External walls

If you want to re-render or replace timber cladding to external walls, building regulations may apply depending on the extent of the work.

Where 25 per cent or more of an external wall is re-rendered, re-clad, re-plastered or re-lined internally or where 25 per cent or more of the external leaf of a wall is rebuilt, the regulations would normally apply and the thermal insulation would normally have to be improved.

If you want to insert insulation into a cavity wall the appropriate requirements will be applied to ensure the insulation material is suitable, and that in the case of some foam insulants, the risk of formaldehyde gas emission is assessed. Read more about insulation¹.

Walls can be constructed in various ways by using timber frame structure or masonry structure. 

If using a masonry structure then two forms of construction can be used:

Cavity Wall

This is where there are two skins of masonry, the outer skin can be of brickwork or blockwork and the inner skin is generally of blockwork.

The gap between the two skins will vary depending on the type of insulation that is to be used. To stop the two skins from falling away from each other they should be tied together using wall-ties at appropriate centres. These ties should be made from stainless steel and resistant to corrosion.

The bottom of the cavity should be filled with lean mixed concrete with a slant towards the external skin or have a cavity tray installed that also slants towards the outer skin to ensure any moisture that could get inside the cavity will be directed away from the inner skin.

Solid Wall

This is where there is only one skin of masonry which can consist of brick/blockwork

The high standards of thermal insulation needed in buildings means that it is more difficult to achieve those standards with solid masonry wall construction. Solid blockwork constructions may meet the requirements if allied with other insulation products and surface finishes.

Existing External Walls in Conversion Projects

Existing walls will need to be checked for their adequacy in terms of:

• Weight (Loading) and structural stability
• Weather resistance (including Damp-proofing)
• Thermal resistance and changes to 'thermal elements'

If they need to be upgraded, this may well involve the addition of a new internal skin - possibly constructed of lightweight studwork. The detail at the foot of the new skin will need careful planning to ensure that damp-proofing arrangements are sound and that any new timbers are protected from damp.

Further Information

The following pages give an indication of some of the elements normally required to satisfy the requirements of the Regulations for external walls.

Getting your job done safely.²

1. https://www.planningportal.co.uk/permission/common-projects/insulation
2. https://www.planningportal.co.uk/permission/common-projects/external-walls/getting-your-job-done-safely