I have been wondering...with the recent discussions of insulating north walls, installing reflective surfaces on north walls, insulating foundations, radiative versus conductive heat loss...are the online greenhouse heating calculators accurate?

It is my understanding that they use the standard formula Q=(deltaT*A)/R. But I've got a brick kneewall and an insulated north wall, so how accurate is the result of the calculation? I've wondered about breaking the formula down into Q=((deltaT*A1)/R1)+((deltaT*A2/R2)+((deltaT*A3/R3)...

Are the results of these calculations useful as a starting point? Is there a better way to do it?

Thanks for listening to my ramblings... SB