as far as i know, all of FR's hybrids have fabric-bottomed mattresses. some other brands have vinyl-bottomed mattresses.
the fabric ones tend to get condensation under them because of the warm humans on top and the cold bunk platform underneath, so moisture forms.
numerous things have been tried and i've been one of them.
some have had success with Reflectix underneath. i've had limited success with it. i've also tried an wool blanket under it, again with limited success.
a poster on another hybrid forum suggested foil-backed foam building insulation sheets.
so, that's my new mod for this season. haven't had a chance to try it out. i posted pics of the mod on one of the other hybrid forums.
if it works, i'll post the pics here with an update.