Thermal mass Cm parameter

[vic_in_scotland]

Hi all,

Can anyone tell me how the Thermal Mass Cm value is calculated? It appears in the project construction (opaque) window, and appears to provide a value of thermal mass for each construction.

Looking at the units:
The "Mass" appears to be 'density x depth of construction'.
The "Cm" appears to be 'specific heat capacity x Mass', but this doesn't match with the values that appear.

I'm comparing it with SAP, which calculates Cm differently, so I'm trying to work out which values I should be comparing!

Thanks

[hrydberg]

Hi Vic,

Not sure if VE calculates it otherwise, but the Cm-value should be calculated as below (though called Km instead).

How is the Km value (effective thermal capacity) calculated for a construction?

The Km values are calculated by:

For each construction element:

Calculate the contribution of each layer of construction as density (kg/m3) x thickness (m) x specific heat capacity (kJ/(kgK))

Starting from the layer of the construction closest to the space add these values together until any one of the following conditions is satisfied:

the total thickness of layers exceeds 0.1 m

you have reached the mid-point of the construction

you have reached an insulating layer

http://www.ncm.bre.co.uk/page.jsp?id=9#a40