Use of a rubber sheet

Depends on how often you like to change sheets!  A rubber sheet can trap heat and also may allow some leakage off the sides.  They make different pads that are incredibly absorbent, allow more breathing so it's cooler to sleep on and trap everything inside the pad.  Most major pharmacies, not Walgreens etc, but major local pharmacies particulary servicing elderly carry these pads which can run $20 but last forever.

It depends on how much you wet during the night and the diaper you use. Our son Alexander is a heavy wetter and although he wears thick night diapers and plastic diaper pants his diapers may sometimes leak. To protect his matress he has a rubber sheet in his bed.

/Anna