Using Psychrometric Data to Locate Condensation Risk

Introduction

Condensation is the process by which moisture forms on surfaces. This occurs when air that is fully saturated with moisture is cooled and the excess moisture condenses on a surface. This happens because, as saturated air cools, its ability to hold moisture is reduced. This is most commonly found on interior surfaces of building exteriors that are in excessively humid, or moist, environments. The greater the frequency and quantity of condensation, the greater the risk to indoor air quality and building structure.

Avoiding Condensation

The basic methods to avoid condensation in buildings are:

  1. Reduce the water vapour (decreasing absolute humidity) by:
    1. minimizing sources of water vapour such as showers, boiling water, and water infiltration; and
    2. removing water vapour using building ventiallation, or dehumidifiers.
  2. Increase the heating level (decreasing relative humidity)

Detecting Condensation

Using the principles of psychrometry are used to determine the effect of water-vapour mixtures and the application of heating, ventillation, and air-conditioning. A psychrometric chart graphically depicts the thermodynamic features of air at various moisture levels and is used to calculate those effects.

Psychrometric Chart

The following ASHRAE-style graph shown below is a graphical equalation of state by representing the physical and thermal properties of moist air. By finding the intersetion of two measurements, four others can be determined:

"PsychrometricChart.SeaLevel.SI" by ArthurOgawa - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0-2.5-2.0-1.0 via Wikimedia Commons -

Using Electronic Instruments

While pyschrometric charts are essential learning tools, they are cumbersom and time consuming to use in real-world situations. Highly specialized and precise electronic instruments may be used to record and automatically calculate these properties. In the following image, surface temperatures were plotted relative to the dew point (at which condensation occurs) at multiple intervales. The fourth reading in this chart shows a high risk of condensatio while the third and fifth readings were a cause for conern because the temperature differential was only 4 degrees Celcius.

Summary

Psychrometric analysis is an essential diagnostic tool used for evaluating the performance of building designs and existing conditions. It is used to identify conditions of condensation risk and is useful in formulating solutions to mitigate the causes.

 

 

 

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