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Building Science

Flux: A Building Science Term Defined

Understanding the intricacies of how moisture, air, and heat flow through surfaces

Flux describes the rate of flow per unit area and applies to anything that flows, like heat, air, and moisture [Image by Chetvorno, public domain]

At first glance, the word “flux” looks like it may not be appropriate in polite company. In fact, though, it’s a word used in physics all the time. Flux comes from the same Latin root that gives us “flow.” The two are still intimately connected because things that flow can be quantified by their flux. And that’s where building science comes in.

You may have already dealt with flux and not realized it. One of the fundamental principles of building science is controlling the flow of moisture, air, and heat. Let’s zero in on heat. When we talk about heating or cooling loads, we specify them—using those annoying imperial units—in British Thermal Units per hour (BTU/hr). That’s a rate characterizing the flow of heat per unit time.

Taking it a step further, we can divide the time rate of heat flow by the area that it flows through. For heat, we’d end up with BTU per hour per square foot (BTU/hr/sf). And that quantity is the heat flux. But it doesn’t stop with heat. We can calculate the flux of air, moisture, electric field, energy…anything that flows. That’s all there is to it. (Well, OK, there may be a bit more to it, but this isn’t the place for a lesson on vector calculus.)

I was reminded of this recently when I was reading some old research papers from the Florida Solar Energy Center about attic ventilation as I was preparing my article on powered attic ventilators. One of the things they studied back in the 1980s and ’90s was the heat flux through the ceiling for attics under various conditions: unvented, passively vented, and power vented. Naturally, the reason behind the studies was that everyone—building scientists, occupants, and especially the people who have to go up there—wants to get the flux out of the attic.

Bloodletting flux
Watching the flux of blood spill into the barber’s bucket

As mentioned above, we can calculate the flux for any quantity that flows. Air flow is another important building science concern. The result of a blower door test is an air flow rate: cubic feet per minute at 50 Pascals of pressure difference (cfm50). We can factor in the size of the building in various ways, but one of the best is dividing the air flow rate by the building enclosure surface area. The (I-P) units for this would be cfm50 per square foot, and that tells you the air flux through the building enclosure. Phius uses that as a passive house metric for airtightness.

Now, whether you wanted to or not, you know this term that occasionally pops up in building science. Feel free to use it in polite—or other—company and explain the intricacies of moisture, air, and heat flow through surfaces.

If we lived in 14th century Europe, you would probably want to be careful how you used the term. Apparently it meant “abnormally copious flow” back then, and given what we know about how weird those people were, you might end up watching the flux of your blood into the barber’s bucket if you said the wrong thing.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.

One Comment

  1. Charlie Sullivan | | #1

    I appreciate the history of this term (as well as the explanation of it).

    Warning: what follows is a long tangent of no practical importance in building science.

    I find the distinction between how the term is used in thermal and fluids, vs. in electromagnetism an interesting annoyance. In both fields, it's precisely defined so as to distinguish between the total amount vs. the per-unit-area amount. But which one is called "flux" is opposite in the two fields. In thermal and fluids flux, is the per-unit-area flow, whereas in magnetics, flux is the total, obtained by integrated the per-unit-area quantity, called flux density.

    So how did this split arise? The 14th century meaning seems to be total, not per unit area, so that seems to legitimize the electromagnetic convention, but on the other hand, Maxwell, who worked in both fields, seems to use the per-unit-area definition for both fields--he talks about integrating the flux to get the total. The previous reference for heat flux in the OED is from Tyndall, whose mentions of it are often qualitative enough that it could be either. However, he closes his "Heat Considered as a Mode of Motion" with,

    "The law of conservation rigidly excludes both creation and annihilation. Waves may change to ripples , and ripples to waves , -- magnitude may be substituted for number , and number for magnitude , asteroids may aggregate to suns, suns may resolve themselves into flora and fauna, and flora and faunæ melt in air, -- the flux of power is eternally the same . It rolls in music through the ages , and all terrestrial energy , —the manifestations of life , as well as the display of phenomena , are but the modulations of its rhythm ."

    Conservation applies better to flux as the total quantity than flux as a density, so I'm putting him in the total camp and blaming Maxwell for defining it as density in both thermal and electromagnetics. But if Maxwell is the father of modern electromagnetics, why has electromagnetics reverted to using "flux" to refer to the total? It seems that Oliver Heaviside, who reformulated Maxwell's tangled mess of equations into the compact set of four we call "Maxwell's Equations", also chose the modern use of flux as total and "flux density" for the per unit area quantities. He even devotes most of a page (p. 27) of his 1894 "Electromagnetic Theory" to explaining that choice.

    Oh, and the first use I could find for the term "thermal flux" was referring to heat-induced diarrhea in sheep.

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