Is polystyrene porous then David?

No but if you have damp sand sandwiched between the polystyrene and a top
layer of polythene then it will stay damp.
The top layer will stop the water from going into the sand
If the sand dries then the heat doesn't spread evenly through the bench.
I have had this working now for many years.
I also put the wires 6 inches apart with the thermostat probe between the
wires and just below the surface (about 1/2 inch)
The polythene also stops the plant rots going into the bench.

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David Hill
Abacus Nurseries
www.abacus-nurseries.co.uk