Try a HyperVent Pad
Recommend you consider trying a HyperVent pad under your existing mattress. It is made of stiff plastic that will provide both cushion as well as breath-ability to help prevent your mattress from getting filled with moisture.Also, it doesn't cost that much when compared to quality foam or innerspring mattresses. Plus, even if you upgrade your mattress it can still be used.We've tried it and it's the best bang-for-the-buck of anything we've come across.http://www.hyperventmarine.com/product.htm