (Lyemance, Lock-Top, or store-brand) that fits your frame type. Stainless steel lasts 3× longer than cast iron in high-moisture climates.
3. Remove the old damper plate—most lift out once you disengage the hinge pins or unscrew the control arm bracket.
4. Scrape rust and creosote off the frame with a wire brush.
5. Drop the new plate into the frame, align the hinge pins, attach the control arm, and test the swing. It should open and close smoothly with no binding.
6. If the frame itself is cracked or missing, you'll need a sweep to reframe the throat with refractory mortar (adds $150–250 to the job).

A new throat damper won't seal as tightly as a balloon or top damper, but it's code-compliant for active fireplaces and handles 300+ open/close cycles without warping if you buy stainless.

Do you need a permit or inspection after installing a chimney seal?

No permit is required for removable seals (balloons, silicone blockers) or throat damper replacement—these count as routine maintenance.

Top-sealing dampers sometimes require a permit if local code treats them as a chimney cap modification, especially in wildfire-prone counties with strict spark-arrestor rules. Call your building department; most issue an over-the-counter permit ($50–100) and require a photo of the installed cap with mesh screen visible. A failed inspection usually means the cap is mounted too low or the cable routing pinches at the crown—fixable in 20 minutes.

If you're also repairing the chimney crown or flashing, that may trigger a separate roofing permit. Bundle the work with a licensed sweep who pulls permits as part of the bid.

FAQ

How much does an open chimney cost per year in lost energy?

A 10-inch flue loses 300–500 CFM of conditioned air through stack effect, costing $200–300 annually in a 2,000-square-foot home with central heat and AC. Sealing it saves 90% of that loss, or roughly $180–270 per year.

Can you install a chimney balloon in a gas fireplace?

Yes, if the pilot light is off and the gas valve is closed. Direct-vent gas fireplaces use a sealed combustion chamber and don't need a balloon. B-vent (natural-draft) gas units do benefit, but you must remove the balloon before relighting the pilot. Install a lockout tag on the gas valve as a reminder.

Do chimney balloons work in round or oval flues?

Yes—most balloon models expand to fit irregular shapes. Measure the widest and narrowest dimensions, then order a balloon rated for that range. If your flue tapers sharply, a silicone blocker with an adjustable spring may grip better than an inflatable.

How often do you need to replace a chimney balloon or flue blocker?

Chimney balloons last 5–10 years if you deflate and remove them before every fire. UV exposure (if sunlight enters the flue) and rodent chewing can shorten lifespan to 2–3 years. Silicone blockers last 10–15 years; the wool insulation may compress but the silicone disk stays intact. Inspect annually and replace if you see cracks, tears, or loss of spring tension.

This is educational information, not construction or energy-efficiency advice. Consult a CSIA-certified chimney sweep before sealing a flue that has never been inspected—hidden creosote buildup or structural damage can create hazards even in unused chimneys.