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Should we eliminate the term “breathable” from our discussion of walls and buildings?

Diagram of a breathable wall

As much as building scientists and energy nerds hate it, “breathable” is a term that does not seem to go away. While this terminology is on the decline, its still a favorite for those interested in mostly natural building techniques.

This isn’t a debate between natural building versus conventional methods as there are many paths to green and healthy building. Energy geeks point to a building’s monthly energy use as the biggest source of our environmental footprints while its tough to beat the sustainability aspects of a small, owner-built home of natural materials that is easily changed or remodeled in the future.

What does breathable mean to you?

The biggest problem with breathable, is that it means different things to different people.  Are we talking about air movement, moisture or Indoor Air Quality? Its common for proponents to make claims that breathable materials are the answer for all these characteristics.  If so, its important to be able to measure said “breathability” to evaluate, duplicate or improve such breathable components.

I think the misuse of this terminology falls into two main camps: “buildings need to breathe” and “walls need to breathe”. While most people accept the broad terminology replacement of  ”houses need to breathe” with build tight, ventilate right there is still plenty of confusion surrounding the benefits of wall and building assemblies that can dry out or “breathe”.

“Buildings need to breathe” is extinct..  almost.

Its been an uphill climb but most folks now recognize the difference between houses need to breathe and build tight, ventilate right.  This school of thought and direction of the building codes revolves largely around Indoor Air Quality IAQ. Reducing Indoor Air Pollution should be a major goal for anyone involved in building and remodeling even for those that don’t care much for reducing environmental impacts. Everyone can appreciate healthy indoor air.

Building tight is one of the most cost effective approaches for reducing monthly energy use and it also gives us more control over how we ventilate a home or building to provide the best possible Indoor Air Quality. Control is the main goal and key word in this strategy. Random, uncontrolled air leaks reduce comfort and durability, increase energy costs and can negatively effect indoor air quality in some fairly serious ways.

“Walls need to breathe”

Other terms used for “breathable” walls include “air open” and “dynamic”. Its tougher to dismiss the term “breathable” from our discussions of wall performance but most experts will avoid its use. To me, breathing involves two different but often related forms of physics when discussing wall properties: Air movement and moisture transport. One of them is generally a good guy while the other one is generally a bad guy (or girl). So much for the de-personification of the physics relating to our structures..

Uncontrolled air movement through walls is almost always a bad thing.  Infiltrating and exfiltrating air can carry large amounts of water vapor which can deposit and accumulate on cold surfaces inside the wall or building assembly. This can result in mold, reduced durability and premature structural failure. Air movement is the second biggest moisture concern after bulk water leaks like leaky plumbing and poor flashing. Uncontrolled air movement is generally more dangerous than moisture transfer through vapor diffusion.

Diagram of breathable wall with drywall and amount of water resulting from small air leak.

Amount of potential water introduced by air leak of a hole as small as 1″!

Amount of potential water from vapor diffusion.

These diagrams are from Joe Lstiburek’s Builder’s Guides. This is a great visual representation of why vapor diffusion is almost irrelevant compared to uncontrolled air leaks.

Moisture transport or water vapor diffusion is where things get more technical and confusing to most people. The persistence of the term “breathable” is most often associated with these characteristics when discussing walls and building assemblies. The main thing to remember is that air leaks are the main moisture concern to be aware of when talking about the “breathability” of walls.

Change your terminology from “breathable” to “vapor permeable”.

This is the easiest switch in terminology for those concerned with these issues. Vapor permeability is one of the most confusing and misunderstood properties in building science. The essence is this: the more vapor permeable a material, the easier it can dry out.

Other than below grade applications where you want to block moisture movement from the ground, vapor permeability is almost always a good thing. One of the biggest differences I see between natural focused builders and more conventional ones is that natural builders generally do not want ANY vapor impermeable membranes or layers in their above grade building assemblies. This is smart as natural materials are often more prone to damage from repeated wetting events and moisture.

Most building experts are fine with one layer of impermeable (semi permeable or semi impermeable) material along with the right details. The unanimous advice is to never create a sandwich of moisture intolerant materials between two layers of impermeable membranes. The wall or assembly should be able to dry to at least one direction.

I hope that wasn’t overly technical but if you enjoy the technicalities of vapor permeability and desire a better understanding along with the appropriate concerns in our climate, be sure to check out Amy’s blog entry on Hybrid walls.

The main idea of getting away from the term “breathable” is to use terms that have an agreed upon meaning and are easily measured.

Air movement is measured by CFMs (Cubic feet per minute).

Vapor permeability is measured in Perms.  Building Science.com  Info sheet on Perms

Its easier to have debate and discussions on wall and building performance when everyone is using the same language and has a basic understanding of the specific details being discussed. Now if we could only come to an agreement on what “green” means but I wouldnt dare touch that one.

Brian Knight

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