Welcome to the Sto Corp. Dew Point Analysis Application
This application is designed to predict possible condensation in exterior walls as a result of water vapor diffusion. In order to acquire the necessary data, the application will guide you through a series of steps. Once you have completed each step press the "Continue" button located in the top right portion of the screen. PLEASE REVIEW THE INFORMATION BELOW CAREFULLY. It outlines technical information as well as the limitations of this application.
 
 

This application utilizes some of the key material properties of many of the most common building materials. The data has been compiled from:

  • ASHRAE Fundamentals - 2001 *
  • Moisture Control in Buildings  **
  • NOAA  ***
  • Manufacturer data

When using this application, the following limitations should  be understood:

While a conscientious effort has been made to obtain the most current and accurate properties of the most common building materials, material properties, specifically, Permeance and R-Value, are difficult to measure and very sensitive to small changes in materials. Therefore, these values should be considered a reasonable representation of the material property, and not an absolute measure. Where applicable you may substitute your own data in place of the default data provided. When substituting, make certain that the units are represented in the same US Customary units as the default data. Keep in mind that R-Value and Permeance values are often "thickness dependent." Be sure to provide values that are appropriate for your material thickness.

Climatic data is based on historical records and climate maps maintained  by US and Canadian goverment agencies. Design default values for exterior temperature and relative humidity are mean monthly values for various climatic zones within the US and Canada. Design default values for interior temperature and relative humidity are based on ASHRAE recommendations for occupant comfort. Where applicable, you may substitute your own data in place of the default data provided.

 

This application utilizes the "Dew Point Method" to calculate the dew point of a given wall assembly. The usefulness of the Dew Point Method not only depends on the judicious selection of boundary conditions and material properties, but recognition that it is a steady-state method. In other words it provides a "snapshot" of water vapor diffusion under specific climatic conditions at an instant of time, opposed to a "motion picture" through daily and seasonal weather change. Furthermore, the effects of other moisture transport mechanisms such as gravity, kinetic energy, and air leakage are not taken into account. Analytical tools such as WUFI, available through Oak Ridge National Labs, are better suited for these types of wall analyses.

Other factors can and usually do have a far greater influence on the potential for moisture accumulation in wall assemblies than condensation from water vapor diffusion. Design and construction detailing (particularly involving joints, flashings, and cladding terminations), components such as windows, doors, or HVAC units, and the effects of air leakage are generally more important considerations than water vapor diffusion. For more information refer to the following Tech Hotlines at www.stocorp.com :

1. Critical Details Checklist for Wall Assemblies

 2. Moisture Control Principles for Design and Construction of Wall Assemblies

If you have questions or comments on this application please visit the StoKnowledgeBase where you can find answers from previously asked questions or "Ask a Wizard" and receive a response from us within the next business day.

*. Handbook of Fundamentals, American Society of Heating, Refrigerating and Air Conditioning Engineers, 1791 Tullie Circle, NE, Atlanta, 2001.

**. Moisture Control in Buildings, edited by Heinz R. Treschel, American Society for Testing and Materials, Philadelphia, 1994 .

***. National Oceanic and Atmospheric Administration, www.noaa.gov.