With the recent hot weather, I thought it worth rerunning an experiment I did a few years ago by making a direct comparison of a John Stimpson Zeist box, made of 12mm plywood, with one modified with an additional roof. I replaced the original roof with a close fitting piece of ply, then I spaced the original roof off this by using 3 strips of ply. I didn't bevel the new roof, as it conveniently provided 2 small triangular ventilation holes. I then painted it white. Both boxes were placed side by side in the sun and 2 max-min thermometers were placed inside at the ends furthest from the entrances.
|The boards on top of the boxes are intended to simulate the shading effect of narrow eaves.|
|A view end on, showing the additional roof and air gap.|
Part of this effect is the white paint - it certainly feels a lot cooler than the unpainted wood, and part is the shading effect and the air gap between the 2 layers of plywood.
The additional roof not only defends the box from the sun, but also from the rain, which is important for a wooden box.
We actually described this idea some years ago here and here