I want to build a small 1x1x1 m insulated (wooden) box, with the purpose of testing an internal coating (the idea is to buy 5 square meters of it instead of 60 for a full scale room). My idea was to place an electrical heating element and a small fan inside, and raise the temperature from approximately 20 to 40 degrees Celsius. Given the air conditions inside, I used the Carrier diagram to find the thermal power I would need to heat up the humid air volume in 0.5 hours. I then tried to consider transient heat losses through the walls of the chamber, of which I know the transmittance: to do so, given the relatively small temperature differences between inside (from 20 to 40 C) and outside (20 C), I figured I would reduce it to a steady state case with a constant internal temperature of 30 C (arithmetic mean between initial and final temperature). The thermal power lost through the walls and calculated this way is then, in my ideas, to be summed to the power needed to heat up the air volume, thus obtaining the power I need from the heating element.
My questions are:
1- Is that of the average temperature a good assumption?
2- Apart from that, am I doing anything wrong?
I reckon this is quite basic stuff, I'm sorry, but I never faced this kind of issues and second opinions would be of great help.