• Here’s the rap I’ve given a hundred times. Can you get condensation on a mirror? (Show a mirror. Heavy breath on the mirror. Note the fog.) Yes, you can get condensation on a mirror. 

    Can you get condensation on a sponge? (Show a sponge. Heavy breath on the sponge. Note that the sponge is unchanged. Second heavy breath, still unchanged.

    My colleague Paul Francisco says yes you can get condensation on a sponge. How? (Take a mirror.) Take a mirror. (Heavy breath on the mirror.) Get condensation on the mirror. Wipe the mirror with a sponge. Now you have condensation on a sponge. Otherwise, NO, you can’t get condensation on a sponge. Something happens to the sponge. What happens is not condensation.

    Water molecules have slightly unbalanced positive and negative sides, so they act like little magnets. They can attach to things, and to one another. They also are very active, and have been since the big bang, maybe a little later. The most active water molecules are unattached to one another and form a gas in the atmosphere. Less active water molecules are liquid, loosely attached to one another, and the least active form is ice. 

    They can also attach to binding sites on things in the environment. The binding sites on mirrors are very weak, weaker than the water-to-water bonds in fact. The binding sites on sponges are very strong, so strong they may suck vapor molecules from the air.

    Image from T. S. Rogers Thermal Design of Buildings (1938). The author was showing the dependence of condensation formation on the indoor humidity. Note that the condensation was on glass, not on sorptive building materials.

    Condensation is a phase of pure water. (Physicists of course have a much broader definition of condensed matter for all matter, not just water.) It may occur in the atmosphere as rain, or on a weak-bond surface such as glass, metal, plastics, glazes and some coatings such as epoxy and lacquer.

    What happens on sponges is not condensation. Sorption is a good word to describe the attachment of water to strong-bond surfaces, and I like to call such water bound water. Many building materials such as wood, brick, concrete, paper and gypsum have strong-bond surfaces, so water binds to them.

    I’ve heard some say the problem then is not condensation but “moisture accumulation.” Let’s make this easy. Walls, roofs, building assemblies are composed of materials that have moisture contents. And the moisture content changes, it goes up and down. Moisture accumulation is a phenomenon that occurs exactly half the time, naturally. Just think if all moisture change had to be moisture loss–not very practical.

    The period immediately before and after World War II was the heyday of bad building science, in the US at least (more on that later). We have not yet undone the damage to our common understandings of buildings that was done back then, often by well-meaning but sales-oriented building professionals. In the late 1930s, “foil” insulation was sold, which provided a series of radiant barriers. They cited an advantage of their products over fiberglass as restricting the movement of water vapor, which, according them, would otherwise cause “condensation”. Tyler Rogers was a Technical Director at Owens Corning Fiberglass beginning in 1938. His product was the target of the “condensation” accusation. So he essentially invented the addition of a “vapor barrier” in insulated wall assemblies, and prescribed attic ventilation—sort of a vapor barrier for the ceiling. 

    Did anyone actually see condensation in building assemblies? Of course not, any excess water got quickly absorbed, almost always with complete safety. Looking for condensation is a snipe hunt. What’s that? Ask a Boy Scout from the 1950s; none of them ever found a snipe. What, after all, would condensation in a building assembly actually look like?

    No, the prediction of condensation hinged not on visible phenomena, but on a crude analytic tool that became enormously popular. I call it the “profile method”, or “steady-state diffusion analysis”, though others call it the “Glaser method” or (I cringe at this) the “dew point method”. It was codified in the 1946 ASHRAE Handbook. The actual copy is shown in two figures below.

    It goes like this. Pick design indoor and outdoor conditions of temperature and humidity. List the insulating R-value of the components in series. Construct a temperature profile through the wall. Assign a “saturation vapor pressure” profile to the temperature profile (more on this later). Calculate the indoor and outdoor vapor pressure (more on this later), and construct a vapor pressure profile through the wall using the permeance of the materials (more on this later). 

    Ninety-nine percent of users will note that, without Rogers’ measures of vapor barriers and attic ventilation in place, the two lines will cross, somewhere in the middle of the insulation. They intone “AHA!”, with actual vapor pressure in excess of saturation vapor pressure, and that becomes the brand-new definition of “condensation” or “reaching dewpoint”. We now have a definition (for these 99%) of “condensation” that is entirely divorced from phenomena in the real world. And these 99% are awfully smug.

    The other 1% will have actually read the rest of the ASHRAE example. It goes like this. If these two lines cross, then the vapor pressure value at the inside of the sheathing is lowered down to the saturation value at that location, in order that the lines not cross. Using the new “actual” vapor pressure at that location allows a user to calculate moisture flow to that plane and away from that plane, and the difference is a rate of moisture accumulation at the plane. There is an unfortunate use of the term “condensation” in the ASHRAE description of the method. But the tool works in a way that the absence of condensation—sorption instead—is expected, and the rate of accumulation is the principal output. Nice.

    But hold on. Isn’t that cheating, to artificially lower the actual vapor pressure in the insulation cavity down to the saturation vapor pressure at the cold boundary? Who on earth would do such a thing? Mother Nature would do such a thing. Measure the actual vapor pressure in a building cavity, and you’ll find that the upper bound for vapor pressure is the saturation vapor pressure at the coldest surface—the sheathing. The air of a cavity is VERY permeable to vapor flow, so the sheathing surface temperature governs what the maximum dewpoint in the cavity can be.

    The 1952 Condensation Conference, which showcased the new methods of vapor barriers and attic ventilation, was kicked off by Leonard Haeger from the National Association of Homebuilders, who came on board with the practices. He said “I suppose in the beginning we should come up with a definition of condensation. To a working man, condensation is what happens on a highball glass at 5:30 in the afternoon.” He failed to note that his constituency does not build houses out of highball glasses. He might have also pointed out that building materials have a lot more in common, not with the highball glass, but with its pressed-board coaster protecting the furniture from condensation damage.

    I will be writing much more about the topics raised here. These include: psychrometrics, history of building practices, vapor permeance of materials, analytic methods (2 and 3 dimensions, and transient (video) instead of steady-state (photo)), and fear as a marketing tool. I apologize to newcomers to building science for using what are likely unfamiliar terms. Hang in there.

    It drives me up the wall to still hear people say, three generations later, “where do you reach dewpoint?” In the 1990s, ASHRAE Handbook removed the profile method. I was chair of the committee at the time. I question that decision now, made at the behest of users and developers of more helpful transient analytic tools, although required input values for the example were no longer supported by ASHRAE. The profile method is a building block for more advanced methods, and any transient modeler must be intimately familiar with steady-state methods. It is an indispensable teaching tool, provided the vapor pressure line adjustment is assigned at least as much importance as the initial two-line comparison.

    I tell my students–your ability to solve water problems in buildings is inversely proportional to the frequency of your use of the term “condensation.”

    Example of the profile method shown in ASHRAE Handbook Fundamentals from 1946 to 2001. Note that there are two lines shown of vapor pressure–the upper one providing flow continuity and the lower one showing equivalence of saturation vapor pressure and actual vapor pressure at X – X. A “rep” is a unit of permeance resistance, the reciprocal of permeance.
    Continuation of the ASHRAE Handbook Fundamentals example. Note that the outcome of this calculation is not the appearance (or not) of condensation, but rather the rate of accumulation at the critical X – X plane. The unit “gr” is not grams; it is “grains”, an avoirdupois measure equal to 1/7000 of a pound.

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