Heat will travel through the 1/2 and eventually reach the other side. Takes more time to heat up however compared to 1/4 inch thick material. On the flip side, it holds heat a bit longer. More mass.

If the air is cool, the heater needs to have some wattage behind it.

For example, I have some awesome heat mats from Amazon dot com - they look very much like Big Apple's "Intelli-Temp" mats though I do not know if they are the exact same device (yet - I just ordered some Intelli-Temps as well). They are perfect for glass tanks and storage box cages in that they do not get overbearingly hot. But for a 1/2 inch thick PVC cage, these heat mats will not do the trick unless the ambient air temp is in the mid 70s at least. Not enough watt-density.

My guess is the 20 watts/ft Flexwatt will have no problem on 1/2 inch material. That stuff will heat through 3/4 inch melamine.
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Mark