Vapor barriers and vapor retarders can be a very tricky subject matter. Using the wrong
product in the wrong location can lead to mold and mildew inside your walls. That’s
why vapor control is an important part of building construction and wall assemblies.
In this video, we’re going to discuss how vapor moves through these two, their different
perm ratings, where they are used and how climate zones affect their installation.
Let’s start with the science behind vapor movement. Diffusion is the movement of water
vapor molecules through porous materials, typically from the side with high vapor pressure
or warm air to the side with low vapor pressure or cold air. In cold climates, water vapor
moves from the warmer interior to the cooler exterior. In hot climates, water vapor moves
from the warm exterior to the cooler interior. Vapor barriers like this plastic sheet keep
rain out and stop vapor diffusion. Vapor retarders stop water but slow down vapor diffusion.
Perm ratings measure the ability of a material to let water vapor pass through. In the American
system, a perm rating of 1 means that in 1 hour, when the vapor pressure difference between
the cold side and warm side of a material is 1 inch of mercury, 1 grain of water vapor
will pass through 1 square foot of the material. In the metric system, a perm rating of 1 means
that in 1 day, when the vapor pressure difference between the cold side and warm side of a material
is 1 mm of mercury, 1 gram of water vapor will pass through 1 square meter of the material.
1 metric perm is equal to approximately 1.5 US perms.
Now let’s look at US perm ratings of the most common building materials. The lower
the number, the less likely it is to allow water vapor through. Class 1 materials are
vapor impermeable or vapor barriers and have a perm rating of 0.1 or less. Some examples
are glass, aluminum foil, cork, rubber or vinyl flooring, 6mil polyethylene, sheet metal
and foil faced polyiso
Class 2 materials are semi-impermeable and have a perm rating of 0.1 to 1. Some examples
are vapor retarder latex paint, oil based paint, asphalt coated kraft paper, foil faced
fiberglass insulation, 2” closed cell spray foam, 1” XPS and kraft paper on insulation
Class 3 materials are semi-permeable and have a perm rating of 1 to 10. Some examples are
3” EPS, OSB or Oriented Strand Board, tar paper, 2” MDF or Medium Density Fiberboard,
fluid applied vapor retarders, latex paint and plywood sheathing
Class 4 materials are vapor permeable and have a perm rating of over 10. Some examples
are wood siding, zip coating, plaster on metal lath, rockwool, cellulose insulation, brick
and Tyvek
To summarize, Class 1 materials with a perm rating of 0.1 or less are vapor barriers,
while Class 2, 3 and 4 materials with a perm rating of over 0.1 are vapor retarders.
Now let’s discuss where these are used. In the past, builders used plastic sheets
behind drywall. Unfortunately, these vapor barriers caused mold to build up behind the
wall, because vapor couldn’t escape. Products like thick XPS, polyiso and closed cell spray
foam can also act as vapor barriers. There are certain instances where you’d
want a vapor barrier in your wall. Interior pools or museums, which have high humidity
levels need a vapor barrier to stop an excessive amount of vapor entering the walls.
Even though vapor barriers aren’t used in walls anymore, they are still used underneath
slab-on-grade foundations, under the concrete slab of basements, and under crawl spaces.
Plastic sheets protect concrete from moisture in the ground. They are also resistant to
corrosion and punctures. Two popular brands are ISI Building Products and Stego.
Vapor retarders that also act as air and water resistive barriers are widely used on walls
nowadays. They can either be peel and stick membranes or attached with staples to a substrate.
Some examples are Siga, Delta and Zip system sheathing.
Before we move on to the code requirements for using vapor barriers and retarders, I’d
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Finally, the code requirements for vapor barriers and vapor retarders. These vary depending
on the climate zone you live in. In warm climate zones 1,2,3 and 4, the International Residential
Code does not require or prohibit the use of vapor retarders. You can use one exterior
side to slow the movement of vapor. In cold climate zones 5,6,7 and Marine 4, code requires
either a Class 1 or 2 vapor retarder on the interior side to limit the movement of vapor
through the wall assembly. Any water vapor on the outside can still dry out provided
your cladding is properly vented.
So, the consensus is to avoid plastic sheets and vapor barriers. Water vapor is going to
get in either through air or through your foundation. You can’t avoid it, so let the
walls dry out by using vapor retarders. Another trend in the building science community is
to prioritize air barriers and stop air infiltration. A study has shown that in most cold climates,
over an entire heating season, a 4’x8’ sheet of drywall will let just ⅓ quart of
water through by diffusion. But if that same sheet of drywall has a tiny 1 inch by 1 inch
hole, moisture-laden air can sneak through and bring in 30 quarts of water.
Continuous exterior insulation like Rockwool Comfortboard can stop air infiltration and
reduce thermal bridging. I hope I was able to simplify this complex and highly debatable
topic. There is a lot of confusing information out there on air barriers, water resistive
barriers and vapor retarders. If you have any questions on the topic, leave me a comment
below. Let me know if I should make a follow up video on specific products or related topics.
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it. A big thank you to everyone already supporting me. Don’t forget to hit that like button,
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