Best substrate?

I have a 20gal glass tank with a uth (on a thermostat) and a heat lamp with a 60w red bulb. Im currently using aspen for a substrate. The uth is having a hard time making it through the aspen. I thought it might be broken, then I moved the aspen and touched the glass and dang near burnt myself. What substrate would help with the heat transfer? I dont want to have to use the lamp when shedding comes around, cuz it kills the humidity, but the uth cant keep the air or the bed warm by itself.

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Aspen is a really good substrate to use and is probably the most common. How thick do you have the aspen? You might want to thin it out a bit, especially on the hot side so it can get through to the top. You want about 1/2" of bedding.