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The impact of Canada's 2010 VOC regulations on under-slab vapour barriers
What's
the difference between a vapour barrier and retarder?
By
Brian Rooney
To some people there is no difference.
They use the terms interchangeably. They shouldn't. The two
terms have taken on distinct meanings in the world of vapour-proofing.
In order to be considered a vapour
retarder, the membrane must meet all of the requirements of
ASTM E 1745. Those requirements include strength and permeance
thresholds. The strength thresholds determine the "Class"
(A, B or C). The permeance threshold is expressed as perms
(Imperial). The permeance threshold is the same for all three
classes: less than 0.10 perms after conditioning.
A vapour barrier must also meet all
of the requirements of ASTM E 1745.
So, a barrier is a retarder...it's
one with exceptionally low permeance. The commonly accepted
permeance threshold for a vapour barrier is: less than 0.01
perms after conditioning...or 10% of the permeance of a retarder.
In essay #5, I stated that: to be
effective, the vapour barrier needs to be at least as impermeable
as the flooring that it's there to protect. How impermeable
is that?
Tough question, but published industry
literature indicates that you want to get below the permeance
of a vapour barrier, i.e., after-conditioning permeance less
that 0.01 perms.
In order to avoid ambiguity, it's
recommended that you add another decimal place making it "less
than 0.010 perms". If you express the permeance threshold
in metric units, it's recommended that you parenthetically
include the Imperial equivalent. In a specification, it might
look like this: Permeance as tested after conditioning (ASTM
E 1745 paragraphs 7.1.2 - 7.1.5): less than 0.010 perms (gr/ft2/hr/in-Hg)
[0.570 ng/(Pa * s *m2)]
For more information on Stego, contact
marla@dre.ca.
I'd like to thank Brian Rooney with
Stego
Industries for providing this information.
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