I am trying to model a thermal mass for a greenhouse design. A thermal mass is basically a box of mass with high Cp buried in the ground with the top exposed to the inside of the greenhouse and all the other sides insulated. My model is based on the fact that during the day, there is a net heat input on the top side of the termal mass via radiation from the sun and during the night there is heat loss due to conduction to the colder interior of the greenhouse. I am looking for the equation to solve to get heat distribution. Assume the radiation from the sun is constant and the greenhouse temperature is also constant. From what I have search this is a one-dimensional heat transfer problem since the heat distribution is only a function of the depth (x) and time (t).