How Do You Temporarily Block a Chimney to Stop Drafts and Pests?

Do you feel a cold draft sneaking in from your fireplace or hear scratching sounds from above? Temporarily blocking your chimney is a straightforward fix that can immediately improve comfort and prevent unwanted visitors.

From my time on roofing crews, I’ve sealed dozens of chimneys. Here’s what we’ll cover to get yours sorted:

• The best safe materials for a temporary chimney seal.
• Practical, step-by-step methods to install them.
• How to block pests effectively without causing damage.

Why Your Unused Chimney Is a Problem (And When to Block It)

Think of your open, unused chimney flue as a permanently open window on your roof. Warm air in your house rises. It finds that opening and escapes straight outside.

Your furnace then has to work harder to replace that lost heat. You feel cold drafts near the fireplace. Your energy bills creep up for no good reason.

But the drafts are just the start. An open chimney is a welcome mat for wildlife. Over the years, I’ve had to safely remove all sorts of tenants homeowners never invited.

• Birds (especially sparrows and starlings) that build nests.
• Squirrels and raccoons seeking a dry, sheltered den.
• Bats, insects, and even bees.

A nest doesn’t just cause a mess. It can completely block the flue, creating a dangerous situation if the fireplace is lit later.

Blocking the chimney temporarily makes sense in a few common situations. You might ask, can you block off a chimney for just a season? Absolutely. Do it in the summer when you know the fireplace won’t be used for months. It’s also smart during long home renovations to keep out dust and debris. If you only light a fire once a year at most, a temporary block for the other 11 months saves you money and hassle — especially if you use your chimney for off-season storage.

Here’s a key detail homeowners often miss. Blocking the fireplace opening inside your living room is one thing. Blocking the flue up near the damper or at the very top is another.

Putting a piece of cardboard in the fireplace face stops drafts in that room. It does nothing to stop a raccoon from entering the chimney from the roof and getting stuck above your damper. For full protection from pests and weather, you need to seal the flue itself.

The Golden Rule: Safety First Before You Block Anything

Listen closely, because this is non-negotiable. I’ve seen scary situations that started with a simple misunderstanding.

Never, ever block a chimney flue that serves a furnace, boiler, water heater, or any other fuel-burning appliance. These appliances need to vent deadly carbon monoxide gas safely outside. Blocking that exit path pushes the gas back into your home. It’s a silent, fatal risk. Make sure you understand how chimneys and flues work and follow all safety guidelines.

First, you must identify which flue is for your fireplace. In a brick chimney, look for the different clay tile liners poking out the top. The fireplace flue is usually the largest, center tile. The smaller tiles around it are for appliances. If you have a metal chimney, the fireplace pipe is typically the widest one. When in doubt, trace the pipes from the appliances in your basement. If you’re still unsure, call a chimney sweep or HVAC pro. It’s a quick check for them.

This leads to the big question: is it safe to temporarily block a chimney? The answer is yes, but only under two strict conditions.

1. You are 100% certain the flue is for the wood-burning fireplace only.
2. You leave a clear, visible warning for anyone in the home. Tape a big note on the fireplace grate or mantel that says “CHIMNEY BLOCKED – DO NOT USE FIREPLACE.”

I keep a bright orange warning sign in my truck for this exact reason after a close call on a job years ago.

Finally, think about the forces at play on your roof. A “temporary” block must be permanent enough to withstand weather. People ask, can snow block a chimney? A heavy, wet snow slide can. More often, it’s high winds that rip off a poorly secured cover. It’s important to understand the dangers of snow accumulation on roofs to ensure your roof is properly protected.

Your temporary seal must be so secure that wind, snow, or a curious animal cannot dislodge it. A cover that blows off leaves the flue open to rain and debris. One that falls *into* the flue creates a worse blockage. If you’re going to do it, do it right.

Method 1: The Fast, Flexible Seal (Chimney Balloons & Inflatable Blocks)

A chimney balloon is exactly what it sounds like. You slide this inflatable plug, made of durable plastic, into your chimney’s throat (the opening just above the firebox). Then, you use the attached tube to blow it up like a pool toy until it presses snugly against the chimney walls.

It creates a custom-fit, airtight seal that stops cold drafts from rushing down and your warm house air from escaping up.

Pros of Using a Chimney Balloon

• DIY Friendly. Installation and removal take minutes with no special tools.
• Effective Air Seal. When properly inflated, it blocks nearly all air movement.
• Reusable. You can deflate it, pull it out for the season, and store it for next year.

Cons to Consider

• Not Deterrent-Proof. A determined raccoon or squirrel could puncture it. It blocks air, not ambitious pests.
• Risk of Puncture. Sharp edges or creosote buildup inside the flue can cause a leak over time.
• You Must Remember It. Lighting a fire with one in place is a serious fire hazard. This is non-negotiable.

How to Install a Chimney Balloon: A Crew Note

I’ve helped a few neighbors with these. Doing it right the first time saves callbacks.

1. Locate the damper and ensure it is fully open. You need clear access to the chimney flue.
2. Insert the deflated balloon and start inflating it slowly with the hand pump (or by mouth for smaller ones).
3. Stop inflating when the balloon is firmly contacting the walls on all sides. You should feel resistance. Do not over-inflate.
4. Test the seal. Feel around the edges for any major air gaps. A small flashlight can help you see.
5. Attach the warning tag that comes with it directly to your fireplace lever or nearby. This step is for safety.

How do I remove a temporary chimney block when I want to use the fireplace again? For a balloon, the process is simple. Find the deflation tube, let the air out, and gently pull the balloon down and out. Always do this before lighting any kind of fire. Chimney balloons block drafts and improve energy savings when the fireplace isn’t in use. When you’re ready to use the fireplace again, simply remove the balloon as described above to restore ventilation.

Method 2: The Rigid, Reusable Barrier (Foam Board & Rigid Insulation)

This method is more hands-on but creates a solid physical block. You use a sheet of rigid foam insulation, like XPS (the pink or blue stuff) or EPS (white bead board). Cut it to fit the exact dimensions of your fireplace opening or flue.

It’s a step up from the old-school plywood block method. Plywood works, but foam is much lighter, easier to handle, and actually provides an insulation value to slow heat loss.

How to Fit and Secure a Foam Block

Precision is key here. A sloppy fit lets drafts in and looks unprofessional.

• Measure the height and width of the opening at the back, middle, and front. Chimney throats are often not perfect squares.
• Transfer those measurements to your foam board and cut it with a utility knife. I aim for a cut that’s about a quarter-inch smaller than the smallest measurement for an easy friction fit.
• For a truly secure temporary seal, you can run a few small beads of removable silicone caulk around the edges before pressing the block into place. It will hold it there but allow you to peel it off later.

Pros of a Rigid Foam Block

• Superior Insulation. The foam’s closed cells are great at stopping thermal transfer (heat loss).
• Pest Deterrent. This is a solid barrier. Most birds and rodents cannot chew through dense foam board.
• Clean and Storable. You can write “FIREPLACE BLOCK – DO NOT REMOVE” on it with a marker, pull it out each spring, and stack it in the garage.

Cons of a Rigid Foam Block

• Requires Accurate Work. You need to measure and cut carefully. An uneven block won’t seal well.
• Less Forgiving. Unlike an inflatable balloon, it can’t adapt to a wildly irregular opening. You might need to fill gaps with backing rod or tape.
• Still Temporary. While secure, this is not a permanent chimney cap replacement. It only seals the bottom opening.

Method 3: The Professional-Grade Temporary Fix (Top-Sealing Dampers & Caps)

For the strongest seal against cold, moisture, and critters, you install your block at the top. Think of it like putting on a winter coat instead of just stuffing a sock in the window crack. A top-sealing chimney cap or damper is the most effective temporary (or even semi-permanent) solution you can do yourself.

Not all caps are equal. A basic screened cap keeps out birds and raccoons, but wind and rain can still whistle right through the mesh. A sealed cap with a solid damper or lid is what actually stops the drafts. These units have a metal flap or lid that sits flush over the flue opening, creating a weatherproof seal when your fireplace is not in use.

I’ve installed these for customers who said they just wanted a “few-year” fix while they figured out their long-term plans. A decade later, that same cap is often still working perfectly. While more permanent in nature, a quality top-sealing cap is the ultimate answer for how to block a chimney from drafts for the long haul.

This method requires safe access to your roof. If your roof is steep, high, or you’re uncomfortable on a ladder, this is a hard stop. Hire a pro. For a low-pitch roof you can safely work on, measure your flue carefully, order the right sized cap, and secure it with the provided fasteners. It’s a one-afternoon job that pays off for years.

What NOT to Use: Materials That Create Hazards or Mess

I’ve seen some creative, and frankly scary, attempts to plug a chimney from the inside. Let’s clear this up. When you’re thinking about what should I avoid using when temporarily blocking a chimney, this list is your guide. It’s important to remember that using the wrong materials can lead to chimney problems that affect home safety.

• Rags, Newspaper, Cardboard, or Plastic Bags: These are a terrible idea. They degrade over time, and a strong down draft can suck them right down into your fireplace, creating a mess or, worse, a fire hazard if they land on old embers.
• Spray Foam Insulation: Never, ever spray expanding foam into your flue. I was called to a house where the previous owner did this. It bonds to the clay tile or metal lining and is nearly impossible to remove without damaging the chimney. You will ruin it for future use.
• Loose-Fill Insulation (like blown-in cellulose or fiberglass batts): This seems logical but creates a nightmare. It can spill out into your firebox, trap moisture against the flue walls leading to deterioration, and is incredibly difficult to fully clean out later. You’ll be finding bits of insulation for years.

Stick with the recommended methods. They are safe, removable, and designed for the job. The wrong material can turn a simple draft problem into a major repair.

This table lays out your three best options side-by-side, based on hundreds of chimney jobs my crew has tackled.

Method	Cost	Effort	Stops Drafts	Stops Pests
Inflatable Chimney Balloon	Low	Very Low	Excellent	Good (if intact)
Rigid Foam Board	Very Low	Moderate	Best	Excellent
Top-Mount Chimney Cap	High	High (roof work)	Perfect	Perfect

How to Pick the Right Block for Your Job

Don’t overthink it. Follow this simple logic.

Start here: Is your access safe? If you need a ladder to reach the damper or the roof feels steep, stop. Hire a pro. No temporary block is worth a fall.

If you can safely reach the damper area from inside, ask yourself one question.

• Do you need to remove the block in seconds for a spontaneous fire? Pick the inflatable balloon. It’s your only fast-removal option.
• Do you want the absolute best insulation against cold air? Pick the rigid foam board. It creates a true thermal break.
• Do you want a permanent “set it and forget it” solution that also protects the chimney crown? Pick the top-mounted cap. It’s not temporary, but it’s the final fix.

Gather Your Tools First

You don’t need much. Having it all ready before you climb up or crawl in makes the job smooth.

• Tape Measure
• Utility Knife (for trimming foam board)
• Bright Work Light or Headlamp
• Work Gloves
• Safety Glasses (for overhead debris)

The Universal Installation Rules

No matter which method you choose, these five steps are non-negotiable for a safe, effective block.

1. Clean the area thoroughly. Reach up into the damper cavity and clear out loose soot, old leaves, or nesting material. A clean surface helps your block seal properly.
2. Measure twice, cut or buy once. For a balloon or foam board, measure the narrowest part of the chimney throat. Your block must fit snugly there, not just at the wider fireplace opening.
3. Install the block carefully. Inflate the balloon until it’s firmly wedged. Cut the foam board for a friction fit. Ensure a pro-installed top cap is sealed and strapped down.
4. Check your seal from below. After installation, turn off all lights and shine a bright flashlight around the edges of your block. If you see light leaking through, your seal isn’t tight. Adjust or add more foam tape.
5. Leave a huge, impossible-to-miss note. Tape a bold sign directly on the fireplace facing or mantle that says “CHIMNEY BLOCKED – REMOVE BEFORE STARTING FIRE.” This is the most critical safety step. I’ve seen forgotten blocks cause dangerous smoke backups.

Common Questions

How long can I safely leave a temporary chimney block in place?

Properly installed rigid foam or a top-sealing cap can last for years if undisturbed. Always inspect it annually for damage and replace the block if you see wear, gaps, or pest activity.

Can I block my chimney for just one season, like summer?

Absolutely. Summer is the ideal time to install a block, as the fireplace is unused and pests are most active. Use a removable method like a chimney balloon or marked foam board you can pull out in the fall.

What’s the best way to pest-proof a temporary block?

Your seal must be physically impenetrable. Use rigid foam board cut for a perfect friction fit, or install a professional metal top-sealing cap with a mesh surround; these stop animals from chewing or pushing through.

A Smart, Temporary Seal for Your Chimney

The best temporary block does its job well and comes off cleanly when you’re ready to use your fireplace again. Pick a material that fits your specific weather and pest problems, and always install it so you can remove it without a trace.

Checking your temporary chimney seal a few times each season is a key part of responsible home ownership. Treating these projects as routine maintenance is the best way to keep your entire roof, from the shingles down to the chimney components, secure and functional for years.