Porotherm Hollow Core Block Work

[marcus_dea]

Howdy all!

I am modelling a school building which is proposing to use Porotherm Hollow Core blockwork on the internal skin of the walls and in the internal walls. (Google "Porotherm 38 Sth Blocks" to see the ones)

I want to model the thermal mass of the building accurately as this has a huge impact on my heating and cooling needs as well as BREEAM ENE1 Credits.
I know the density of the blocks (900kg/m3), and the thermal conductivity (0.141 W/mK), but am struggling with the key element - the Specific Heat Capacity.

If I use the S.H.C for clay, IES will assume the clay is the full thickness of the block and ignore the cavity - higher thermal mass. How can I incorporate these cavities into the construction? Is it a case of adding a number of clay/cavity layers or is there a way I can find an average value??

Many thanks for any help!