Page 10 - CodeWatcher Winter 2017 Issue

Energy Code Minute

“Back When” a Vapor

Barrier was a Vapor Barrier

If you want to build high-performing, durable

wall assemblies, you need to rethink some

old terms and practices.

BY STEPHEN CARR

HILE LISTENING TO Tim
McGraw’s song “Back When,”
I got to thinking about how
words and terms that we
have used for years may not
mean what they did years ago.
W Take the term vapor barrier.
Remember back when we built homes as shelters, not indoor
entertainment studios, back when the only moisture we saw
on the window panes was from cooking or teenagers taking
long showers. Today we expect more out of our homes, yet
we are still set in our old ways.
Ask a seasoned building official what a wall or ceiling
assembly needs, and vapor barrier will be mentioned. Huber’s ZIP System sheathing and tape is an innovative structural
Depending on your climate zone and/or assembly this could roof and wall system with an integrated water resistant and air
be correct, but due to “vapor barrier” being only part of the barrier that streamlines the weatherization process. It is
important to remember that water-resistive barriers are designed
story we need to look into the detail of this term. to prevent liquid water intrusion but still allow water vapor to pass
No Clear Guidance through. The use and location of vapor barriers depend on
different variables like climate, geography, building design and
Try to find “vapor barrier” in the IRC or IECC (residential) local code requirements.
when talking about walls and ceilings; it’s not there. You will in Climate Zones 5, 6, 7, 8 and Marine 4 (IRC 2009 – R601.3,
find the term Vapor Retarder, which has three Classes. One IRC 2012 and 2015 – R702.7)
of the classes happens to be a Vapor Barrier. Exceptions:
Here’s the definition of Vapor Retarder (IRC 2009, 2012, 1. Basement Walls
and 2015): A measure of the ability of a material or assembly 2. Below grade portion of any wall.
to limit the amount of moisture that passes through that 3. Construction where moisture or its freezing will not
material or assembly. Vapor retarder class shall be defined damage the materials.
using the desiccant method with Procedure A of ASTM E Notice that there is no mention of Climate Zones 1 thru
96 as follows: 4 (except Marine) in this requirement of the code. Building
■ Class 1: 0.1 perm or less (Vapor Barrier) (Sheet Polyethylene, science practices tell us when a vapor retarder is placed on the
■
unperforated aluminum foil) “winter warm side” of the assembly then that material helps
■ Class 2: 0.1 ≤ 1.0 perm (Kraft-faced fiberglass batts.) control moisture transfer from getting into the assembly.
■
■ Class 3: 1.0 to 10 perm (Latex or enamel paint) By the Climate Zone definitions, these are all locations that
■
In Section R702.7 gives more guidance as to when and primarily heat their buildings for the occupants comfort.
where a vapor retarder is required for walls: Look at Figure R702.7 from the 2015 IRC. You will find this
Section R702.7 Vapor Retarders. – Class 1 or 2 vapor figure in previous code versions too; it describes exactly what
retarders are required on the interior side of frame walls cold climate building officials talk about, yet notice even in

10 CodeWatcher / January 2017 www.codewatcher.us