Hi all, I bought this roll on membrane to install on hardi backer board I have already put 4 mill plastic behind the backer board. Bathroom will be tiled from floor to ceiling and is only 5' x 8' ( lots of moisture ). Question is do I need to go ahead and put this on the floor and walls as far up as I can go. Its a 1 gallon pale so I'm not sure just how many SF. it will cover. Please advise. Bob