Building Envelope Moisture and Heat Transfer Analysis Tool
Zhipeng Zhong, Lingfeng Ma, and James E. Braun
Table of Contents
MOIST 4.0 predicts heat and moisture performance for a single exterior wall of any construction. It is an updated version of MOIST 3.0 that was developed at the National Institute of Standards and Technology (NIST) and is based on one-dimensional heat and moisture transfer. The user can specify a wall construction and orientation, a location, and an indoor condition. The program uses hourly weather data for ambient temperature, humidity, wind speed and direction, and solar radiation to estimate time and spatial variations in wall material temperature and moisture content. In particular, the program is useful in identifying situations that can lead to mold problems.
Currently the Building Moisture and Heat Transfer Analysis Tool program is available for use on Microsoft Windows Platforms only. It is compatible with all current versions of Windows till Windows Vista. JAVA Runtime Environment (JRE) is a prime requirement for the software to work on and the computer you are installing the software on must have the JAVA Runtime Environment (JRE) installed. Installer for Building Envelope One Dimensional Moisture and Heat Transfer Analysis Tool is available with integrated JRE. It is recommended that the most updated version of JRE be installed, look at the Downloading the Application section for details.
The user can download the Application Installer from the following link for free:
The user will have the option to download a plain installer and a installer with integrated JAVA Runtime Environment (JRE). Choose the installer depending upon the current status of JAVA environment in the userís computer after consulting the Compatibility section.
For Getting the updated JAVA version user can refer
to the following web site:
1) Click on the installer the user has chosen to launch the application. Click on I Agree to accept the conditions laid by the developers and proceed with the installation.
2) Choose the Install components and Click on Next to proceed.
3) Browse for the Directory in which the user needs to Install the files and click Next to Start with the automated process.
4) Click on Close Button to finish the Installation process. Now the user can access the program from the start menu.
**** If the user chooses to Install through the JAVA integrated Installer then the JAVA installer will pop up in Between the above process and the user needs to complete it before proceeding to the next step in the above installation.****
1) Start the Uninstall program either from the Start menu Shortcut item or from the Control Panel ---> Add or Remove Programs
2) Click on the Uninstall button to start the automated uninstall process.
3) Click on the Close button to finish the uninstall process. Building Envelope One Dimensional Moisture and Heat Transfer Analysis Tool has been successfully uninstalled.
The File menu contains the following functions:
Open: Open a previous project file that has been saved.
Save: Save the current input parameters and calculated results.
Exit: Quit the program and exit to windows.
The Help menu contains the following functions:
Help: Open the Help file in the default web browser on the userís computer.
About: Display the current version number for the program, a copyright notice and credits.
In the various Input Tabs explained below, the user can specify various parameters used in calculating heat and moisture transfer performance of a wall construction.
The Main Page is used to specify general simulation parameters.
Project Name: The project name is included with the information that is stored using the Save feature.
Location: The user can select from 239 TMY2 weather files provided by the U.S. National Renewable Energy Laboratory (NREL) for the United States and its territories. The TMY2 data are derived from the 1961-1990 National Solar Radiation Data Base (NSRDB) by NREL. The hourly data is used by the program for the external wall boundary condition and includes ambient temperature and humidity, solar radiation, and wind speed and direction.
Time Step: The user can choose a time step that is employed for the numerical simulation. In general, smaller time steps result in more accurate results but require greater computational time. Different wall constructions and node specifications (see Wall Description) require different time steps for acceptable accuracy. For a particular wall construction, the user can determine an appropriate time step by reducing it until the results do not change.
Compute Temperature: The user can on/off toggle the calculation of temperature profiles within the wall. When this feature is toggled off, then only moisture transport is considered.
Capillary Transfer: The user can on/off toggle moisture transport by liquid capillary transfer. When this feature is toggled off, then only vapor diffusion is considered as the mechanism for moisture transport.
Latent Heat: The user can on/off toggle the energy effects of phase change of water. When this feature is toggled off, then the energy effects associated with moisture condensation, evaporation, freezing and melting are not considered.
Indoor Temperature: This specifies a fixed indoor temperature boundary condition for the wall.
Indoor Relative Humidity: This specifies a fixed indoor relative humidity boundary condition for the wall. BMOIST should be employed to consider variable indoor conditions.
Wall Azimuth: This specifies the orientation of the wall relative to north measured in a clockwise direction. North facing is 0 degrees, east is 90 degrees, south is 180 degrees, and west is 270 degrees.
Wall Tilt: This specifies the slope of the wall relative to a horizontal surface. A vertical surface has a 90 degree tilt.
Users can select an existing wall construction from a library or create their own walls and have them added to the library. Each wall includes parameters associated with the interior and exterior surfaces and properties for the different layers that make up the wall. When using an existing wall, the user may edit any of the parameters for the existing wall layers or interior/exterior surfaces but may not change the number or type of layer materials. To consider alternative constructions, a new wall should be created and added to the library.
The following text describes the properties that are used in specifying a wall.
Surface Convective Heat Transfer Coefficients: Both interior and exterior surfaces include a convective heat transfer coefficient. The heat transfer coefficient for the interior surface should be appropriate for natural convection, whereas the exterior surface coefficient includes the effects of wind.
Permeance: The permeance is a measure of the moisture transport capabilities of a layer or surface coding operating under steady conditions. Values of permeance are used for interior and exterior surfaces and layers within the wall that do not have significant moisture storage capabilities but have significant resistance to moisture transport. The interior and exterior permeance values are associated with surface coatings, such as paint or wall paper. Permeance is measured in units of perm, which is a water vapor transmission rate for a given differential in partial pressures across the material or membrane. The US perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury. 1 US perm ≈ 57.2 ng∑s−1∑m−2∑Pa−1. The table below gives permeance values for some common non-hygroscopic materials.
Table of Permeance values for some common non-hygroscopic materials (Burch, 1995)
19-25 mm Vertical Air space
Building Insulation R19
Building Insulation R11
Solar Absorptance: The exterior wall surface properties include a solar absorptance, which is the fraction of incident solar radiation that is absorbed by the surface.
Layer Properties: The wall layers are ordered from the interior to the exterior side of the wall. The layer names refer to common materials used for walls and have hidden properties associated with them. Thermal and moisture properties are stored within a database for a number of common layer materials. For layers that have the ability to store energy and moisture, the user can change the thickness of the layer and the number of nodes used in the numerical model for that layer. Materials that do not have significant storage are specified as "NON-STORAGE MATERIAL" and utilize overall thermal resistance and permeance as parameters (instead of thickness and number of nodes). For example, an air gap could be modeled as a non-storage material. Also, insulation is often modeled as a non-storage material.
A wall can have up to 8 layers. Layers are specified from interior to exterior by selecting from a database of layer materials. Each layer has default values for thickness and nodes if it is hygroscopic (i.e., has significant moisture storage capability) or thermal resistance and permeance for non-hygroscopic (non-storage) materials. There are also default values for the interior and exterior surface properties (convection coefficients, permeance, and solar absorptance). The user needs to specify a name for a new wall before it is stored within the database.
Deleting a Wall Construction:
This allows the user to delete any existing or new wall construction from the library.
Annual Average Temperature: Ambient air temperature averaged over the entire year (in units of įF).
Annual Average Relative humidity: Ambient air relative humidity averaged over the entire year.
Annual Total Number of Hours of Rain: The total number of hours when rain occurred.
Annual Total Precipitation Amount: Annual total rainfall (in inches).
Number of Hours over Threshold for Mold Growth: Mold growth can occur within a building material when the moisture content is above a threshold for extended periods of time. A typical threshold for mold growth would be an equilibrium relative humidity for the layer of 80%. Equilibrium relative humidity is a relative humidity of air surrounding the layer that would yield no moisture transport at its existing moisture content. This particular summary result gives the number of hours during which any layer within the wall was above an 80% equilibrium relative humidity threshold.
The Output tab allows viewing of plots for wall and ambient quantities with different time scales. The user can specify average hourly, daily, weekly, or monthly outputs and can vary the period for displaying the plot (see zooming in and out).
Inside Wall Outputs: Plots of temperature or moisture performance are generated for different locations within the wall. By default, wall results are averages for each of the layers within the wall. However, results for any layer can be expanded to show individual nodes within that layer (see expanding wall layers into nodes). Two different measures of moisture performance are presented. When "Moisture Content" is selected then results are expressed in terms of the mass of water stored as a percentage of the dry mass of the material. An alternative choice for moisture performance results is "Equilibrium RH". Equilibrium relative humidity is a relative humidity of air surrounding the layer (or node within a layer) that would yield no moisture transport at its existing moisture content. Since different materials have different abilities to store moisture, equilibrium relative humidity is a more useful measure of the relative moisture content for any material. In fact, an equilibrium RH greater than about 80% for an extended period of time can lead to mold growth regardless of the absolute amount of moisture that exists within the material.
Ambient Outputs: Choices include outdoor air temperature, outdoor humidity ratio, and rain intensity.
Zooming In: The user can zoom in by selecting the zoom range window (left click on the chart and drag the mouse towards the right and down over the area where you want to magnify). The enlarged result will show detailed information within the reduced range.
Zooming Out: Return to the original output scale by left clicking on the chart and dragging the mouse towards the left and upwards.
Expanding Wall Layers into Nodes: The default for plotting wall results is to show averages for each of the layers within the wall. In order to view spatial profiles within each layer for hygroscopic (i.e., moisture storing) materials, the user can expand the nodes within each layer by clicking the mouse on the plot line corresponding to the layer of interest.
Collapsing Wall Nodes into Layers Plot lines for individual nodes of a layer can be collapsed by clicking on any of the plot lines for the layer.
free to contact the undersigned for any questions or information regarding the
Prof. James E. Braun
Ray W Herrick Laboratories
School Of Mechanical Engineering
West Lafayette, Indiana 47906
Email : firstname.lastname@example.org
The user can also visit our webpage for download links, documentation, posting comments and more updated information regarding the software at http://engineering.purdue.edu/MOIST.