the backing on the professional framed piece is call a "dust cover" this does not seal against water. it is used mainly to "clean up" and finish off the back so the hardware and back is not exposed to have to look at. the whole framed piece is allowed to "breathe" and should be moisture free in doing so. If your bathroom or kitchen area is so damp that condensation gathers on the inside of a framed piece you need to look into a fan in the room not so much as worry about the framed pieces. Youve got a bigger problem with home ventalation.
I used to work as a framer for embroidery and I can tell you that it is definitely true. Also, gegebee43 is correct that the framer uses materials that allows the area to "breathe" - this actually helps against mould and condensation. The best thing you can do for your embroidery is to stretch it well using acid free board as the backing, do not use adhesives instead of lacing, use an acid free mat or spacer and do not hang in direct sunlight even if you have UV protection glass. I know it sounds complicated, but it can make a big difference on how long your embroidery will look as beautiful as when you finished it. :)
I spoke to a really great framer and you have to shop about, but the one I used framed my King Arthur using perspex instead of glass, it is just as clear but avoids accidents with glass such as scorching if caught in direct sunlight, f the piece falls by accident broken glass can damage the fabric and also protects a stitched piece from fading faster :)
chekccard