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[DaveC]

I have recently submitted an external wall detail to my local Planning and Building Control Office as part of a Building Notice for a Kitchen Extension. The cavity is 50mm (with insulation fill) and I have been advised that this should be detailed as 100mm (with 100mm insulation fill). Is this correct? I can find nothing in Part A or Part L of the Building Regs that stipulates 100mm. Also, does the cavity size depend on the wall construction? I am intending to use blockwork inner leaf and rendered blockwork outer leaf for one wall and blockwork inner leaf and brickwork outerleaf for a second wall.
Any comments / advice would be appreciated.

[bco]

Hi not entirly true this! It would depend on the type of block and insulation you were intending to use!

Cavity Thickness generally comes from Parts C and L of the Building Regulations but you will not find specific refererance to cavity dimensions!

What you will find is a U value to be achieved 0.3W/m2K. This related to the thermal performance of the wall from its construction.
100mm cavities are now stabdard with concrete blocks and stabdard cavity insulation however you could reduce this down to 75mm with a higher spec of construction like;

Cavity wall construction to be 102mm facing brickwork outer skin, 75mm cavity fully filled with Crown Dritherm Cavity Slab Plus , 100mm Thermalite Turbo light weight aircrete (or similar block with minimum thermal conductivity of 0.11 W/mK), with 12.5mm plaster boards on dabs with skim finish. Minimum “Uâ€