Chimney draught proofing

Summary

There are a number of ways of reducing heat loss and draughts through chimneys.

One method is to ‘cap and fill’ the chimney by sealing the chimney flue at low level with a strong board or metal plate to stop draughts and to prevent the insulation fill from spilling out. Each flue is then filled with inert granular insulation from above. A new weather-shield top prevents water ingress while allowing some limited ventilation. This can be implemented where an internal fireplace is still in place as well as where fireplaces have been removed. It must be fully sealed at the bottom so that the insulation material cannot escape.

A simpler but less effective method is a ‘chimney balloon’ which is a draught excluder which can be installed by a homeowner to help prevent draughts and reduce heat loss from an unused chimney flue. It can also help to reduce noise infiltration.

Cost/difficulty

LOW/MEDIUM COST – Typically around £30 for a chimney balloon/sheep.

Typically around £500 for a cap and fill measure with scaffolding to reach the chimney stack likely to be the biggest cost, so it may be sensible to do this measure when other works requiring scaffold are carried out.

LOW/MEDIUM DIFFICULTY – A chimney balloon can be installed by a homeowner with competent DIY skills.

A cap and fill measure should be completed by a competent contractor/builder.

Applicability in Letchworth

Applicable – No specific requirements.

Insights from ASBP

“Ensure the chimney balloon is fitted loosely to ensure some air flow. This is important to reduce moisture build up and potential damp issues. Consider using a felted sheep’s wool balloon or blocker instead of plastic as the breathable material helps to release trapped moisture.”

Insights from People Powered Retrofit

“For the cap and fill measure, care should be taken to ensure that the flue is no longer used and not connected to any other flues still in use. This can be particularly important where chimneys are shared with neighbours. A party wall agreement may be necessary in some cases.”

Case study

“Chimney sheep – a low cost way to stop chimney draughts“

A blog post from Zero Chippenham.

Energy Efficiency Guide for Letchworth Garden City residents (2024), page 3
The Surrey Home Energy Improvement Guide (2025), page 38
HCCSP Retrofitting Your House Guide (2024), page 12
Retrofitting Your Home – Cambridge Guide (2022), page 11

Image credits – Beige Concrete Houses With Cloudy Sky (Pexels); Zero Chippenham.

This guidance is part of Draught proofing

See more guidance notes on this topic below:

Window draught proofing