Cavity Wall below DPC

[jamie91279]

I am building a garden room, two of the external walls are constructed of blue engineering brick externally, 110mm cavity, dense concrete block internally.
The cavity from DPC to concrete foundations is 350mm.

I am finding conflicting information regarding filling the cavity, filling some of the cavity and not filling the cavity at all until above DPC. Can anyone point me in the direction of the correct method which is building regulations compliant?

 

[Wayners]

I have a few blocks built on concrete footings then it's brick's.
My cavity goes down well below dpc.
All the way to footings.

Is that what you are asking?

 

[jamie91279]

My query is what is can the cavity wall below DPC be left empty i.e. Air only or does the cavity need to be partially filled with concrete? I have found an LABC document from 2023 that suggest that the void should be left free a minimum of 225mm below DPC. My cavity is currently 350mm below DPC. I have used AI tools such as Copilot and Chat GPT to see what they reccomend and they both reccoment different solutions, one suggests leaving the cavity free, as installing anything within the cavity would act as a thermal bridge and the other suggests filling the cavity with concrete to 50mm below DPC to stop the cavity filling with void. Both explanations make sense to me but im not sure which way to go.

 

[stuart45]

It used to be 150mm or 2 courses below DPC, but got increased to 225mm or 3 courses to allow for the amount of mortar subbies were dropping down the cavity.