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ASHRAE’s Vapor Pressure Profile

Josh Eaton | Posted in GBA Pro Help on

When using ASHRAE’s Vapor Pressure Profile (Handbook of Fundamentals 2009, 27.8) for a particular wall construction. If multiple material interfaces in the wall construction show a vapor pressure (VP) exceeding the saturation pressure (SVP), then is the most likely condensation plane the interface where VP exceeds SVP by the greatest amount?

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  1. Bill Rose | | #1

    The ASHRAE profile method begins with a calculation of SVP using R-values of the components, and VP using permeance of the components, as you noted. Those lines may cross, and if they do, you’re half-way through with the ASHRAE profile method. Vapor pressure, as we know, cannot exceed saturation vapor pressure. So the following steps involve 1) correcting the vapor pressure value to the saturation vapor pressure value, 2) calculating the flows to and from the plane under consideration and 3) assigning a net rate of accumulation to the surface where this occurs. The output of the ASHRAE profile method is a “rate of accumulation” under the design conditions, not a prediction of “condensation”. The method still contains use of the term “condensation” but that usage is likely going to go away in the 2013 version.

    Cheating? Not at all. This “correcting” is exactly what nature does. Vapor pressure of air in a rather airtight cavity will be in equilibrium with the moisture content of whatever is the coldest surrounding surface. If the critical surface is fully saturated, then correcting VP to SVP represents what happens in nature. More likely, the moisture content of the critical surface is at less than saturation. Then the moisture content-relative humidity (from which you'd derive vapor pressure) relation is expressed in the sorption isotherm; values for the wood sorption isotherm are available through the US FPL Wood Handbook.

    I hope this helps and is not too confusing. Good luck working through the examples. What happens in actual assemblies is very poorly characterized using the ASHRAE profile method. Transient modeling gives more reliable values. There is a strong clamor to abandon the example and the method because it is so flawed. It leads to perceptions of "condensation" even in capillary active materials, in which condensation as the change in phase from vapor to liquid is practically impossible.

    The idea that the ASHRAE profile method "predicts condensation" is wrong. This idea is very widespread, damn near universal. The ASHRAE profile method predicts a rate of accumulation, under steady-state conditions imposed on the example. There's nothing wrong with accumulation--every porous material is accumulating moisture half the time and losing it the other half of the time.

    Bill Rose, (outgoing) Handbook Chair Technical Committee 4.4, ASHRAE Fundamentals, ch. 25, 26 and 27.

  2. GBA Editor
    Martin Holladay | | #2

    Josh,
    I'm glad that Bill Rose showed up to answer your question first, since he is the foremost authority in the country on the topic at hand.

    Now that Bill has explained why the ASHRAE profile method does such a poor job of characterizing real walls, you may want to read Are Dew-Point Calculations Really Necessary?

  3. Josh Eaton | | #3

    Thank You Bill. Your response was a very helpful.

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