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Building Water Vapor Transmission & Condensation Analysis

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1. This computer program is designed to calculate the heat loss or heat gain across constructions such as walls, roofs and floors using the R values of the entire construction.. Based on those calculations the program then calculates the surface temperatures across the construction. Using the input air properties on each side of the construction and the perm values the program determines the water vapor transmission rate and the surface dew point temperatures to check for possible condensation. This program has been used to look for potential problems in new constructions, and it has been used to determine why there is condensation in existing constructions and to identify a cure to the problem.

Understanding the Program

1. This computer program is designed to calculate the heat loss (or heat gain), the surface temperatures of the materials used in the construction, the amount of water vapor flowing through the construction and last of all it determines if there is condensation in the construction. The program can be used on walls, roofs and floor constructions.

2. The program requires the input of the materials of construction, the thickness of each component in the construction, the "R" Value (Resistance to Heat Transfer) for the specified thickness, the perm (or permeance) of each component at the specified thickness, the outdoor dry bulb and relative humidity, the indoor dry bulb and relative humidity and the Elevation above sea level to establish the barometric pressure. The program calculates the indoor vapor pressure and the outdoor vapor pressure from the dry bulb and relative humidity.

3. The program adds up the column of component "R" Values (Resistance to Heat Transfer) to get the total resistance to heat transfer, and it adds up resistances to water vapor transmission.

4. The quickest way to spot condensation in the construction from a computer printout data sheet is to look for a series of four stars on the right side of the program next to the column called Surface Dew Point. If four stars appear that means that there is condensation in that area of the construction.

5. Another way to look for condensation is to compare the Surface Temperature to the Dew Point Surface Temperature. If the Surface Temperature is colder then the Dew Point Temperature condensation will occur. Each component in the construction can be broken down into smaller components to help determine how deep the condensation is in that particular component.

6. Another way to look for condensation is to compare the Calculated Surface Vapor Pressure (Pw –inches of mercury) to the Saturated Surface Vapor Pressure (Px –inches of mercury). If the Calculated Surface Vapor Pressure is greater than the Saturated Surface Vapor Pressure condensation occurs.

Understanding the Water Vapor Transmission

Vapor Migration Calculations (PDF)
Condensation Analysis of Supermaket Walk-In Cooler Floor (PDF)

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Green Bay, WI 54308