In an experiment I heated a room and logged the temp change as it went up. One of the walls in the room is made of plywood. I also logged the temp on this surface (on the heated side). When I looked at the graph after the experiment I noticed that as the air temp of the room went up, the surface temp of the plywood was reluctant to follow the change. It only increased slightly for a while before suddenly skyrocketing about ten degrees within a few minutes. When I did the whole thing again the next day, the same thing didn't happen. Now the surface temp rose evenly with the room temp. I'm confused as to this "reluctantness" in the first experiment. Any explanations?