What are Vapor Barriers, Retarders & Perm Ratings?

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Vapor permeance, permeability, vapor barriers, vapor retarders and perm ratings. It’s all pretty confusing, isn’t it? In this video, I’m going to explain the science of vapor movement, the perm ratings system and rating of specific building materials.
Link to Patreon page: https://www.patreon.com/Belinda_Carr

Chapters
0:00 Introduction to water perm
0:15 4 control layers of a building
1:57 Relative humidity
2:27 Dew point
3:05 Vapor diffusion
3:34 Perm rating system
4:26 Building material perms
5:53 Rockwool perm graphic
6:22 Permeability
6:34 Conclusion

The exterior walls of a building are responsible for controlling 4 main differences between the conditioned indoor and unconditioned outdoor environments. The first and most important is water which is controlled with cladding or siding, windows, doors, flashing and a water resistive barrier like Tyvek and ZIP system. The second is air which is controlled with plywood or OSB sheathing, some types of insulation, plastic sheets, caulk, tape, and fluid applied weather resistive barriers. The third is temperature which is controlled with insulation like fiberglass, Rockwool, spray foam, denim, hemp wool and cellulose. The fourth is water vapor which is controlled by the arrangement of many material layers.

There are a couple of terms we need to clarify before moving on. Relative humidity is one of them. It is the percentage of water in the air, relative to the maximum the air can hold at a given temperature. The relative humidity changes when the air temperature changes because warmer air can hold more water than cooler air. Ideally, the relative humidity inside your home should be 30 to 50%. Anything higher can lead to rot, mold and structural damage.

Dew point is another important term in this discussion on vapor movement. It is the temperature at which water vapor condenses into liquid. If warm air comes in contact with a cool surface, the molecules slow down and the gaseous vapor turns into liquid water droplets. You don’t want that condensation to happen inside your walls. You want to place a vapor barrier or vapor retarder at a certain part of your wall, before the warm air has a chance to cool down and condense.

Vapor diffusion is the movement of water vapor molecules through porous materials. To achieve an equilibrium water vapor typically moves 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 in the warm interior air is driven to the cooler exterior. In hot climates, water vapor in the warm exterior air is driven to the cooler interiors.

Now let’s look at the US perm ratings of the most common building materials. The lower the number, the less likely it is to allow water vapor through. The first classification includes materials with a perm rating of 0.1 or less. They are considered vapor impermeable or a vapor barrier, they barely allow any vapor through. They include glass, 1 mil aluminum foil, cork, rubber or vinyl flooring, 6mil polyethylene, sheet metal, foil faced polyiso. The second classification includes materials with 0.1 to 1.0 perms. They are considered semi- impermeable to water vapor like vapor retarder latex paint, oil based paint, asphalt coated kraft paper, foil faced fiberglass insulation, 2” closed cell spray foam, 1” XPS, plain kraft facing on insulation. The third classification includes materials with 1.0 to 10 perms. They are considered semi-permeable to water vapor like 3” EPS, OSB, tar paper paper, 2” MDF wood, some fluid applied vapor retarders, latex paint, plywood sheathing. The last group of materials have a perm rating over 10. They are considered vapor permeable like wood siding, zip coating, plaster on metal lath, mineral wool, cellulose insulation, brick veneer, Tyvek.

SOURCES:
Rockwool graphic: https://www.patreon.com/posts/demistifying-45857926?utm_medium=clipboard_copy&utm_source=copy_to_clipboard&utm_campaign=postshare
https://www.youtube.com/watch?v=-6T3ICXWqjc
https://www.finehomebuilding.com/2019/01/03/crash-course-in-control-layers
https://www.rockwool.com/siteassets/o2-rockwool/documentation/technical-guides/commercial/vapor-diffusion-and-moisture-management-technical-guide.pdf
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#buildingscience #permeability #diybuild #permratings #permeance #construction #architecture #design #diy

Видео What are Vapor Barriers, Retarders & Perm Ratings? канала Belinda Carr