Masonry cavity walls
What are the regulations and guidance for insulation in masonry cavity walls and which products could be used?
What are the Building Regulations for fire in Wales?
Most building work being carried out in Wales must comply with the Building Regulations 2006. The Building Regulations state the following for external fire spread:
B4-(1) The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.
(2) The roof of the building shall adequately resist the spread of fire over the roof and from one building to another, having regard to the use and position of the building.
Regulation 7 (4) sets out requirements for materials which become part of an external wall, or specified attachment on ‘relevant buildings’. For more information take a look at our article on relevant buildings. Note that on 20th December 2025, the definition of 'relevant building' will be extended to cover hostels, hotels and boarding houses.
How can you meet the requirements of the Building Regulations?
Approved Document B gives practical guidance on common building situations to meet the requirements of the Building Regulations 2006 for Wales.
Note that complying with the guidance in the Approved Document does not guarantee that building work complies with the requirements of the regulations. There may also be other ways to comply with the requirements than those described in the Approved Document. The project Principal Designer (during design stages), Principal Contractor (during construction) and relevant consultants (such as Fire Engineers) should be consulted to ensure compliance with the Building Regulations.
Our guides to the regulations
Looking for more information?
Book our CPD on fire safety for insulation
Our CPD covers the guidance, regulations and testing for insulation products. It takes a look at essential terminology for understanding how a fire develops and how products need to perform as a fire develops.
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Page updated 21/10/2025