AWCI’s Member Products Catalog 2017 - Par4

Vapor Permeability
A key factor in moisture management of a building is
understanding the atmospheric conditions impacting
it. Moisture vapor is always present, with different
temperatures and concentrations of moisture (relative
humidity) in the air, creating a difference in vapor
pressure from interior to exterior environments.
Nature always tries to equalize this imbalance, which
means that the water vapor will try to migrate from
a location of higher concentration to one of lower
concentration. This vapor drive flows from high
pressure to low pressure across the wall assembly.

WARM INSIDE AIR

COOL OUTSIDE AIR

DEW POINT
DEW POINT
PLANE:PLANE:
POTENTIAL CONDENSATION IN WALL
POTENTIAL
CONDENSATION
amount
of vapor permeance, or breathablity
IN WALL

The
required for a wall assembly varies depending upon
climatic conditions and internal environmental
requirements . The vapor drive from higher
temperatures and pressures to lower plays a key
role in determining the most appropriate type of
membrane, permeance requirements, and proper
placement within the wall assembly.

ClAss
I - Vapor Barrier

4

The most significant determining factor in proper
selection of membrane type and position within the wall
assembly design will be the geographic location of a
structure, which determines temperature and humidity
exposures and seasonal conditions. For most wall
constructions in most climate zones, vapor permeable
products like WeatherSeal should be considered.
Low permeance barriers: for some advanced wall
designs, such as those for cold climate zones with
metal studs, continuous external insulation only and
no batt insulation between studs, a low permeance
barrier coating like Weather Block on the exterior
sheathing may be appropriate as an alternative to
a vapor retarder on the indoor side of the framing.
That is because in such designs, materials to the
interior of the continuous insulation are all "warm
in winter". Some special hot-humid climate zone
wall designs may also benefit from a low vapor
permeance barrier. NOTE: WeatherBlock is not to be
used when another vapor retarder is on the indoor
side of the framing; a double vapor retarder can trap
moisture in the wall.
In cold climate zones, a vapor retarder, like Parex
USA's WeatherBlock, stops water vapor leakage of
conditioned air to the exterior. For hot, humid climate
zones, it stops infiltration of hot humid air to the
interior. For mixed climate zones, vapor permeable
products, like WeatherSeal, should be considered.
The Weather Tech line offers products that
accommodate the need for water-resistive barriers
and air barriers with a variety of permeance ranges.

PerMs

WeAtherteCh ProdUCts

0.1 perm or less

WeatherSeal BG

II - Vapor Retarder

1.0 perm or less and greater than 0.1 perm

WeatherBlock

III - Semi Permeable

10 perm or less and greater than 1.0 perm

WeatherSeal Trowel-On

greater than 10 perm

WeatherSeal Spray & Roll-On

www.parexusa.com I 866.516.0061
http://www.parexusa.com
Table of Contents for the Digital Edition of AWCI’s Member Products Catalog 2017
