Difference between Air
Barrier & Vapor Barrier & Water Resistive Barrier
Vapor Barriers
are not to be confused with an air barrier. A vapor barrier is
designed to restrict the flow of water vapor through a
material, just the same as a air barrier material restricts the flow
of air through a material.
Vapor barriers or vapor
retarders are intended to control the rate of diffusion into a
building assembly. As a vapor barrier they will control the rate of
moisture flow where they are placed. Therefore the vapor barrier
does not have to be continuous, does not have to be free of holes,
does not have to be lapped, does not have to be sealed, etc. A hole
for example in a vapor barrier will simply mean that there will be
more vapor diffusion in that area compared to the other areas where
the vapor barrier.
Water vapor permenance
is measured by the amount of water which will work its way through a
material. This is normally reported in ng/(Pa•s• m2) Many
areas require a vapor barrier which has a maximum water transmission
rate of 60 ng/(Pa•s•2).
Much work is being done
and much discussion is being held on whether vapor barrier should be
used at all and if they are used should the water transmission rate
be. There is discussion on the need to allow buildings to dry. Keep
in mind that during this time period where vapor barriers are being
discussed you still need to meet the local building code which may
demand it even when Building Science would indicate that it should
not be used.
Water vapor may be
transported by air leakage but you deal with this by installing a
proper air barrier.
Vapor barriers are
designed to be installed on the warm side of the insulation.
Water Resistive
Barriers are materials which are primarily designed to be used
to keep liquid water from entering the building enclosure. Water
resistive barriers are specifically designed not to be a vapor
barrier. The minimum water vapor permeance for a water resistive
barrier is 300 ng/(Pa•s• m2). Water resistive barriers
are combined with flashing and other materials to ensure that there
is a shingled assembly to direct liquid water which passes on the
cladding system to be directed to the exterior.
Water resistive barriers
are designed to be installed o the cold side of the insulation.
Combined air
barriers, vapor barriers and water resistive barriers can be
provided in a single material. There are also vapor permeable air
barriers, and there are water resistive barriers which are not air
barriers. Please understand the three separate functions and then
determine whether the material you choose provides more that one
function and then you need to decide whether you will design your
building so it actually performs more that one function.
As an example, you can
have two, three or even four air barrier materials in a wall
assembly but it will depend on which material you have chosen and
how you have connected the air barrier materials together. Gypsum
wallboard, polyethelyne film, exterior grade drywall, self adhered
membranes and spray polyurethane foam insulation are all air barrier
materials in a specific wall assembly but you need to choose which
one is the air barrier and then ensure that you have the air barrier
material chosen has been connected.
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