Roofing-felt-under-cement-board, yes you ll need to put the felt just behind the cement board.. behind the green board thats above your cement board wont hurt but you don t need to. all you need to do is tack it up or staple it.... Almost. the moisture barrier is installed because moisture will get through the backer board and you want it to be able to find its way to the tub. no rocket science involved in the installation. if using felt, hang it in shingle fashion with at least a two-inch overlap at the horizontal joints., for a tub surround the kerdi may be a bit of overkill. it really depends on how much use it will see as a shower. kerdi is a surface applied membrane, any water that penetrates the tile and grout will either evaporate back out through the tile and grout or drain into the tub..

Quoted text here when i redid my bathroom, the building inspector told me to use a permeable membrane behind the cement board, like 15 lb felt.