On Nov 24, 11:59 am, Wayne Whitney wrote:
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

Taking logic a step further, using a non-permeable membrane and the
appropriate adhesive (avoiding staples), there will be no moisture
transmission problems. Paul F, is spot on in his recommendations in
this regard.
The building inspector who regarded roofing felt as permeable is
puzzling. If that were true, then Tyvek and other brands
used as house wrap today could be replaced by cheaper felt.

Joe