The information I am given is : a door has two steel layers both are .47 mm thick, the door itself is 725 mm by 1800mm. The question asks, how thick of a layer of wood (oak) would have to be put in the door to limit the heat loss to 740kJ per hour? Temp inside is 18C and outside is -20C All work is done with only three significant digits. I have been working with the equation ; H=area(temp1-temp2)/((thickness1/k1)+(thickness2/k2)) Where; k is the thermal conductivity of the material X is the unknown I have run threw the equation several times but every time I either can't isolate the variable or end up with a wrong answer. 0.205kJ/sec= ((1.30m^2)(38C))/((9.40*10^-5m/43W/mC)+(Xm/0.17W/mC)) Is this the right equation and/or have I made a simple error with conversions ? Please don't post full answer I want to find it but tips or suggestions would be awesome !!!