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How to Block a Chimney From Inside?

Open fireplaces are a charming feature in numerous UK homes, especially since approximately 60% of the residences were constructed before 1960. However, even though the warmth and ambiance of a roaring fire can be incredibly inviting during chilly days, open fires and wood burners are not the most eco-friendly heating options.

To maintain warmth in our homes, doing more than just lighting up a fire or turning on the heating is essential. Chimneys, in fact, can let out warm air, even when there's no fire burning.

In this guide, we'll delve into how to block a chimney from the inside to help conserve that much-needed warmth.

How To Block A Chimney From Inside?

Chimney insulation is essential for homeowners wishing to retain warmth and prevent drafts or leaks from unused fireplaces. Inside the home, one can insulate the chimney using a fireplace balloon or a fireplace glass door. For those who prefer a DIY approach, using a plastic garbage bag combined with insulated material can be an effective solution.

Many homeowners decide on chimney insulation mainly when they've stopped using their fireplaces. An inactive chimney can act as a gaping hole in one's house, allowing warmth to escape. Given this, insulating it becomes a practical decision.

Let's dive into each chimney-blocking method in detail:

General Steps for Chimney Insulation

  1. Decide the duration you want the chimney blocked (temporarily or permanently).

  2. Install a flue cap at the chimney's top.

  3. Insulate the bottom of the chimney.

  4. Securely seal the clean-out.

Using a Fireplace Balloon as a Chimney Draught Excluder

The fireplace balloon stands out as an efficient chimney draught excluder for those who prefer a ready-made solution over DIY methods. This device is designed to prevent warm air from leaving your home and stop the cold exterior air from entering, aiding energy conservation.

How does it function? Chimney balloons inflate, typically using a plug-in wall outlet, expanding to press against the chimney's four walls. This ensures they are large enough to stay in position without additional support.

Notably, these balloons come with a ventilation gap to permit a minimal amount of airflow, ensuring the chimney remains sufficiently ventilated and avoids excessive dampness.

You can expect to invest around $50 for a fireplace balloon.

Fireplace Glass Door

A fireplace glass door is another viable option as a chimney draft excluder. As implied by its name, it's a glass door that can be easily opened and closed. While it permits a certain amount of airflow to ensure the chimney remains ventilated, closing the doors effectively seals off the chimney.

Regarding cost, fireplace glass doors are generally pricier than fireplace balloons. Entry-level models typically range from $150 to $200.

Whether you opt for a fireplace balloon or a glass door, measuring your fireplace accurately is essential to ensure a firm fit!

DIY Chimney Blocker

It's entirely feasible to make a chimney blocker using household items. Here's a step-by-step guide:

  1. Start by selecting two sturdy plastic bags, preferably contractor bags. Avoid the thin, lightweight bags typically used for kitchen waste.

  2. Take some insulation material, like fibreglass or rock wool, and encase it within one of the plastic bags. Ensure only insulation fills this bag.

  3. Securely tie off the bag.

  4. For added protection, insert the filled bag into the second plastic bag, creating a double-layered barrier.

  5. Position this dual-bagged insulation packet near the chimney's interior opening.

And there you have it! Your chimney is effectively blocked from the inside!

Chimney Draught Excluder

Another way to block a chimney from the inside is a chimney draught excluder, which is a cost-effective solution. Installed inside the chimney or around the fireplace, this device effectively prevents warm air from escaping upwards while blocking cold draughts from entering.

Starting at a modest price of around £20, it's an investment that pays for itself in no time. Our estimates show that by using a chimney draught excluder, you could save up to £90 annually.

You have options in choosing the type of chimney draft excluder. The inflatable variant can snugly fit inside your chimney, but ensure it has a small vent for airflow. Alternatively, you might opt for one tailored to your chimney's dimensions. Materials like wool, which are breathable yet efficient in retaining heat, are ideal.

One crucial reminder: always remove the draught excluder from the chimney if you light a fire. And, of course, remember to do so on Christmas Eve for Santa's visit!

Reasons Why You Should Consider Blocking A Chimeny From Inside

There are multiple motivations behind the decision to insulate your chimney from the inside. From the challenges of maintenance to concerns about energy efficiency, here are some reasons to consider chimney insulation:

  1. Maintenance Challenges: A fireplace and its associated chimney demand regular upkeep. This means procuring fuel, addressing dampness issues, and cleaning up ashes and soot. You can reduce the maintenance tasks on your checklist by insulating your chimney, even intermittently.

  2. Infrequent Fireplace Usage: If your fireplace isn't in regular use, it's worth considering chimney insulation. A non-utilized chimney becomes a conduit for energy loss within the home.

  3. Energy Efficiency Concerns: Chimneys can inadvertently act as energy drains. When heat from the fireplace is exhausted through the chimney, it often takes along with it the conditioned air from your home—whether it's warmth in winter or coolness in summer. This leads to an inefficient energy usage pattern and potentially higher utility bills.

  4. Space Optimization: A fireplace can sometimes occupy significant room space. By insulating the chimney and sealing the fireplace opening, you can rearrange furniture or make better use of that wall space.

Is It Safe To Block Chimney From The Inside?

Blocking a chimney is safe when done appropriately. However, sealing a chimney can lead to issues related to moisture or condensation within the flue, fireplace, and chimney. Even when sealed off, ducts require regular maintenance, cleaning, and ventilation.

Is It Possible To Block A Chimney Temporarily?

While it's possible to temporarily block off your chimney, especially when it's not in use, it might be more beneficial to leave it unblocked.

The main concern with temporarily sealing a chimney is that it disrupts the natural balance of air intake and outtake. This airflow is crucial in maintaining the internal conditions of the chimney.

Even with a tightly sealed chimney, precipitation water can accumulate inside the chimney and fireplace. Over time, this can lead to significant condensation problems.

By blocking the chimney, you hinder its proper ventilation, potentially inviting the moisture-related complications mentioned earlier.


Blocking a chimney, whether temporarily or permanently, requires careful consideration due to potential condensation issues.

While it can offer energy-saving benefits, disrupting the chimney's airflow might lead to moisture problems. Always ensure proper ventilation and maintenance when sealing a chimney to avoid long-term complications.

Read More: Why Is My House So Cold? Understanding the Chill Factor

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