It is inevitable

Posted 1/18/23

Whenever I talk with someone about the need to make our built environment more airtight and highly insulated, the question always pops up: “But houses have to breathe!”

Allison A. …

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It is inevitable


Whenever I talk with someone about the need to make our built environment more airtight and highly insulated, the question always pops up: “But houses have to breathe!”

Allison A. Bailes III, Ph.D., in his new book “A House Needs to Breathe… Or Does It?,” thoughtfully challenges that question. This is a book about building science, a much misunderstood field of study that is slowly finding its way into helping our building industry build better, healthier, more resilient structures in both the commercial and residential markets.

We started to see health problems—for both our buildings and their occupants—in the mid-to-late ‘70s; and in the ‘80s, we saw an explosion of issues with indoor air quality (IAQ). As we insulated to counter the financial pain of the rising cost of heating fuels, we began to experience the growth of mold and mildew in our homes. The occupants of our buildings began to experience more respiratory problems, and the structural elements of our buildings—notably the wood studs that created our exterior walls—began to succumb to decay.

We discovered that this was caused by water vapor migrating to the interior of the walls. So we “fixed” the problem. To keep water vapor out of our walls, vapor barriers were invented and building codes required their application to the warm-in-winter wall surface. Problem solved!

Some reading references

“Essential Building Science,” by Jacob Deva Racusin. Published by New Society Publishers,

“A House Needs To Breathe… Or Does it?,” by Allison A. Bailes III, Ph.D.,

“Pretty Good House,” by Dan Kolbert, Emily Mottram, Michael Maines and Christopher Briley,

Or was it?

As the building boom of the ‘80s spawned more structures, building codes sort of evolved. They focused on important stuff—“life safety”—and centered their code requirements around how to keep our buildings from falling down on us (structural failure) and how to keep our buildings from burning down (fire prevention). And knowing that even the best-designed building could catch fire, the codes focused on egress, how to get the occupants safely out of a burning building.

Life safety—I like it!

But what about water as a destroyer of buildings?

As we continued to experience water-related damage to our buildings and the subsequent health effects, the ‘80s became the era of Stachybotrys chartarum, the infamous black mold that grew on walls and ceiling.

How could this be? Codes began to focus more on how to keep bulk water out of building assemblies, but were still not recognizing the relationship of water vapor to building resiliency—or the lack of resiliency.

Simply put, our codes did not focus on an awareness of building science, the study of the physics of how air, water, water vapor and heat move in and through our built environment, a body of knowledge now called hygrothermal dynamics.

Sir Isaac Newton’s first and second laws of thermodynamics lay the foundation for building science. The first law simply states that energy is neither created nor destroyed; it just changes from one form to another. Visualize a tray of water transforming to ice and then back to water, or the steam rolling off of a boiling pot of pasta water. That water molecule is doing something: transforming, moving, settling on a cold wall and condensing.

Building science helps us understand how this amazing source of life—water—can transform from solid to liquid to gas, and find its way into places that we do not want it to go.

Such as into unseen wall, floor and ceiling cavities.

The second law is termed “enthalpy,” which very simply put, means that energy seeks equilibrium. Warm air always moves to cold air, wet surfaces always move to dry surfaces, high pressure always moves to low pressure.

Understanding how air, water, water vapor and heat move through our buildings opens the door to a way in which we design and build to control the unwanted movement of water and water vapor through our structural assemblies.

Knowing and understanding the relationships of defined control layers (thermal, air, water, vapor) is the stuff of building science that guides us to healthier and more resilient buildings. Building science requires us to check our egos at the design door. If we know and accept that we cannot stop the second law of thermodynamics, then we can learn the best methods to manage it.

And no, buildings do not need to breathe. They need to be ventilated, so that the occupants can breathe healthy, clean air!

insulation, building science, indoor air quality


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