Question about under tank heaters

I'm not quite a BP owner yet, but am getting one soon, and it will be my first herp. I'm trying to sort out my viv setup at the moment.

Are under tank heaters supposed to be used outside/underneath the enclosure, or inside the enclosure under the substrate? If it's outside altogether, do they only work when the enclosure has a glass base, or could they pass enough heat through a wooden base?

Sorry if my questions are really stupid!

Most of the UTH's are made to go on the outside of the cage. Here is a list

OUTSIDE:
Zoo-Med
Flexwatt
Exo-Terra
Cobra

INSIDE:
Cobra
Kane
Stanfield

You can use any UTH with wood depending on the thickness of the wood but as a general rule using them with glass, plastic or inside the cage covered with plexiglass(for the outside models) is best..
-Matt