On 2007-11-24, Wayne Whitney wrote:

When I redid my bathroom, the building inspector told me to use a
permeable membrane behind the cement board, like 15 lb felt.

Just to follow up, the logic behind using a permeable membrane is
this. Water will eventually make its way through the membrane,
e.g. through the holes from the staples holding it up. So given that
there will be some moisture in the wall cavity, the permeable membrane
will allow some water vapor to escape back into the shower.

Cheers, Wayne