Wood mulches block out heat. If you want to stick with a wood substrate Aspen will let the most heat through, but even still you want to have as thin of a layer as possible. Eco-Earth in thin layers will also work well.
Air is an excellent insulator. The air pockets between wood shavings/mulch is what blocks out most of the heat. By eliminating these air pockets paper based substrates like paper towels and newspaper transfer nearly all of the heat from a UTH to the snake. Which saves electricity as an added bonus!