there is no direct regulations saying you cant....
But NICEIC info suggests it is difficult for installer to ensure compliance on all of the following areas inside a cavity wot you cant see into!
522.8.1: Damage to cables during installation
Obstructions in cavity e.g. metal wall ties, mortar projections, create a risk of unseen damage.
522.8.4: Strain on cables lacking support.
Long unsupported vertical drops may put strain on conductors.
522.5.3: Materials liable to cause mutual or individual deterioration.
Expanded polystyrene sheets, granules or foam used in buildings for insulation if comes into contact with thermoplastic (pvc) cable sheathing, plasticiser can migrate from the thermoplastic to the polystyrene.
523.7: Cables in thermal insulationThermal insulation installed in cavity walls during or after construction, the cable current-carrying capacity is to be taken as half the �clipped direct� (Reference Method C) rating unless more precise information is available.
522.9.1 and 522.10.1: Presence of flora and fauna
Designers and installers may not necessarily detect or predict the presence of flora or fauna in a cavity wall. The wiring system should be selected to withstand all the external influences expected, or damage to cables may occur from, for example, mould or rodents.
Building Regulations
Building Regulations 2000, Approved Document C, 2004 Edition, Section 5: Walls, paragraph 5.13 b Cavity external walls, requires, amongst other things, that a cavity external wall is at least 50 mm wide, and the cavity is to be bridged only by wall ties or by cavity trays provided to prevent moisture being carried to the inner leaf.
A cavity is intended to provide a gap to prevent water penetration. Cables could bridge this protection if they touch both the inner and outer leaves of a cavity wall. Furthermore, the cables could provide a route for water to drain directly into accessories, with potentially dangerous results.
NHBC Requirements
It is noteworthy that the National House-Building Council states, in Section 8.1 - S2 of NHBC Standards, that no cables other than electricity meter tails are to be located in the cavity of an external wall. Where meter tails do have to pass through the cavity, however, this does not preclude the requirements of BS 7671 referred to in this topic having to be met.
