If it is 100% humidity(raining), said CCF pad will have gained weight anyways on its surface. Same with your sleeping bag/clothes etc.
On a CCF pad, you gotta shake it out, likewise this is why I carry my CCF pad on the BOTTOM or TOP my pack, not generally strapped on the side. This guarantees I don't even have to shake it out as there is a guaranteed dry surface at all times. Note I do NOT carry those crappy CCF pads made by Thermarest with all of there moronic cups and ridges just waiting to open their arms and catch all available free water.
PS. In summer and no rain, I will take a half Z-rest as they make quick wonderful chairs, sunshades, cooking wind screens, but in rainy season or a long treck? NO!
If it is raining that hard, then you better have a rain cover/poncho to go over your pack. If you have one of those nice WP packs or just use a packliner like most folks do, then a single end cap bag that tucks under the strap holding your CCF pad rolled works as well. I will admit I did this way exactly once and decided to ditch the weight as a tightly rolled up CCF foam pad will maybe be wet on the outside 1-2 inches on the ends even in a downpour if carried under your pack or on top of your pack. Key, CCF pad MUST be TIGHTLY rolled. If you are trying to carry your CCF pad without a strap and just "get by" by using the straps on the bottom of your pack, you sir will be SOL generally.