Thermal Mass Cm - How is it calculated? Where is it used?

[blumentopferde]

I am doing a series of simulations trying to simulate trombe walls.
A trombe wall is a passive solar wall, consisting of a glazing, with an air cavity and an absorber with high thermal mass behind it.

I was trying to find out the effect of the thermal mass of the absorber on the Heating Load and stumbled upon the derived value "Thermal mass Cm [kJ/(m²K)]" (see picture).



Obviously this value is the thermal mass, related to the area of the wall, and thus should be calculated this way: Thickness [m] * Denisty [kg/m³]* Cpecific Heat Capacity [J/kgK] / 10000

But here comes the question: No matter which values I chose for my internal wall, Cm always equals 50% of the calculated thermal mass, when I use single-layer-constructions. Only with multi-Layer-constructions I can reach higher thermal masses than 50% of the calculated. Now I am not sure whether 100% or only 50% of the thermal mass are considered in the calculation.

Does anyone know how this value is derived and how it is used in the calculation?

Thank You!

blumentopferde

[mshaynes1992]

I believe IES calculates thermal mass based on the heat capacity of the construction layers in use, from inside surface to outside, and STOPS measuring from whichever condition occurs first:
-halfway through the construction assembly (based on length, I believe. So if you only have one material layer, it will calculate the thermal mass based on half of the length of that layer, assuming one of the other conditions below hasn't already occurred)
-an insulating layer
-a depth of 100mm (so if your construction assembly only has one material layer that is 300mm thick, it will calculate thermal mass based on the first 100mm of thickness, not half of 300mm).