different temps

I have a thermometer in each coldframe now. And I'm surprised. I thought the new one, with solid wood sides and glass lid, would stay warmer than the upright one that's wrapped in plastic sheeting. Nope. The coldhouse warms up sooner, and stays warm later in the evening (by at least two hours) than the traditional cold frame. I thought it was mainly because the coldhouse, being taller, catches sun earlier in the day. But today was cloudy and overcast, and my coldhouse was 50° while the coldframe still below freezing. This makes me want to build another upright coldframe, and sell the window-topped one. (It's also much easier to get plant trays in and out of the coldhouse, as I don't have to prop the lid open, or kneel on the ground.)