Why Unblocked Chimneys Are Costing UK Homes Energy and Comfort

Why Unsealed Chimneys Are Costing UK Homes Energy and Comfort

Across the UK, many older homes still feature traditional chimneys—once symbols of warmth and domestic life. But today, these unused and unsealed chimneys are quietly draining heat from our homes and driving up energy bills. The culprit? A physical process known as the stack effect.

What Is the Stack Effect?

The stack effect happens when warm air inside a house rises and escapes through an open chimney. As it leaves, it pulls in colder outside air through gaps around doors, windows, and other openings. This creates draughts, makes rooms feel colder, and forces heating systems to work harder to maintain a comfortable temperature.

In winter, when indoor temperatures typically sit around 19–21°C and outdoor temperatures average just 7.6°C, the difference is enough to trigger significant airflow. A study by the Building Research Establishment (BRE) found that unsealed chimneys can leak around 80 cubic metres of warm air per hour—roughly the equivalent of replacing the air in a room several times a day.

Why It Matters for Net Zero

The UK has committed to reaching net zero carbon emissions by 2050. With buildings responsible for around 23% of the country’s CO₂ output, tackling inefficiencies like unsealed chimneys is essential. Simple solutions—such as installing chimney draught excluders or permanently sealing unused flues—can make a real difference.

These small fixes not only improve comfort but also reduce energy use and emissions. According to the BSRIA report, sealing a chimney can save up to 1,300 kWh of energy per year—about 11% of the average household’s gas usage. That’s equivalent to cutting nearly 0.3 tonnes of CO₂ annually per chimney.

How Common Are Unsealed Chimneys?

The UK’s housing stock is largely made up of older properties. Over half of homes were built before 1965, when chimneys were standard for coal fires and stoves. That’s around 15–16 million homes with chimneys.

Not all of these are still open. Many have been sealed, capped, or repurposed. But surveys suggest that around 30–40% of pre-1965 homes still have disused, unsealed chimneys—roughly 5 million across the UK. These chimneys continue to leak heat even when they’re no longer in use.

The Cost of Doing Nothing

Leaving chimneys unsealed means more than just chilly rooms. It leads to higher heating bills, increased carbon emissions, and reduced comfort. In older homes, which often lack modern insulation, the impact is even greater. BRE’s airflow study found that 15 out of 20 monitored homes had open fireplaces, with 8 never used—yet still leaking heat.

Across the UK, the total energy loss from unsealed chimneys could exceed 6.4 billion kWh per year—comparable to the output of a large power station. That’s a huge opportunity for savings, both financially and environmentally.

What Can Be Done?

Installing a chimney draught excluder or permanently sealing unused flues is a simple, low-cost fix. These measures can dramatically reduce airflow, improve comfort, and support the UK’s net zero goals. Historic England and the Energy Saving Trust both recommend sealing disused chimneys to prevent unnecessary heat loss and draughts.

As we move towards a more energy-efficient future, addressing overlooked issues like the stack effect is a practical step that benefits households and the planet alike.

What You Could Save: Energy, Money, and Emissions

Blocking off the stack effect isn’t just about comfort—it can lead to real savings. Based on updated calculations from BSRIA and the University of Liverpool, sealing an unused chimney can prevent around 1,300 kWh of heat loss per year. For older terraced homes, the savings range from 1,143 to 1,481 kWh annually. The Energy Saving Trust estimates this could save households around £60 a year.

When we factor in 2025 energy prices—6.33p per kWh for gas and 8.4p for wood—the financial benefits become clear. Here are three typical examples:

Across 5 million homes, that adds up to a national saving of around £1.78 billion every year.

Cutting Carbon Emissions

The environmental impact is just as compelling. Using standard emissions factors, each sealed chimney could reduce CO₂ output by roughly 1–1.5 tonnes per year, depending on the heating system. Nationally, that’s a potential reduction of 5.8 million tonnes of CO₂—an important step towards the UK’s net zero goals.

And the cost of action? Chimney draught excluders typically range from £20 to £50 and pay for themselves quickly through lower heating bills.

How to Stop Chimney Draughts

There are two main ways to tackle chimney draughts:

• Chimney Balloon: These inflatable devices are made from durable laminated plastic. They’re inserted just above the fireplace opening and partially inflated to block airflow, while still allowing a small amount of ventilation to prevent condensation. BSRIA tests show they reduce airflow by up to 87%. Liverpool’s modelling found they’re nearly as effective as permanent sealing, with the added benefit of being reversible.
• Permanent sealing: This includes masonry infill or installing a damper. It’s best suited for chimneys that are no longer in use. Historic England recommends dampers for listed buildings. Both approaches can reduce draught-related discomfort by 33–50%, according to parliamentary reports.

Supporting the UK's Net Zero Mission

The UK’s commitment to net zero by 2050 means buildings must cut emissions by 80–85% by 2035. Reducing heat loss through chimneys directly supports this goal. Liverpool’s projections suggest that sealing unused chimneys nationwide could cut emissions by 2 million tonnes a year—about 2% of all residential emissions.

Government schemes like ECO help fund these improvements, and public support for net zero remains strong. When combined with other upgrades like insulation and heat pumps, chimney draught-proofing is a simple, effective way to accelerate the UK’s transition away from fossil fuels.