Summary

Sealing gaps

Some simple measures can be undertaken to improve the airtightness of homes and reduce air leakage. Holes are often made within the wall fabric for cables and pipes when installing new utilities and building services, such as waste pipes, electricity cables, gas pipes and broadband cables. These holes are rarely sealed properly, if at all, so often leak air. These holes can be relatively easy to seal using an appropriate sealant or airtightness tape or a traditional lime based filler for larger gaps.

Repointing brickwork

Damaged mortar is a common source of leaks and a cause of damp in homes in need of wider renovation. Repointing brickwork is recommended to keep the wall dry. This is especially important if installing cavity or internal wall insulation.

Lime-based mortars should be used for traditional buildings as they are ‘breathable’ and less likely to trap moisture within the building fabric. Cement mortars are more brittle and less porous than lime. They also tend to be more susceptible to salt and sulphate attack, while mortars containing only lime and sand are not vulnerable to this kind of damage.

Cost/difficulty

LOW COST (Sealing gaps); MEDIUM COST (Repointing brickwork)

LOW DIFFICULTY (Sealing gaps); MEDIUM DIFFICULTY (Repointing brickwork)

Applicability in Letchworth

There are no specific requirements in the Letchworth Heritage or Modern Area Design Principles in regard to these measures. However, efforts should be taken to avoid measures or the use of certain materials which may negatively impact the aesthetic of external facades.

If the brickwork repointing is extensive, it may require an application to the Heritage and Stewardship team at Letchworth Garden City. See here for more information on receiving free pre-application advice before submitting an application.

Insights from ASBP

“Lime mortars can be more expensive upfront than cement but they are highly recommended to use for traditional buildings to retain the breathability of the building fabric. If inappropriate cement mortars are used on a solid wall building fabric and are causing moisture issues they can be costly and highly difficult to remove.”

Case study

Links to further information

• HCCSP Retrofitting Your House Guide (2024), page 12
• The Surrey Home Energy Improvement Guide (2025), page 51

Image credit – Cornish Lime

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This guidance is part of Walls

See more guidance notes on this topic below:

• Cavity wall insulation
• External wall insulation
• Hybrid wall insulation
• Internal wall insulation