Thermal mass, building shell air volume, and heat losses over time

[dansphere]

Hello,

I'm looking for some feedback on and an analysis of a spreadsheet calculator I've made.

It looks at temperature changes over time, given starting volumes of a building shell and thermal mass. The calc takes into account building fabric and ventilation heat losses.

First I've derived the available heat energy at a given starting temperature and used the specific heat capacities of the air and thermal mass to calculate the total available heat energy at 20 degrees Centigrade relative to absolute zero. Does this make sense?

Then I've calculated the losses over one hour and deducted that from the total energy, and done this recursively for several hours.

The change in internal temperature I'm deriving from a simple ratio.. ((heat energy lost/total heat energy) * original temperature in Kelvin). Although what I've realized is that the energy lost is only translatable to a temperature change via the specific heat capacities of the air and thermal mass, is this correct? Or can it be done via a ratio?

Attached is the spreadsheet I've made.

 

[dansphere]

I've realized that the calculator is going to be very inaccurate due to the Thermal Time Constant of the thermal mass..
I'm not sure how to include the thermal time constant into the calculations (the calculations of which I'm thinking would best be done through some sort of exponential function.. eek I don't have a clue) Any help on this would be greatly appreciated :)

Dan