Posted on Oct 12, 2022

Draught proofing is a key step to improving efficiency and reducing home energy bills. People often worry about how to stop draughts from doors, but many of us overlook the biggest hole in a building’s fabric: the chimney and fireplace.

Happily, fitting a chimney draught excluder is one of the most cost-effective energy-saving upgrades.

‘A 2013 study by the University of Liverpool showed 5% of a household’s heat can be lost up an open chimney,’ says Sally Phillips, inventor and director of Chimney Sheep(opens in new tab). ‘Blocking off a fireplace with an easy-to-use draught excluder can cost as little as £30 and give you payback in less than six months.’

How to draught proof a chimney

Open chimneys are designed to suck air up and out of the house, drawing a supply of oxygen for the fire below. In turn, replacement fresh air is pulled into the living spaces through trickle vents or gaps in the building fabric (such as around windows and doors).

Known as the stack effect, this siphoning of warm air causes much of the sensation of draughts we feel in our homes.

‘It doesn’t really matter what you do in terms of insulating elsewhere if you leave the chimney open. It’s just like having a permanently open window,’ says Simon Wild, inventor and managing director of Chimella(opens in new tab).

There are many DIY draught-proofing ideas for your house. In this instance, the quickest DIY fix is to fit a removable chimney draught excluder. This step-by-step guide will help you choose and correctly install the best type for your chimney...

1. Measure the chimney

Clean up any loose debris, lay a dust sheet and wear goggles in case you disturb any dirt within the chimney flue. Measure the inside cavity. For a round chimney, you can just take the diameter. For an oblong shape, take the width and depth.

2. Choose the right size of draught excluder

Avoid one-size-fits all versions and instead choose a design that matches your chimney dimensions. Most suppliers offer size charts to make this easy.

There are three main types of chimney draught excluder. They’re all DIY-friendly, but the process is slightly different, so we’ll look at each in turn.

3. Fit a Chimney Sheep

Chimney Sheep (available on Amazon(opens in new tab)) is basically a double layer of felted sheep’s wool insulation (a natural and sustainable product) attached to a plastic handle.

Select a model that’s a few inches larger than the chimney flue, to help it grip in place once installed. The wool head is malleable, so it forms to the shape of the opening. Simply push the head up into the flue, give a gentle tug down on the handle to ensure a good fit, and you’re done.

4. Fit a Chimella

A Chimella looks like an inverted umbrella and works in much the same way. All you need to do is hold it up in your open chimney, press the button and it will deploy. You can then make minor adjustments to ensure the canopy stretches taut across the flue. A benefit of Chimella is it won’t move even if a significant amount of debris falls down the chimney.

5. Fit a chimney balloon

This type is installed by partially inflating a specially-designed balloon, using the tube provided, and nestling it into place before pumping up to a firm fit. An advantage is you can get the excluder into tight spaces, such as through narrow fire surrounds. Check the balloon every three months or so to see if it needs topping up.

Could draught proofing my chimney cause damp?

The main risk is forming too tight a seal. How to get rid of damp, condensation and mould in your home is a common worry. We'd advise you don’t just wrap a pillow in bin bags and ram it up the chimney, completely blocking much-needed ventilation.

‘Chimella is designed to turn the air around in the chimney. It features tiny gaps so there’s sufficient ventilation and you don’t get any damp,’ explains Simon Wild.

Chimney Sheep deals with the issue in a different way. ‘The wool insulation is naturally breathable, so it blocks about 93% of the airflow but allows 7% through as ventilation,’ says Sally Phillips.

Can I draught proof the chimney if I still use my fireplace?

Yes; just remove the draught excluder before you light the fire. Chimney Sheep and Chimella can both be repeatedly reinserted. ‘If you want to use the fireplace, just click a button and take the Chimella out,’ says Simon Wild. Balloon types are less convenient, as they must be partially deflated in situ before removal.

Draught excluders are designed for open chimneys, not stoves with dedicated flues. If you’re feeling a chill from a stove, the seal around the appliance door probably needs replacing.

Should I cap my chimney?

Closing off the top of the chimney can reduce draughts, while also preventing dirt, debris and wildlife from entering the cavity. Choose a product that allows some ventilation, such as C-Cap(opens in new tab) from Amazon, to prevent condensate forming. Alternatively, if you want to continue using your fireplace, go for a mechanical damper operated by a hearthside handle. Installing a cap or damper is usually best left to a qualified trade.

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