I am going to be putting down a cermic tile floor in my kitchen. I ripped out the old floor all the way to the sub-floor which is constructed of 3/4" thick by 4" wide planks which run perpendicular to the joists. I want to use plywood for the underlayment because I can't use the hardy backer because it has to be mortared to the sub floor which will make a mess because it is planks..I plan on using 1/2" AC Douglas Fur for the underlayment. Is this sufficient??