I am going to have a fiberglass tub installed with a plastic surround. I want to make sure it's done right & I want overkill. It's very wet here in the winter & I will not get the house sided this year. My outside walls are 1/4 " hardwood - un sealed. I decided on Wonderboard under the tub & behind the plastic surround which will be on an outside wall on 2 sides. What is the proper procedure for this installation ? Thanks.