Hi edh,
Thanks for the update ā¦ I appreciate it 
If you mean a mattress protector then post #89 here has more information about the different types that are more commonly available and the tradeoffs involved in each. A wool/cotton cover is probably the best choice for humidity control and temperature regulation.
If you mean a mattress pad (which is thicker than a protector) ā¦ then post #10 here may be helpful in making comparisons between the many options available. I would bear in mind though that the thicker the mattress pad you use the more it will affect the feel, response, and performance of the memory foam underneath it.
Phoenix