Shower Installation Guide

26 VAPOR — The importance of water vapor management
Why is water vapor management important?
It is important to consider that water exists as both liquid and vapor in showers, and
must be managed in both forms. The prime example of this is the steam shower.
Steam showers that are not designed to manage water vapor have caused mold growth
in wall cavities, rot in wood framing, and damage to moisture-sensitive wall coverings
on the opposite side of the stud cavities.
Do all waterproofing membranes manage water vapor?
The ANSI A118.10 standard for bonded waterproof membranes was developed to
provide a framework for determining the suitability of these products to serve as
barriers to positive liquid water migration only. As such, the standard does not include
a minimum criterion to establish the ability of products to limit vapor transmission.
Thus, simply selecting a membrane that meets the requirements of the ANSI A118.10
standard for a steam shower application is not enough. The ability of the product to
limit vapor transmission must be considered as well.
How can I identify a waterproofing membrane that
manages water vapor?
Water vapor permeance is a measure of how quickly water vapor is transmitted through
a material and can be determined according to the ASTM E96 Standard Test Method
for Water Vapor Transmission of Materials. The higher the vapor permeance, the less
effective a material is at slowing vapor transmission. Materials with a water vapor
permeance of 1.0 perm or lower are generally considered to be vapor retarders and will
manage vapor effectively in showers and steam showers.
Does Schluter
®
-KERDI manage water vapor?
The vapor permeance of Schluter
®
-KERDI, as measured per ASTM E96, is 0.75 perms.
Thus, this product functions as both a waterproofing membrane and vapor retarder to
manage water vapor in showers and steam shower applications.
VAPOR
The importance of water vapor management