NAT4-.INST." OF STAND & TECH R.I.C.

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PUBLICATIONS

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology

NIST Special Publication 917

MOIST

A PC Program for Predicting Heat and

Moisture Transfer in Building Envelopes

In Release 3.0

Douglas M. Burch and Dr. Joseph Chi

QC

ICQ

.U57

N0.917

1997
rhe National Institute of Standards and Technology was established in 1988 by Congress to "assist industry in the development of technology . . . needed to improve product quality, to modernize manufacturing processes,

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'At Boulder, CO 80303. ^Some elements at Boulder, CO.
NIST Special Publication 917

MOIST

A PC Program for Predicting Heat and

Moisture Transfer in Building Envelopes

Release 3.0

Theory of MOIST developed by

Douglas M. Burch

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

Visual Basic Program developed by

Joseph Chi, PhD, PE

University of District of Columbia

Washington, DC 20008

September 1997

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Gary R. Bachula, Acting Under Secretary for Technology

National Institute of Standards and Technology

Robert E. Hebner, Acting Director
National Institute of Standards

and Technology

Special Publication 917

Natl. Inst. Stand. Technol.

Spec. Publ. 917

47 pages (Sept. 1997)

CODEN: NSPUE2

U.S. Government Printing Office

Washington: 1997

For sale by the ' Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
ABSTRACT

This report is a User's Manual for a computer program called MOIST (Release 3.0). MOIST is a

user-friendly, personal computer program that predicts the one-dimensional transfer of heat and

moisture in building envelopes. With MOIST, you will be able to easily define a wall, cathedral

ceiling, or low-slope roof construction. You will subsequently be able to predict the temperature and

moisture content (or relative humidity) of the individual construction layers as a function of time of

year. In addition, you will be able to investigate the effects of various parameters on the moisture

accumulation within layers of the construction. For example, you will be able to conduct computer

runs for different U.S. and Canadian cities, thereby investigating the effect of climate on moisture

accumulation. You will be able to determine if a vapor retarder is needed, and if needed, where it should be placed relative to the other materials. MOIST allows the user to vary the building

materials and their relative placement and analyze the effect of the water-vapor resistance offered

by paint layers, wallpapers, and vapor retarders.

KEYWORDS: building technology; heat and moisture transfer; heat transfer; moisture analysis;

moisture control; and moisture transfer.

iii

TABLE OF CONTENTS

DISCLAIMER AND CONDmONS vii

SYSTEM REQUIREMENTS vii

ACKNOWLEDGMENTS vii

CHAPTER L INTRODUCTION 1 Brief Description of MOIST 1 Features of MOIST 1 Applications of MOIST 1 Limitations of MOIST 2

Getting Help 2

CHAPTER 2. INSTALLE^JG MOIST 2

Microsoft Windows 95 2

Microsoft Windows 3.1 3

CHAPTER 3. GETTING STARTED 3

Starting the Program 3

MOIST Main Dialog Box 4

Select Units 5

Command Buttons 5

Pull-Down Menus 6

Tool Icons on the Tool Bar 7

CHAPTER 4. REVIEWING THE EDIT DATA SCREENS 9

Construction Data Entry Box 9

Input Parameters Dialog Box 11

Selecting Weather Data for the Analysis 13

Data Box for Specifying Analysis Intervals and Indoor Parameters 14

Data Box for Selecting Output and Analysis Options 18

Material Database Dialog Box 20

CHAPTER 5. RUNNING A MOIST ANALYSIS 23

CHAPTER 6. PLOTTING RESULT GRAPHS 25

CHAPTER 7. GETTING A HARDCOPY OF THE INPUT DATA 26

CHAPTERS. MANAGING FILES 26

Saving a Project to File 27

Opening an Existing Project File 28

CHAPTER 9. ACHIEVING CONVERGENCE OF THE MATHEMATICAL SOLUTION . . 28

CHAPTER 10. MODELING LOW DENSITY INSULATIONS AND AIR SPACES 29

CHAPTER 11. FEEDBACK AND COMMENTS 31

NOMENCLATURE 33

REFERENCES 34

APPENDIX A. THEORY OF MOIST (RELEASE 3.0) 35

APPENDIX B. DESCRIPTION OF FILES IN MOIST DIRECTORY 41

APPENDIX C. USING NON-WYEC WEATHER DATA 45

DISCLAIMER AND CONDITIONS

The input and output processor of MOIST (Release 3.0) is compiled in Microsoft Visual Basic (Version 3.0).

The mathematical calculation algorithms are compiled in Microsoft Fortran (Version 5.0). The American

Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) has consented to permit

NIST to use their WYEC hourly weather data. The MOIST program comes with weather data for forty-six

U.S. cities and five Canadian cities. These weather data will permit you to learn how to use the program and

perform a significant amount of analysis for the United States and Canada.

Certain trade names and company products are mentioned in the text to specify the computer products and

equipment needed to use MOIST. In no case, does such identification imply endorsement by the authors of

these computer products and equipment, nor does it imply that the products are necessarily the best available

for the purpose. Microsoft is a registered trademark, and Windows is a trademark of the Microsoft

Corporation.

The authors have made a concerted effort to find and remove errors in this publication and the MOIST 3.0

software. However, we make no guarantee that they are free from errors or that the results produced with the

software will be free of errors. We assume no responsibility or liability for the accuracy of the software or for

the results which may come from its use.

Some copies of this publication are being disseminated without the diskettes containing the computer software.

For more information or to request the software, contact Paula Svincek; NIST; Room B320, Building 226;

Gaithersburg, MD 20899 or telephone (301) 975-5648 or e-mail moist@nist.gov.

SYSTEM REQUIREMENTS

MOIST 3.0 is designed to run on any IBM-compatible personal computer having a Microsoft Windows

operating system. To run MOIST 3.0, you need at minimum the following equipment:

1) a DX or Pentium microprocessor;

2) Microsoft Windows 3.1, Microsoft Windows 95, or Microsoft Windows NT;

3) Five hundred kilobytes free memory available to execute a program from a DOS Window; the MOIST

Program may run under less memory but problems may occur.

4) a hard disk with seven megabytes of available memory space;

5) a VGA or higher monitor; and

6) twelve or more files specified in the CONFIG.SYS file.

ACKNOWLEDGMENTS

The authors would like to thank the National Institute of Standards and Technology for funding the

development of this program. In addition, the authors would like to thank ASHRAE for agreeing to permit

us to release their WYEC hourly weather data with this program.

vii

CHAPTER 1. INTRODUCTION

Brief Description of MOIST

MOIST (Release 3.0) is a user-friendly, personal computer program that predicts the onedimensional transfer of heat and moisture. With MOIST, you will be able to easily define a wall,

cathedral ceiling, or low-slope roof construction. You will subsequently be able to investigate the

effects of various parameters on the moisture accumulation within layers of the construction. For

example, you will be able to conduct computer runs for different U.S. and Canadian cities, thereby

investigating the effect of climate on moisture accumulation. You will be able to determine if a

vapor retarder is needed, and if needed, where it should be placed relative to the other materials.

MOIST allows the user to vary the building materials and their relative placement and predicts the

resulting moisture accumulation within each as a function of time for the selected climate. MOIST

permits the evaluation of the effect of the water-vapor resistance offered by paint layers, wallpaper,

and vapor retarders. Finally, MOIST can be used to generate guidelines and practices for controlling

moisture.

Features of MOIST

The theory of MOIST is given in Appendix A. The algorithms in the program can predict moisture

transfer in the diffusion regime through the capillary flow regime. The important effect of water

evaporating from one place in the construction and condensing in another place is included in the

model. The model predicts the moisture content, relative humidity, and temperature of the

construction layers, as well as the heat and moisture transfer rates at the inside and outside surfaces

of the construction. The program contains a heat and moisture property database for common

building materials, and uses ASHRAE WYEC hourly weather data which is available for 51 cities

of the United States and Canada. Data is input in a user-friendly Microsoft Windows environment.

Result graphs are readily generated from within the program. During a simulation, the user can

either specify a constant indoor temperature and relative humidity, or permit the winter indoor

relative humidity to vary and be calculated from a moisture balance of the whole building.

Applications of MOIST

MOIST may be used for the following wall, unventilated cathedral ceiling, and low-slope roof

applications:

1) predicting the winter moisture content in exterior construction layers and determining whether a vapor retarder is needed;

2) predicting the surface relative humidity at the construction layers in hot and humid climates,

thereby analyzing the potential for mold and mildew growth;

3) determining the drying rates for materials containing original construction moisture;

4) investigating the performance of cold refrigeration storage rooms; and

1
5) analyzing the effect of moisture on heat transfer.

Limitations of MOIST

One of the most significant limitations of MOIST is that the model is one dimensional. This means

that the model does not include the effect of framing members and two- and three-dimensional

effects such as the vertical movement of moisture in an earth coupled wall. Moreover, the model

does not include the exterior wetting of a construction by rain and the insulating effect and change

in roof absorptance from a snow load. In addition, the model does not include the transfer of heat

and moisture by air movement. The construction is assumed to be air tight.

Getting Help

After reading this manual, if you need help in using program MOIST, you may contact Doug Burch

at (301) 975-6433 or at dburch@nist.gov.

CHAPTER 2. INSTALLING MOIST

Microsoft Windows 95

Insert the MOIST 3.0 Program Disk in your floppy disk drive. From the task bar at the bottom of

your computer screen, click the Start Button, and then click Run. After the Run Dialog Box is

displayed, enter "a:\setup" in the Open Text Box. Here "a:" is the floppy disk drive from which you

are installing the program. If you are installing from drive "b:", then you would enter "b:\setup."

Click the OK button to commence the installation process. During the installation, the following

dialog box is displayed.

NIST MOIST 3.0 Setup

If you want to install the test application in a different directory

and/or drive, type the name of the directory.

Install To: C:\MOIST

To quit Setup, choose the Exit button.

Continue E.xit Setup

Figure 1 . NIST MOIST 3.0 Setup Dialog Box.

If you want to install the MOIST program in a different directory and/or drive, type the name of the

directory and/or drive in the Install To text box. Then, click Continue to proceed with the program

installation. During the installation process, you will be prompted to insert the second installation

disk into your computer.

2
Microsoft Windows 3.1

Insert the MOIST 3.0 Program Disk in your floppy disk drive. On the top menu bar of the Program

Manager, cHck File. Then cHck Run. After the Run Dialog Box is displayed, enter "a:\setup" in

the Open Text Box. Here "a:" is the floppy disk drive that you are using. If you are installing from

drive "b:", then you would enter "b:\setup." Click the OK button to commence the installation

process. During the installation, the NIST MOIST Setup Dialog Box shown above is displayed. If

you want to install the MOIST program in a different directory and/or drive, type the name of the

directory and/or drive in the text box. Then, click Continue to proceed with the program installation.

During the installation process, you will be prompted to insert the second installation disk into your

computer.

A description of the program files installed in the MOIST directory is given in Appendix B.

When MOIST is installed weather data for six cities (i.e., Madison, WI; Boston, MA; Portland, OR;

Washington, DC; Atlanta, GA; and Lake Charles, LA) are installed in the MOIST Directory. Before

you conduct a moisture analysis for another city, you must first install a weather data file for that

particular city in the MOIST Directory of your hard drive. Compressed weather data files for other

WYEC cities are contained on five disks supplied with the MOIST software package. Before using

one of these files, you must copy it into the MOIST directory and extract it by entering its name.

After extracting it, you may delete the compressed file to save space on your hard drive.

CHAPTER 3. GETTING STARTED

Starting the Program

After the installation process is completed, the MOIST Program Folder (Group) shown below is

displayed on your computer screen.

File Edit View Help

mm.

WIST

MOIST Reset MOIST

Release 3.0 3.0

[2 obiect(s) 572 byt.^

Figure 2. MOIST Program

Folder (Group).

Two icons are displayed in the MOIST Program Folder (Group). Double-clicking the MOIST

Release 3.0 icon, starts the program. Double-clicking the Reset MOIST 3.0 icon, resets the input

3
data files to their original state as installed.

Go ahead and start the program by double-clicking the MOIST Release 3.0 icon. After the program

starts, the Title Screen shown below is displayed.

MOIST Main Dialog Box

After a few seconds, the title screen will automatically disappear, and the MOIST Main Dialog Box

Nfoist Theorv; Dou2 Burch

Building and Fire Research Laboratoxy

National Institute of Standaids and TechnologyVisual Basic: Joseph Chi

Department ofMechanical Engineeiins

UnireTsit)' of the l>istxict of ColunOiia

Figure 3. Title screen of the MOIST Program.

Sjj^NIST MOIST PROGRAM - Release 3.0 HO

nil

Select units:

(J- SI Units C English Units

Edit data:

Material Database

Input Parameters

Building Canstractlan

Interval/Indoor Prmtrs

Output/Anal. Options

Piocess dn^lysis::

Run Analysis Prog.

Present Results

Hanage projects:

Open Project.. \

Save Project...

Reset

Plat graphs:

Dbl click to select Boondny cmditioiis Ii»fantane«as layer HC

Ii)stanl«iie«ii$ snrface UC

InstantaneffDS surface Kfl Time-evg. svrfece SH

Surface beat finzes Surface moisture fhaes

Time-avg. thermal resistance Surface Ten^eratures

Exit

Figure 4. MOIST Main Dialog Box.

4
shown in Figure 4 is displayed. From this screen you may direct and launch a wide range of heat and

moisture transfer projects.

Select Units:

Click on the appropriate button to select either SI Units or English Units.

From the MOIST Main Dialog Box, you may run the program by clicking command buttons, using

pull-down menus, clicking tool icons on the tool bar, or by using short cut keys. These different

ways to run the program are described below.

Command Buttons:

Single-clicking one of the command buttons takes you to a particular input data screen. Descriptions

of the command buttons are given below:

Material Database. This command button takes you to the Material Database Dialog Box where you

may enter a new material into the database, edit the properties of an existing material in the database, or delete a material from the database.

Input Parameters . This command button takes you to the Input Parameters Dialog Box where you

can modify various input parameters for your simulation and select the WYEC hourly weather data

used for the analysis.

Building Construction. This command button takes you to the Construction Data Entry Box where

you can create a new building construction or modify an existing building construction.

Interval/Indoor Prmtrs. This command button takes you to the data box for specifying analysis

intervals and indoor parameters. At this screen, you can specify the intervals over which you want

a moisture analysis carried out. In addition, you can modify the parameters for the indoor climate

model which permits the indoor relative humidity to vary and be calculated from a moisture balance

of the whole building.

Output/Analysis Options. This command button takes you to the data box for selecting output and

analysis options. At this screen, you may select which output files you would like the program to

generate. In addition, you may select various analysis options such as activating the latent heat term

in the temperature calculations.

The MOIST program may also be run by using pull-down menus and clicking on tool icons on the

tool bar. The function and uses of the pull-down menus and tool icons on the tool bar are explained

below.

5
Pull-Down Menus

There are four pull-down menus: File, Edit, Analysis, and Help. You may click on one of the

menu options of a pull-down menu, or you may press the Alt Key and then the underlined letter of

the menu option. The menu options of the pull-down menus are described below.

File Menu:

Reset Reset MOIST to its initial state (as loaded)

Open File...

Save File...

Load saved project files to the MOIST working area

Save input files and analysis results of current MOIST session

Exit Quit the current MOIST session

Edit Menu:

SI Units Select SI system of units

English Units Select English system of Units

Material Database Update the material database

Input Parameters Update the input parameters and select the weather data file

Building Construction Update a building construction

Intervals and Indoor

Parameters

Enter the analysis intervals and indoor climate model parameters

Output and Analysis

Options

Select output files and analysis options

Analvsis Menu:

Run Analysis Program Start model analysis sequence

Present Results Present the results (listing of all input data and nodal potentials, if full

hardcopy documentation is selected)

Plot Graphs Initiate graphs plotting sequence

6
Help Menu:

Introduction

Tutorial

Introduction to help (Users should read this first)

This will initiate a demonstration of the MOIST run, which takes about

fifteen minutes to complete.

Select Units Help in selecting units

Choose number of Nodes Help in choosing number of nodes

Update Material Database Help in updating the material database

Enter Input Parameters Help in entering input parameters and selecting the weather data file

Enter Building Construction Help in entering and modifying a building construction

Enter Analysis Intervals Help in specifying analysis intervals

Enter Indoor Parameters Help in entering parameters for the indoor climate model

Select Output Options

Run Analysis Program

Present Results

Plot Graphs

Open and Save Files

Reset and Exit

(variable RH)

Help in selecting output files for an analysis

Help in starting a moisture analysis sequence

Help in viewing the output (listing of all input data and nodal

potentials, if full hard copy documentation is selected) and getting a hardcopy

Help in plotting and printing graphs

Help in opening and saving MOIST project files

Help in resetting MOIST input files to their original state (as installed)

and exiting the current session.

About MOIST program Tells about the development of the MOIST program

Tool Icons on the Tool Bar

The tool bar at the top of the screen contains various tool icons that provide additional flexibility and

convenience in using MOIST. Functions for the various tool icons and their equivalent menu

options are listed in Table 1 . Placing the cursor over a tool icon from the MOIST Main Dialog Box

will cause a tool tip to be displayed.

7
Table 1. Tool Icons on the Tool Bar

Tool

Icons

Equivalent

Menu Items Functions and Use

Reset Reset MOIST input data files to their original state (as

loaded)

Open Proiect Lx)ad saved proiect files to the MOIST program

Save Proiect Save data and analysis results of current MOIST proiect

Material Database Update the material database

1

Input Parameters Update input parameters and select weather data file

ligl Building Construction Update building construction

nil

Analysis Intervals and

Indoor Parameters

Update analysis intervals and indoor climate model

parameters

O R

T X

Outniit Files and

Analysis Options

Select outnut files to be venerated and analvsis ontions

Run Analysis Program Start model analysis sequence

Plot Graphs Enter graph plotting sequence

Present Results Present the output (listing of all input data and nodal

potentials, if full hard copy documentation is selected)

[iEXJ.ll Exit Quit the current MOIST session

The Reset command button resets the input data files to their original state (as installed). The Exit

Command Button causes you to leave the MOIST program and to return to the Windows operating

system. All input data are saved to files.

The purposes of Process analysis, Manage project, and Plot graph are explained later in the

manual.

8
CHAPTER 4. REVIEWING THE EDIT DATA SCREENS

We will now take you on a tour through the various screens used to edit the data for a moisture

analysis session. The manual is written in SI units. Therefore, before beginning this chapter, select

SI units by click the SI Units button on the MOIST Main Dialog Box.

Construction Data Entry Box

From the MOIST Main Dialog Box, single-click on the Building Construction command button.

The screen shown in Figure 5 is now displayed on your computer screen.

£E!e £dtt Analysis Help

iiii

h| Building Consduclion '

CticV alaycr l.D-brttftn.therntype aicWftr:

£.D.I. or X. (Clict m«us« (or details^

Note, CcmshDrtifTnlaiiet"/ crre RrtCTedtrominsifieto uatiiiJe

2

I lOI-STOBAGE MATESIAL

FIBESBOASD. ASFMALT

SDGAI rilE

Igypsdm boaid

ThicXBtss. cm Intial Tmrp^C Initial PttnM««ft«* Molxlur* Contmt (X)

1.2T0O

tS4

1.2708 U700

21.0

t.^SE-OI

21.0

21.0

7.0

7.0

Modes

Figure 5. Construction Data Entry Box.

A four layer wall is shown in the Construction Data Entry Box. From the inside to the outside, each

layer is displayed as a row in the table. In using this program, building constructions must always

be enteredfrom inside to outside, and the number of layers in a wall must be ten or less. This wall

construction has the following layers: (1) gypsum board, (2) glass-fiber insulation (modeled as a nonstorage material), (3) asphalt fiberboard, and (4) sugar pine. Layers 1,3, and 4 are treated as storage

layers. That is, they store both heat and moisture. The glass-fiber insulation is treated as a nonstorage layer. That is, the storage of heat and moisture is neglected, and capillary transfer is

neglected. Other examples where a layer may be treated as a non-storage layer include a vapor

retarder, an air space, and a building paper.

Note that for each of the storage layers, the following four parameters are specified: thickness in cm

(in.), initial temperature in °C (°F), initial moisture content (%), and the number of nodes. The

initial moisture contents and temperatures are used to initialize the simulation results. It is

recommended that the initial temperature be the indoor temperatures of the simulation. The initial

moisture content of the materials should correspond to the equilibrium moisture content (EMC) at a relative humidity of 50%, unless the purpose of the MOIST simulation is to determine the rate of

drying for initially moist construction. Recommended EMC values are given in the Table below.

9
Material EMC Percent at 50% rh

Wood Base Materials 4.0-7.7

Masonry Materials 0.2-1.4

Glass Fiber Insulations 0.6

Gypsum Board 0.3

Plastic Insulations 1.9-3.8

EMC values may also be calculated by the sorption isotherm function [see eq (1) covered later in

the manual]. Choosing a suitable number of nodes to achieve convergence of the mathematical

solution is covered in Chapter 9.

For a non-storage layer, the following two parameters are specified: the thermal resistance in

°C-m^AV (h-ft^-°F/Btu) and the permenace in kg/s-m^-Pa (perm) for the material layer. To modify

a parameter, click on the parameter, and use the backspace, del, and numeric keys to change the

number. Another option, is double-click on the parameter. The parameter becomes highlighted.

When you enter a new parameter value, it replaces the old parameter value.

You may delete a layer by clicking the layer number to select the layer. Then type D. You may

insert a layer above another layer by first selecting the layer by clicking its number. Then type I. The program now displays the Material Selection Box shown in Figure 6 on your computer screen.

Help D. 1 in mi 0 P T % s i£xni

1 H Select Building Materials..

Select a Material Eor layer Ko: 3

BWChcV 1o select inAterials: Mirtenal idected (or layvr 13; OI-STOIAGI MATEIIAl

All BAIIIES (FULYOLEFIII ALOMIIOM SIDIKG BSICE

BOILDIIG PAPEB. ASPHALT

BOILT-OP lOOFIMG

COMCIETE

COICKETE BLOCK

FIBEIBOAID. lOOFIIG

FIBEIBOASD. SHEATHIMG FOAM COEE SHEATHIXG

GLASS-FIBEI BOAIB

GIAVEL GYPSDH BOAIB

ICYMEVE DiSDLATIOI XIAFT FAPEI METAL lOOF OECX

OIIEITEB STBAID BOASO

PAinCLE BOAKO

PEILITE BOABB

PLYHOOB. EXTEBIOB GIADE

FIBEBBDABB. ASPHALT

Figure 6. Material Selection Box.

You will now be prompted to select a new material. In the scroll box of materials, double-click on

a material to select it. It will now appear in the material dialog box. Click the OK button to accept

the material. The program will take you back to the Construction Data Entry Box (see Figure 5).

You may now enter parameter values for the inserted material.

10
If you want to replace one of the materials with another material, then select the material by clicking

its number. Then type E. The program will take you to Material Selection Box (see Figure 6).

Select a new material, click on the OK button, and then enter its parameter values.

You must obey several rules in using the Construction Data Entry Box. First, the layers must be

specifiedfrom inside to outside. Second, the first and last layers must be storage layers. It is worth

mentioning that you may put several non-storage layers adjacent to each other. However, ifyou do

that, the analysis algorithms of the MOIST model will combine the adjacent non-storage layers into

a single layer. This will cause the depiction of layers on the plot screen to be renumbered

accordingly.

When you have completed updating the building construction, type an X function to exit the editing

sequence if a layer number is still highlighted, and then click on the OK button. This will

incorporate the updated construction into the current moisture analysis session. Alternatively, you

may click on the Cancel button to cancel the revisions to the building construction.

Input Parameters Dialog Box

From the MOIST Main Dialog Box (see Figure 4), click the Reset button to restore all input

parameter values to their original state (as installed). Now click the Input Parameters command

button. The program now takes you to the Input Parameters Dialog Box shown in Figure 7.

File Edit Analysis Help

MM

m nil

O P

T H

it. Input Parameters OK Cancel

Enter Correct Paraineler Vahies:

Convectiim coefficient at inside surface (Hfm*2.C) |2.B0

Convection coefficient at outside surface (Hhn'2.C) 11.10

Solar absoiptance of eKterior surface [Fraction) 0.70

Surface tilt anqle (Degrees) $0.00

Surface azimuth orientation (Degrees) p.oo

Indoor temperature (C) 21.00

Indoor relative humidity (X) 30.00

Inside Surface Faint Fermeance (Kg/s.m*2.Pa) S.TOE-10

Outside Surface Paint Permeance (Kg|s.m*2.Fa) l.lOE-tO

Time Zone (for nan HYEC users) 5

Latitude (for non HYEC users) #2.00

Longitude (for non HYEC users) 75.00

Time Step (1 hour or less) 1.00

DW Oicl To Select

HeaChef Files: vymadin

wyatlga

wybosma

wylkcia

wymatJwi

wyporoi

wywasdc

Figure 7. Input Parameters Dialog Box.

11
The input parameters are defined below.

Convection coefficient at the inside surface . This is the coefficient of heat transfer due to the

movement of air at the inside surface of the construction. The coefficient does not include the

transfer of heat by radiation. The authors recommend that you use a value of 2.6 W/m^-°C

(0.46 Btu/h-ft2-°F).

Convection coefficient at the outside surface . This is the coefficient of heat transfer due to the

movement of air at the outside surface of the construction. The coefficient does not include the

transfer of heat by radiation. The authors recommend that you use a value of 1 1.4 W/m^-°C (2.0

Btu/h-ft^-°F).

It is worth mentioning that the surface conductances given in Table 1 of Chapter 22 of the 1993

ASHRAE Handbook ofFundamentals are total conductances and include the radiation conductance.

When using these ASHRAE conductances, you must subtract the radiation conductance from the

value given in the ASHRAE table. A moisture analysis is usually not very sensitive to reasonable

variations in these coefficients.

Solar absorptance at exterior surface . This parameter is the fraction of the incident solar radiation

that is absorbed by the surface. The program comes loaded with a value of 0.7. We recommend a value of 0.2 for light-colored surfaces, 0.5 for medium-colored surfaces, and 0.8 for dark-colored

surfaces.

Surface tilt Angle. This is the surface tilt with respect to a horizontal plane expressed in angular

degrees. Select 90° for a vertical surface and 0° for a horizontal surface.

Surface azimuth orientation . This is the surface compass azimuth expressed in angular degrees

clockwise from North. A north azimuth is 0°, and a west azimuth is 270°.

The next two parameters are used when the indoor temperature and relative humidity are held

constant during a simulation. This mode of operation occurs when the indoor climate model is not

selected as an analysis option (see the section on the Data Box for Selecting Output and Analysis

Options). If the indoor climate model is selected, then the next two parameters may not be revised

and are not used in the computer analysis.

Indoor temperature. This is the indoor dry-bulb temperature expressed in °C (°F) which is held

constant during the computer simulation.

Indoor relative humidity. This is the indoor relative humidity expressed as a percent which is held

constant during the computer simulation.

Inside surface paint permeance. This is the water-vapor permeance of the inside surface covering

(paint or wallpaper) expressed in kg/s-m^-Pa (perm).

Outside surface paint permeance. This is the water-vapor permeance of the outside surface covering

(paint, stain, etc.) expressed in kg/s-m^-Pa (perm).

12
Some guidelines on choosing an appropriate value for paint permeance is given below. Latex paint

systems (primer and top coat) have permeances ranging from 2.9 x 10"'° - 8.6 x 10"'° kg/s-m^ Pa (5-

15 perm) depending on the manufacturer. If you do not know the exact value of the permeance of

the your latex paint, the authors recommend that you use a value of 5.7 x 10"'° kg/s-m^-Pa (10

perm). Oil-base paints have a permeance ranging from 5.7 x 10" -1.7 x 10"'° kg/s-m^-Pa (1-3 ).

If you do not know the exact value of the permeance of the your oil-base paint, the authors

recommend that you use a value of 1.1 x 10"'° kg/s-m^-Pa (2 perm). Vapor retarder (vapor barrier)

paints have a permeance less than 5.7 x 10" kg/s-m^-Pa (1 perm). If a wallpaper is applied to the

surface instead of a paint, then enter the permeance value of the wallpaper. For vinyl wallpaper, the

authors recommend using a permeance of 2.9 x 10" kg/s-m^-Pa (0.5 perm). If a surface has no

paint or wallpaper applied, then enter a value of 5.7 x 10'^ kg/s-m^-Pa ( 100000 perm).

The next three parameters arefor non-WYEC users only. When an official WYEC weather data file

name is specified, the next three parameters are not used by the MOISTprogram. The procedure

for using non-WYEC weather data is given in Appendix C.

Time zone (for non WYEC users) . The global circumference of the earth is divided into 24 time

zones (each 15 degrees) measured west of the Greenwich Meridian. The time zone is equal to the

longitude of the location divided by 15, rounded to the nearest whole number.

Latitude (for non WYEC users) . The angular position of your weather data location expressed in

degrees north or south of the equator, north positive, south negative (-90° < latitude < 90°).

Longitude (for non WYEC users) . The angular position of your weather data location expressed in

degrees west of the Greenwich Meridian. (0°< longitude < 360°).

Time step . This is the time step used in the analysis expressed in hours. It must be 1 hour or less.

In most simulations, a 1 hour time step will provide reasonable results. However, when MOIST is

used to model low density insulation as a storage layer, then a smaller time step may sometimes be

needed to achieve a reasonable simulation. If too large a time step is used in this case, then MOIST

may become unstable, resulting in unreasonable fluctuations in moisture content. Further

information on modeling low density insulation as a storage layer is given in Chapter 10.

When you have completed updating the input parameters, click the OK button. This will

incorporate the revised parameters into the current moisture analysis session. Alternatively, click

the Cancel button, if you want to cancel the revisions just performed.

Selecting Weather Data for the Analysis

From the Input Parameters Dialog Box, you may also selected the weather data to be used in the

analysis. Weather data is selected by double-clicking on the name of the weather data file contained

in the Weather Data Dialog Box. All the weather data files in the MOIST directory are displayed

in the dialog box. When the MOIST program is initially installed, weather data for six U.S. cities

is installed in the MOIST directory. The six cities include: Madison, WI; Boston, MA; Washington,

DC; Atlanta, GA; Portland, OR; and Lake Charles, LA.

13
Weather data for 45 other U.S. and Canadian cities are provided with the program and may be

installed in the MOIST directory for use with the program. They may be installed by copying the

appropriate compressed file into the MOIST directory. Entering the name of the file from the

MOIST directory will cause the file to be extracted into an uncompressed format. After a file is

extracted, you may delete the compressed file to save hard disk space. Procedures for using nonWYEC weather data with the MOIST program are described in Appendix C.

Data Box for Specifying Analysis Intervals and Indoor Parameters

From the MOIST Main Dialog Box, click the Interval/Indoor Prmtrs command button. The

program will now display the Data Box for Specifying Analysis Intervals and Indoor Parameters

shown in Figure 8.

^ = !ffij<J

File^ Edit Analysis Help m nil

0 P T X Nixil!

\ M Analysis Intervals and Indoor Parameters OK Cancel

I Analysis Intervals:

Analysis fifst day 1

Analysis last day 546

Pfintfplot first day 1«2

Printfplol last day SU

prinlfplot intrrwal (hours) 188

f

Winter Humidity Index

1

! i

r Space Cooling Index: —

Indoor Climate Model Parameters

20.0 Sfijijk coet^icmrsl |C!iui r«»«x& lot urtHs) 1.50E-04

Suwrwe* s«>ipoi»t teirajtsfature (C) 2t.4 Mind citifWicient (CBci; mousf? tof mitsj 1.04£-a«

15.0 Floor ^rea |n4^2) 139.4

S«i»mei( mdoo! BE {X) 56.0 Sarptiott const. (Clicl mouse im w«iS,s} 4.48E-08

Indoor mccli venfiUtiun < ate 0.00E.00 Xna&or moisture gi?^«fa):mn i^ite (%L9ld«t<«jf} 10.9 13.3 Thennai thtw cense^t [h] 72.0

CiiQimti baia«ce -pomt temp, fC] 16.7 Hm4ow tranjimiWirnce fH|im*2-CJ 2.84

77<.2

Figure 8. Data Box for Specifying Analysis Intervals and Indoor

Parameters.

Under Analysis Intervals,

Analysis first day. This is a pointer that instructs the program to carry out an analysis starting with

the Julian calendar day specified in the data field. In the Julian calendar, the days of the year are numbered consecutively. For example, Julian day 45 refers to February 14.

Analysis last day. This is a pointer that instructs the program to carry out an analysis ending with

the Julian calendar day specified in the data field.

14
Print/plot first day. This is a pointer that instructs the program to send print and plot data to

specified output files starting with the JuHan day specified in the data field.

Print/plot last day. This is a pointer that instructs the program to send print and plot data to specified

output files ending with the Julian day specified in the data field.

The above print/plot pointers may be different than the analysis pointers, thereby making it possible

to include a pre-conditioning period in the computer analysis when print and plot data are not sent

to the selected output files. Here the term "pre-conditioning period" denotes a period of time,

placed in front of the period of analysis, which serves to initialize the simulation results so that they

are independent of assumed initial moisture content and temperature. The authors recommend using

a 6-month pre-conditioning period, unless the purpose of the MOIST simulation is to predict the

drying rate of initially wet construction.

Print/plot interval (hours) . This is how often print and plot data are sent to output files. For many

analyses, a print/plot interval of 168 h (or once a week) is an appropriate selection.

When conducting a 1-year moisture analysis in cold climates, it is convenient to put the winter in

the middle of the analysis period. In a similar fashion, when conducting a 1-year analysis in a hot

and humid climate, it is convenient to put the summer in the middle of the analysis period. For the

case of including a 6-month pre-conditioning period, the following parameter values are recommended:

Analysis Winter Summer

First Day 1 182

Last Day 546 730

Printing/Plotting

First Day 182 366

Last Day 546 730

Print/Plot Interval 168 168

If you want to eliminate the pre-conditioning period, then set the first day of the analysis equal to the

first day of printing and plotting.

The other parameters shown in this data box (i.e.. Winter Humidity Index, Space Cooling Index, and

Indoor Climate Model Parameters) pertain to the indoor climate model. To use and revise these

parameters, go to the analysis options (Data Box for Selecting Output and Analysis Options) and

select the indoor climate model. If the indoor climate model is not selected, then these parameters

may not be revised and are not used in the computer analysis. A mathematical description of the

indoor climate model is given in Appendix A. The parameters for the indoor climate model are described below.

Winter Humidity Index. When Constant RH is selected, the winter indoor RH will be maintained

at the value specified in the Indoor Climate Model Parameters. This option allows separate indoor

15
RH and temperature values to be specified during the winter and summer. This option could be used

to simulate a house which is air conditioned during the summer and humidified during the winter.

When Variable RH is selected, the winter indoor RH will be permitted to vary and will be

calculated from a moisture balance of the whole building. Here the indoor moisture generation rate

and building air tightness serve as input. This variable indoor relative humidity model should only

be used for cases where the interior of a residential building may be modeled as a single zone having

a uniform temperature and relative humidity. This featured should not be usedfor non-residential

buildings.

Space Cooling Index . When On is selected, space cooling will be provided during the summer at the summer set point temperature specified in Indoor Climate Model Parameters. When Off is

selected space cooling is not provided and the windows are assumed to be open during the summer.

The indoor temperature and relative humidity are then assumed to be equal to the outdoor values.

Under the Indoor Climate Model Parameters:

Winter set point temperature. This is the set point temperature for space heating.

Summer set point temperature. This is the set point temperature for space cooling.

Winter indoor RH. This is the winter indoor relative humidity used in the analysis when the Indoor

Humidity Index is set to Constant RH. When the Indoor Humidity Index is set to Variable RH,

this parameter is not used in the analysis.

Summer indoor RH. This is the summer indoor relative humidity used in the analysis when the

Space Cooling Index is set to On. When the Space Cooling Index is set to Off, this parameter is not

used in the analysis, and the indoor relative humidity is equal to the outdoor value. The authors

recommend using a value of 56%.

Indoor mech. ventilation rate. This is the volumetric ventilation rate of the indoor space provided

by mechanical equipment during the winter. This value is the actual value rather than the rated

value. If no mechanical ventilation is provided, then use a value of 0.0. During the summer, the

indoor relative humidity is either assumed to be equal to a specified value provided by the air

conditioning equipment or equal to the outdoors value if the windows are opened. In this situation,

mechanical ventilation is not included in the analysis.

Heating balance point temp. This is the balance point temperature for space heating. When the

weekly space heating loads are plotted versus average outdoor temperature, this is the average

outdoor temperature at which the weekly space heating loads become equal to zero. The authors

recommend using a value of 13.3 °C (56.0 °F) for a well insulated house and 16.7 °C (62.0 °F) for a poorly insulated house.

Summer balance point temp. This is the balance point temperature for space cooling. When the

weekly space cooling loads are plotted versus average outdoor temperature, this is the average

outdoor temperature at which the weekly space cooling loads become equal to zero. The authors

recommend using a value of about 19.4 °C (67.0 °F).

16
Effective leakage area. If all the leakage area of the whole house were replaced with a single sharpedge orifice, then the effective leakage area would be the area of the orifice which would produce

the same airflow rate as the actual leakage area in the whole house. The authors recommend using

values of 355 cm^ (55 in^) for a tight house, 710 cm^ (1 10 in^) for a typical house, and 1420 cm^ (220

in^) for a leaky house.

The next two coefficients are used in the Lawrence Berkeley Laboratory (LBL) infiltration equation

(see Eq. A- 12 in Appendix A) to calculate the house infiltration rate from the driving forces of

inside-to-outside temperature difference and wind speed.

Stack coefficient. This is the coefficient for the inside-to-outside temperature difference. The

authors recommend using a value of 0.000145 (I7s)^-cm-^-°C' (0.0156 cfm^-in-^-T ' ). Other values

are given in Table 6, Chapter 23, 1993 ASHRAE Handbook of Fundamentals.

Wind coefficient. This is the coefficient for the wind speed. The authors recommend using a value

of 0.000104 (L/s)^-(cm)"^-(m/s)"^ [0.0039 cfm^-in-^-mph'^l. Other values are given in Table 8, Chapter

23. 1993 ASHRAE Handbook of Fundamentals.

Floor area. This is the floor area of the indoor conditioned space. Unconditioned spaces are excluded. A typical house has a floor area of about 139.4 m^ (1,500 ft^).

Sorption const. This is a moisture storage parameter (per unit floor area) used in the moisture

balance of the whole building to include the effect of moisture storage at interior surfaces. The

authors recommend using a value of 4.5 x 10"^kg/s-m^ (0.33 x 10^ Ib/h-ft^). Other values are given

inTenWolde(1994).

Indoor moisture generation rate. This is the moisture release rate for occupant-related activities

expressed in kg/day (lb/day).

Thermal time constant. This is another moisture storage parameter used in the moisture balance of

the whole building to include the effect of moisture storage at interior surfaces. The authors

recommend using a value of 72 h.

The authors have found that moisture accumulation in the building envelope is fairly insensitive to

reasonable variations in the above two moisture storage parameters.

Window transmittance. This is the center-of-glass thermal transmittance for the windows. The

authors recommend using a value of 6.2 W/m^-K (1.1 Btu/h-ft^-°F) for single-pane windows and

3.2 W/m^-K (0.57 Btu/h-ft^-°F) for double-pane windows. Window thermal transmittance values for

various glazing systems are given in Table 5, Chapter 27, 1993 ASHRAE Handbook of

Fundamentals. In the MOIST program, the window transmittance is used to calculate an inside glass

surface temperature which is the maximum sustainable indoor dew-point temperature, thereby

limiting the indoor relative humidity.

When you have completed updating the Data Box for Specifying Analysis Intervals and Indoor

Parameters, click on the OK button. This will incorporate the revised parameters into the current

17
moisture analysis session,

just performed.

Alternatively, you may click the Cancel button to cancel the revisions

Data Box for Selecting Output and Analysis Options

From the MOIST Main Dialog Box, click the Output/Analysis Options command button. The

program will now display the Data Box for Selecting Output and Analysis Options shown in Figure

9.

A m nil

0 P

T X m ihXIT!

;»sgMri||A|as^Aa-.r

1

II Output File and Analysis Option s

file £dit Analysis Help

Output File Options: Analysis Options:

WiB oimdary conditions]

|7 Instantaneous layer moisture contents

f7 Instantaneous surface moisture contents R Instantaneous surface relative humidities

|7 Time-avg. surface relative humidities

17 Abbreviated hard copy documentation n FuH hard copy documentaion

|7 Surface heat fhizes

17 Surface moisture fluxes

[7 Time-avg. thermal resistances

[7 Surface temperatures

P'^ Temperature calculation inchided

|7 Capillary transfer inchided

|7 Latent heat included

r~ Indoor Climate model inchided

OK

Cancel

Figure 9. Data Box for Selecting Output and Analysis Options.

Under Output File Option, you may select which output files will be generated by the program.

MOIST sends results to the selected files at the print/plot interval. The output file options are described below.

Boundary conditions . This option is a snapshot (at the print/plot interval) of the boundary conditions

(i.e., indoor temperature, indoor water-vapor pressure, indoor relative humidity, outdoor temperature,

outdoor water-vapor pressure, incident solar flux less long-wave sky radiation, and outdoor dewpoint temperature). All values are time-averaged over the print/plot interval.

Instantaneous layer moisture contents . This option is a snapshot (at the print/plot interval) of the

moisture contents of the construction layers. Each moisture content is averaged across the thickness

of the layer. Moisture content is defined as the mass of water in a material divided by its dry weight.

Instantaneous surface relative humidities . This option is a snapshot (at the print/plot interval) of the

surface relative humidities of the construction layers. The surface relative humidities at adjacent

surfaces will not be exactly equal. This is because the surface relative humidity is taken to be the

18
value of the nearest finite-difference node to the surface. If adjacent values are too much different,

then you should increase the number of nodes in the adjacent layers.

Time-avg. surface relative humidities . This option is the same as the above, except that the values

are time-averaged over the print/plot interval.

Abbreviated hard copy documentation. This option is a listing of all the input data parameters

contained in the dialog boxes. After a MOIST session, you can view these input data parameters

in a dialog box by clicking Present Results.

Full hard copy documentation . This option contains a listing of all the input data parameters plus

snapshots (at the print/plot intervals) of the nodal potentials (i.e., temperature, moisture content,

water-vapor pressure, and logarithm of capillary pressure). After a MOIST session, you can view

these input data parameters and nodal potentials in a dialog box by clicking Present Results. In the

sample problem that comes pre-loaded with the program, the size of the file containing the full hard

copy documentation is 58 kbytes. The user is cautioned that this file can become quite large when

an analysis interval less than 168 hours (1 week) is used in the program.

Surface heat fluxes . This option contains the heat fluxes at the inside and outside boundaries of the

building construction at the print/plot interval. The values are time-averaged over the print/plot

interval.

Surface moisture fluxes . This option contains the moisture fluxes at the inside and outside

boundaries of the building construction at the print/plot interval. The values are time-averaged over

the print/plot interval.

Time-avg. thermal resistance . This option contains the time-average thermal resistance of the

building construction. Each value is computed by dividing the time-average (surface-to-surface)

temperature difference across the building construction by the time-average heat flux at the inside

boundary. When the heat flux and temperature difference are close to zero, small computational

errors may propagate into unreasonable thermal resistance values.

Surface temperatures. This option is a snapshot (at the print/plot interval) of the surface temperature

of the construction layers. Since each of the surface temperatures is taken to be the value of the

finite-difference node nearest to that surface, adjacent surface temperatures will not be exactly equal.

If adjacent values are too much different, then you should increase the number of nodes in adjacent

layers.

If the storage space on your hard drive is limited, you may save storage space by not selecting some

of the output file options. However, if an output file option is not selected, the output parameters

in this file may not be plotted after a MOIST analysis.

In this data box, you may also select the following analysis options:

Temperature calculation included. This option includes the temperature calculation in the analysis.

For most practical applications, you will select this option. If this option is not selected, the initial

19
temperatures of the layers specified in the Construction Data Entry Box (see Figure 5) are maintained during the entire simulation. The material temperatures will not vary.

Capillary transfer included. This option includes capillary transfer in the simulation. If this option

is not selected, then moisture transfer will be by water-vapor diffusion and capillary transfer will be

excluded. For most applications, you will select this option.

Latent heat included. This option includes latent heat (i.e., the effect of water evaporating from one

place and condensing in another place) in the temperature calculations. For most applications, you

will select this option. However, the inclusion of latent heat in some applications where low density

insulation is modeled as a storage layer and becomes wet may cause the MOIST model to become

unstable (see Chapter 10).

Indoor climate model included. When this option is not selected, the indoor temperature and

relative humidity will be held constant during the MOIST analysis. The model will use the indoor

temperature and relative humidity given in the Input Parameters Dialog Box (see Figure 7). On the

other hand, if this option is selected, then the indoor climate model will be used in the moisture

analysis. The Winter Humidity Index, the Space Cooling Index, and the Indoor Parameters specified

in Figure 8 will be used in the analysis.

When you have completed updating the Data Box for Selecting Output and Analysis Options, click

the OK button. This will incorporate the revised parameters into the current moisture analysis

session. Alternatively, you can click the Cancel button, if you would like to cancel the revisions

just performed.

Material Database Dialog Box

From the MOIST Main Dialog Box (see Figure 4), click the Material Database command button.

The Material Database Dialog Box shown in Figure 10 will be displayed on your computer screen.

M Mateiial Database

Materiols Listing:

AIR lAIUUER (POLYOLEFIN) ALUMINUM SnUNC

BRICK

BVILSINC; PAPER, ASPHALT

BVILT'lJ? ROOFDIG CONCRETE CONCRETE BLOCK

FIBERBGAItB, ASPHALT nBBRBOARD, RODFIHC

FDBRBOARII, SHEATHINC FOAM CORE SHEATHINC

CLASS-FIBER SOARS

CRAVEL CVPSUMBOARB ICmENE raSVLATION KRAFT PAPER METAL ROOF DECK

ORIENTED STRAND BOARS PARnCLE BOARD

PERLITE BOARD

PLYWOOD, EXTERIOR CRASS POLYETHYLENE

P0LY1S0CYANURATB INS POLYSTYRENE, EXTRUDES

ROOFDIC SHINCLBS, ASFHALT STRAWBALE

STUCCO FINISH

VINYL SIDINC WAFIRBOARS SDINC

Database Update:

Create/Edit Properly File:

Create a New Rle

EdS the Eiitling Fie

Matcrlalt/Propertieg Update:

C Insert new malarial C Add new matefial C Delete old material

(* Revise old mateiial

Accept Changes

Reject Changet

Materiat'c Name: SDOAX Pim

Sorption Coclftcicnts: Ul soiptioB oovf. (Al) .IS2DI-00 2n4 sotftlon co»f. (A21 .2050E*01

3rd loqptlm co»f. (A3) .KSDE.tM

Capillary CnefHcienfs:

lil dlifuxlvlty c»»MCl| .37«iE*09 2nd difiuslwltip eo«l.(C2) .21«0E<01

Sat. Mobt. CvntmCr) .lTtOZ«Dl

.S651E*03

.1(33E*0«

.S6StE-0S OOOOE-OO

Meat Transfer Propeitiet: Dry dmsily

Specific \*M

Diy lh«nn.oonduoli«iiy

T«mp«raliue eoef.(CT|

Moistufe ooeE.(Cy)

Vapor Permeabilily Coeflicients:

Isl penne*bllity eovf. (Bl] .«420E-12

SndMrncabllityco*!. (B2) .9100E.1S

3rd permeaUnty coef. (B3) .9860E*01

Figure 10. Material Database Dialog Box.

20
From this dialog box, you may create a new material database or update an existing material

database. To update an existing material property database, click on Edit the Existing File

command button. You may now perform one of the following options:

Inserting a new material above a selected material . Double-click on Insert a new material. Then

double-click on the material above which the new material is to be inserted. You may now enter the

new material name and its properties.

Add a new material below a selected material . Double-click on Add a new material. Then doubleclick on the material below which the new material is to be added. You may now enter the new

material name and its properties.

Delete an existing material . Double-click on Delete an old material. Then double-click the

material you want to delete.

Warning: Inserting, adding, and deleting a material changes the order of the materials in the

material database. When these operations are performed in the middle of a MOIST session, the

names of the materials given in the Construction Data Entry Box may be altered. Therefore, it is

recommended that you perform these operations at the beginning of a MOIST session.

Revise an existing material . Double-click on Revise properties. Then double-click the material

you want to revise. You may now revise the name of the material and its properties.

After you have completed updating the material property database, click on Accept Changes, if you

want to incorporate the changes into the database. On the other hand, if you want to reject the

changes, click on Reject Changes. Click OK to return to the MOIST Main Dialog Box. The

Cancel button also returns you to the MOIST Main Dialog Box, but the changes you made are canceled.

This dialog box also permits you to create an entirely new material property database by clicking

Create a New File. After making this selection, enter the material name and the property data for

the first material. Then click on Accept Changes. Now to add additional materials, proceed in the

following manner. Double-click on Add New Material. Then double-click on the last material

given in the Material Listing. You may now enter the material name and the property data for the

second material. Then click on Accept Changes. Proceed in this fashion until all the materials have

been added. When you are done, click on OK to retum to the MOIST Main Dialog Box. It is worth

mentioning that you may restore the original material property database at any time by clicking

Reset in the MOIST Main Dialog Box.

The material property data are defined below. From the Material Data Dialog Box, you may view

the equation for the property by clicking on the property name.

Sorption Coefficients . The first three parameters pertain to the storage of water vapor in the

material. They are used to relate the moisture content (y) to the ambient relative humidity ((J)).

21
(1+A,(j))(l-A,({))

(1)

In the above equation, both y and (j) are expressed as a fraction. This equation is called the sorption

isotherm function. The three parameters (A,, A2, and A3) are determined by a fit of measured data

for Y and (J). Here the sorption isotherm is assumed to be independent of temperature.

Capillary Coefficients . The next three parameters pertain to capillary transfer in a material. The

first two parameters (C, and Cj) are coefficients of the following empirical correlation that relates

liquid diffusivity (D^) to moisture content (y ):

= C,exp(C2Y) (2)

The liquid diffusivity (D^) and parameter (Ci ) are expressed in m^/s (ft%). The parameter (C2) is

dimensionless.

The third parameter is the capillary saturated moisture content of the material expressed as a fraction.

It is the moisture content of a material after it is submersed in water and allowed to reach

equilibrium.

Heat Transfer Properties . The next two parameters are the dry density of the material expressed in

kg/m-' (Ib/ft^) and the specific heat of the material expressed in J/kg-K (Btu/lb-°F). The next three

parameters are coefficients in the following empirical equation for the thermal conductivity (k) of

the material:

k = k,^^ CjiT-T^ef) + C^Y (3)

where

kdry = dry thermal conductivity, W/m-K (Btu/h-ft-°F)

Cj = temperature coefficient, W/m-K^ (Btu/h-ft-°F^)

T,^f = reference temperature [24°C (76°F)]

C^ = moisture coefficient, W/m-K (Btu/h-ft-°F)

T = temperature, °C (°F)

Y = moisture content, kg/kg (lb/lb)

Vapor Permeability Coefficients . The last three parameters pertain to water-vapor diffusion in a

material. They are the coefficients for the relation between water-vapor permeability and

ambient relative humidity (([)), or:

|j = 5, + B^Q\p(B^<^) (4)

In the above equation, |i, Bj, and B2 are expressed in kg/s-m-Pa (perm-inch) and (J) is expressed as

22
a fraction. This equation is called the water-vapor permeability function. The three parameters (Bj,

Bj, and B3) are determined by a fit of measured data for |i and (j).

Moisture property data for building materials are published in the lEA Annex 24 - Material

Properties Report (1996), Burch, et al. (1992), Richards, et al. (1992), Burch, et al. (1995), Tveit

(1966), and Hansen (1986).

CHAPTER 5. RUNNING A MOIST ANALYSIS

From the MOIST Main Dialog Box (see Figure 4), click on the Reset command. This will reset the

input data to their original state (as initially installed). Click the Run command to start the MOIST

analysis sequence. The Run I.D. Dialog Box shown in Figure 1 1 will appear on your computer

screen. You will need to enter a run ID number, filename, project title, analyzed by, and date. The

completed Run I.D. Dialog Box is shown in Figure 11.

M Input Project I.D...

Run ID/:

Project Title:

001 Filename: Results

Sample Problem

Analyzed By: Doug Buich

Date: 12/01/9G

OK Cancel

Figure 11. Run I.D. Dialog Box.

After you have completed the entries for this dialog box, click the OK button to proceed with the

run. Alternatively, you may click the Cancel button to cancel the MOIST analysis and return to the

MOIST Main Dialog Box. The Analysis Application Dialog Box shown in Figure 12 will appear

on your computer screen. Click the OK button to proceed with the MOIST analysis.

M Run MQI ST Analysis Progiam

Application to run:

Moist Analysis

OK Cancel

Figure 12. Analysis Application Dialog Box.

23
After clicking the OK button on the Run MOIST Analysis Program Dialog Box, the MOIST

program reads in hourly WYEC weather data file for the analysis. The following message is

displayed: "Please wait... reading in weather data." In the analysis intervals, if you specified a preconditioning period, then the following message will subsequently be displayed during the preconditioning calculation: "Please wait... pre-conditioning calculation in progress."

Note: Ifyou specified the indoor climate model (see Figure 9), the MOISTprogram requires more

time to process the weather data and prepare the indoor temperature and relative humidity

conditions neededfor the analysis.

During the analysis, the MOIST program displays a plot of the moisture content of the construction

layers versus time of year on your computer screen (see Figure 13).

Figure 13. Screen Plot Displayed during Execution.

On a Pentium 166 MHZ computer, the MOIST program takes 50 seconds to run this sample

problem. In the graph on the computer screen, the vertical axis displays the relative moisture content

of the storage layers of the construction. The term "relative moisture content" is defined as the

moisture content of the material layer divided by the moisture content of the material at maximum

sorption. Maximum sorption is the moisture content of the material in equilibrium with saturated

air at 97% RH. The dashed horizontal line in the center of the graph depicts a relative moisture

content of 1 . This is the critical moisture content above which liquid water appears in the large pore

spaces of the material. Relative time is displayed on the horizontal axis. A relative time of 0

corresponds to the start of a simulation, and a value of 1 corresponds to the end of the simulation.

In the above plot, three curves for the sample problem (see Figure 5) are displayed. Layer 1 (lower

curve) is the moisture content of the gypsum board, layer 3 (middle curve) is the asphalt fiberboard,

and layer 4 (upper curve) is the sugar pine. Since the insulation is treated as a non-storage layer

(layer 2), it is not shown in the plot.

Neat Feature: Ifyou would like a hard copy of this computer screen plot, press the Print Screen key

24
after the plot is completed. This copies the screen plot onto the clipboard. After exiting the MOIST

program, you may invoke the Windows Paint Program, and copy this screen plot into the Paint

Program using the Paste Command. This screen plot may subsequently be printed by selecting Print

from the File Menu.

After the screen plot is completed, press the Enter key to continue. The program now will display a Finish-Analyze Box, if you are using a Microsoft Windows 95 operating system. A finish-Analyze

Box is not displayed if you are using either a Microsoft Windows 3.1 or a Microsoft Windows NT

operating system. If the MOIST program fails to complete the simulation, an error message will be

displayed in this box. Press the x button in the upper right comer of the window to continue. The

program returns to the MOIST Main Dialog Box (see Figure 4).

CHAPTER 6. PLOTTING RESULT GRAPHS

After you complete a MOIST simulation, you may view and plot result graphs from within the

program. From Plot graphs of the MOIST Main Dialog Box (see Figure 4), double-click the

output result that you want plotted.

The program now displays the Graphics Editing Dialog Box shown in Figure 14. From this dialog

box, you can click the appropriate check boxes to select the variables you want plotted. In addition,

you may accept or revise the x-axis and y-axis labels. When finished, you may click on the Plot

command to view the plot or click on the Cancel command to return to the MOIST Main Dialog

Box.

w^-— —

£ie £.d!t Analysis ijelp 1 m iiii O P

T M H Customize Graphs "Hot i| Cnol

Check Variables to Plot and/or Edit Legends:

f7 Layer t1 MC

P Layer MMC

r m, m

B

r mr"

r m

Accept or Revise X-axis

r

rr

r

r

r

r

r

r

r

Accept or Revise y-axis

ImtUyetMC

Figure 14. Graphics Editing Dialog Box.

After clicking the Plot command, the Graphing Dialog Box shown in Figure 15 is displayed. From

this dialog box, you may click Print to send a copy of the plot to your printer. If you want to revise

the variables plotted or revise the x-axis or y-axis label, click on Revise. You may also add a title

to your plot by clicking Title and entering a title when prompted. You must click on Revise to

25
incorporate the title into the plot. When you are finished with this dialog box, click OK to return

to the MOIST Main Dialog Box.

Note: When you plot the boundary conditions, two of the parameters are substantially smaller than

the other parameters. Therefore, they are displayed as nearly zero values. Ifyou would like to see a plot ofthese two parameters, then select only these variables on the Graphics Editing Dialog Box.

This will produce a plot of these two parameters.

Re Edit Analysis w nil

0 P

T X M 1

It Plat Graphs

Revile Title

I Print | OR |

,--N / Uyst^lMC

Figure 15. Graphing Dialog Box.

CHAPTER 7. GETTING A HARD COPY OF THE INPUT DATA

After you have plotted the results, you may want to get a hard copy of all the input data parameters.

If you selected a full hard copy documentation, the output potentials (i.e., temperature, moisture

content, water-vapor pressure, and logarithm of capillary pressure) will also be displayed after the

input data parameters. From the MOIST Main Dialog Box (see Figure 4), click on Present Results

and a report of the MOIST simulation will be displayed on your computer screen as shown in Figure

16. You can use the vertical scroll bar to move you through the complete set of input data. If you

would like a hard copy of these results, then click Print and this report will be sent to your printer.

Click OK to return to the MOIST Main Dialog Box.

CHAPTER 8. MANAGING FILES

After you run and analyze a project, you may save the project to a file. In this way, you will be able

to open this project at a later time either to re-run it with revisions or to re-examine the results.

When you save a project, all the input data parameters given in the dialog boxes, including the

property data catalog of the materials, is saved to files (see Appendix B for a complete description

of the files). The procedures for opening and saving project files are described below.

26
m Piesent Result:

Piim IE

MOIST Run Data and Results

INPOT PARAMETERS

1 COHVECTIOH COEF AT IHSIDE SURFACE

2 CONVECriOH COEF AT OUTSIDE SURFACE

3 SOLAR ABSORPTANCE OF EXTERIOR SURFACE

4 SURFACE TILT (DEGREES)

5 SURFACE AZIMUTH ORIENTATION (DEGREES)

6 INDOOR TEMPERATURE

7 INDOOR RELATIVE HUMIDITY, PERCENT

8 FILE TYPE (UYEC=1, SPECIAL=2)

9 BOUNDARY FILE NAME

10 INSIDE SURF PAINT PERMEANCE

11 OUTSIDE SURF PAINT PERMEANCE

12 LATITUDE (FOR HON US USERS)

13 LONGITUDE (FOR NON US USERS)

14 TIME ZONE (FOR NON US USERS)

15 TIME STEP

MODEL OPTIONS

2 600 11.401

.700

90.

0. 21.000

30.000

1 wymadwi

.570E+03

. llOE+03

42.0 75.0

510

1 TEMPERATURE CALCULATION INCLUDED? Y

2 CAPILLARY TRANSFER INCLUDED? Y

3 LATENT HEAT INCLUDED? Y

4 VARIABLE INDOOR RH MODEL INCLUDED? H

HALL CONSTRUCTION

LAY DES L T MC NX R M

1 14 1 270 21.0 . 5 3 2 1.94 1949.94

3 29 1 270 21 . 0 7.0 5

Figure 16. A Report of the MOIST Simulation.

Saving a Project to File

From Manage projects of the MOIST Main Dialog Box (see Figure 4), click on Save Project, and

the Dialog Box for Saving Current Project shown in Figure 17 is displayed on your computer screen.

ISave Files

FuUdovn driver list and

Clicl to j^Iect dfi«es: DblCficV to ^Meci directoii*?: Sive project &s: iBc: HI mm

Cmcd

sample Ijndl sampledjndl

Figure 17. Dialog Box for Saving Current

Project.

Under the Pull down driver list, click and select the drive onto which you wish to save the project.

Then double-click on the directory into which you wish to save the project. To see a list of all the

directories available, double-click on the root directory (i.e., c:\ in this example). You may either

type in a new file name in the Save project as data box or select an existing project name by

clicking and selecting one of the existing project files. Files names must be seven characters or less.

Do not enter afile extension because the MOISTprogram does that automatically. If you select an

existing file, then the new project will be copied over the existing project. The old data will be lost.

Click OK to save the current project as named and to return to the MOIST Main Dialog Box.

Clicking Cancel returns to the MOIST Main Dialog Box without saving the current project file.

Note: You can not save a MOIST project to file until after the MOIST run is completed. This is

because the output results are saved along with a complete set of input data.

27
Opening an Existing Project File

From Manage projects of the MOIST Main Dialog Box (see Figure 4), click on Open Project, and

the Dialog Box for Opening an Existing Project shown in Figure 18 will be displayed on your

computer screen.

Open Project..

Futldoim driver list ntd

Clicl Id select drives: OblClicl to rel»ct diiectmies: Click to j«l»cl fiUv: Be: 3 Piojecl F3et;

Ot

s<rmp1eljnd1_ sampleZ.mdl

Figure 18. Dialog Box for Opening an

Existing Project.

Under the Pull down driver list, click and select the drive from which you wish to retrieve the

project. Then double-click on the directory from which you wish to retrieve the project. To see a

list of all the directories available, double-click on the root directory (i.e., c:\ in this example). Click

on the file name (which you wish to retrieve) in the Project Files data box. Click on OK to open

the specified project file and to return to the MOIST Main Dialog Box. Click Cancel to return to

the MOIST Main Dialog Box without opening the specified project file.

CHAPTER 9. ACHIEVING CONVERGENCE OF THE MATHEMATICAL SOLUTION

MOIST predicts the temperature and moisture content of the construction layers as a function of

time of year using the finite-difference model described in Appendix A. In the model, each layer of

the construction is subdivided into equal-size subvolumes. A heat and moisture balance is

performed on the nodes at the center of these subvolumes. The number of nodes for each layer are

specified in the Construction Data Entry Box (see Figure 5). The heat and moisture fluxes between

nodes are predicted by linear approximation. As the number of nodes is increased, the approximate

solution approaches the "exact" solution. However, the computer time also increases as the number

of nodes is increased. Therefore, it is desirable to find the fewest number of nodes in each layer that

achieves convergence of the mathematical solution.

An optimum number of nodes in a layer is determined by carrying out a series of simulations with

an increasing number of nodes in the layer. The moisture content of the layer is plotted versus time

of year for each of the simulations. An adequate number of nodes is deemed to exist when the

predicted moisture content of the layer is nearly the same as a simulation with twice as many nodes.

This process should be carried out for each of the layers. Some general guidelines on choosing an

appropriate number of nodes is given below. These general guidelines should only be used as a

starting point. It is imperative that the above procedure be applied to make certain that the number

of nodesfor each layer is sufficient to achieve convergence of the mathematical solution.

28
Each layer must have a minimum of two nodes. The total number of nodes in a construction

(including all the layers) must not exceed 500.

When the construction layers operate in the hygroscopic regime (i.e., liquid water does not coalesce

in the large pore spaces), the number of nodes needed in very permeable materials (e.g., gypsum

board, fiberboard sheathing, etc.) is 2 per cm (5 per inch). In relatively impermeable materials (e.g.,

sugar pine and plywood), 8 nodes per cm (20 nodes per inch) are needed. In very thin layers (i.e.,

vapor retarders), two nodes are sufficient.

When the construction layers operate in the capillary regime (i.e., the surface rh of a material rises

above 97% and its moisture content rises above maximum sorption), then at least ten times as many

nodes are needed to accommodate the very large variations in moisture properties that occur in the

wet-dry transition. When a layer gets wet only at one of its surfaces, the layer may be subdivided

into two separate layers: a thin surface layer and a remainder layer. Considerably fewer nodes are needed to achieve convergence in the two separate layers compared with a single whole layer.

CHAPTER 10. MODELING LOW DENSITY INSULATIONS AND AIR SPACES

When modeling low density insulation (e.g., glass fiber blanket insulation and cellulose insulation),

it is recommended that you treat the material as a non-storage layer. That is, it is recommended that

you neglect the storage of heat and moisture in the layer. This will provide the MOIST model with

the maximum amount of numerical stability. As a non-storage layer, you will specify the thermal

resistance of the insulation expressed in m^-°CAV (ft •!i-°F/Btu) and water-vapor permeance

expressed in kg/m^-s-Pa (perm). The permeance for glass fiber insulation is equal to 1.69 x 10 '°

kg/Pa-s-m (116 perm-inch) divided by the thickness in meters (inches), and the permeance for

cellulose insulation is equal to 1.37 x 10"'°kg/Pa-s-m (94 perm-inch) divided by the thickness in

meters (inches).

MOIST also has the capability of modeling low density insulation as a storage layer. However, when

low density insulation is modeled in this fashion, the MOIST model may become unstable. In this

situation, the moisture content in the insulation may experience unreasonable fluctuations or the

become negative. This problem is caused by very large moisture transfer rates occurring in the

insulation due to its very high permeability. Often a stable solution can be obtained by choosing a

large number of finite-difference nodes in the insulation layer (e.g., > 30 nodes) and using a small

time step (e.g., < 0.1 hour). It usually requires some trial-and-error choices for these two parameters

to achieve a stable solution. Material properties for glass fiber insulation and cellulose insulation

are given in the table below.

29
Low Density Insulation Properties

Li. i/jjci vy TTiliPt* XlUCi f^lcicc TTicii1<;i^irkTi VTlM^d XlldUlctilllll V^CIIUIUSC xllsUIallUIl

out IJllUII V^UCAlldCIlid*

1 cf .^ot*rifinn f~^opFFipipTit ^ A 1 ^ 0 001 703 0 2737

^lixX '?iirl Ovll ^ofiition I^ApffiPiPfit V^i/CiildCIII. i'A'?^ \r\^j 0 lF-07 S QS3

0 Qft3 0 QQQS

let rUffiicivitv Pnpf (C1\ 0 0

'^nd Diffiisiivitv Pnpf (C7\ 0 0 0 0

Cof- IVTrkicl'iiT'o ^^rktil'Pti^' /''\/^ 0 0 4t.uf»

¥-fp£il" TTfii Fi cfpt* T^i*i^npt*l"ipc

flCdi X i CtlldiCi IT 1 ilUCI lies*

r^i*v iJij T^pncii'v x/ciiaii'Y Q fill ro 44 0^ 7^^

71 1 8 ^'^ 1

7'»

1^1 J X lit::! Ill* V^UIlilUCilVllj

T^PiYinPfiifiii'P l^npf ^ 0 0 0 0

Mni<2tiirp Pnpf TP 0 0 0 0

\/5it^r4f PpftnP€*f%ilif"\7 ^^/^pf c* T d^Ui X ci lllcaUlIllj V^Uci ^*

1st Permeability Coef. (Bl) 0.1824E-09 (125) 0.1372E-09 (94)

2nd Permeability Coef. (B2) 0.0 0.0

3rd Permeability Coef. (B3) 0.0 0.0

Values in parenthesis are English unit values.

An air space is modeled as a non-storage layer. When the width of an air space is small (e.g., < 0.64

cm (0.25 in), it may be treated as a stagnant air layer having a permeability equal to 1.75 x

10"'°

kg/Pa-s-m (120 perm-inch). The permeance is found by dividing the permeability by the air space

thickness expressed in meters (inches). When the width of an air space is 1.3 cm (0.5 in) or larger,

the air space becomes a convective air space. The exact value of the permeance of a convective air

space may be calculated using the Lewis relationship. However, most moisture analyzes are not very

dependent on the particular value used, providing that the value is reasonable. The authors

recommend using a value 1.3 x 10'^ (230 perm) for all convective air space widths.

30
CHAPTER 11. FEEDBACK AND COMMENTS

The authors would greatly appreciate any comments that you have concerning MOIST. This

feedback will us help develop improved releases in the future. Please send any comment(s) to:

Douglas M. Burch

NIST

Heat Transfer Group

Building 226; Room B320

Gaithersburg, MD 20832

(301) 975-6433

dburch@nist.gov

31

NOMENCLATURE

SI English

Symbol Units Units Definition

A, ft^ House floor area J/kg-°C Btu/lb-°R Dry specific heat

Cw J/kg-°C

W/m^-°C^

Btu/lb-°R Specific heat of water Ct Btu/hft^°F^ Temperature coefficient for thermal conductivity

Cv W/m°C Btu/hft°F Moisture coefficient for thermal conductivity

(I7s)^-cm--''(m/s)"^ cfm^in"'''mph'^

cfm^-in-''-°F'

Wind coefficient for whole house

(L/s)^-cm-*-°C' Stack coefficient for whole house

Y mVs ft'/h Liquid diffusivity

E Emittance factor

*lv J/kg

W/m^-°C

Btu/lb Latent Heat of Vaporization

Btu/lTft^-°R Convective heat transfer coef at inside surf. W/m^-°C Btu/h-ft^-°R Convective heat transfer coef at outside surf. W/m^-°C Btu/h-ft^-°R Radiative heat transfer coef at inside boundary

W/m2-°C Btu/h-ft^-°R Radiafive heat transfer coef at outside boundary

^sol W/m^ Btu/h-ft^ Incident solar radiation onto a surface

k W/m-°C Btu/h-ff°R Moist thermal conductivity

'^drv W/nT°C Btu/h-ff°R Dry thermal conductivity

K kg/m-s-Pa Ib/ffh-inHg Hydraulic conducfivity

II ft^ Effecfive leakage area for whole house

kg/m^-s-Pa perm Effecfive inside surface permeance

Mfi kg/m^-s-Pa perm Air film permeance at inside surface

kg/m^-s-Pa perm Paint permeance at inside surface n Hourly summation index

N _ _ Current hour

Pi Pa inHg Capillary pressure

Pv Pa inHg Water vapor pressure

Pv.i Pa inHg Water vapor pressure of indoor air

t

' s h Time

T °C °R Temperature

Ti °C °R Indoor temperature

T„r °c °R Reference temperature [24°C(24°F)]

T„ °c °R Mean temperature

°c °R Sky temperature

T„0 °c °R Outdoor air temperature

V. m/s mph Average wind speed

V ,n

h,t y

mVs cfm House mechanical ventilation rate mVs cfm House natural ventilation rate mVs cfm Total house ventilation rate m ft Distance

W kg/s Ib/h Moisture generation rate Z(n) Exponential weighting factors Y kg/kg lb/lb Moisture content on dry mass basis

K kg/sTn^ Ib/h-ft^ Sorption constant per unit floor area

Pa kg/m' lb/ft' Air density

Pd kg/m' lb/ft' Dry material density a W/m^-°C* Btu/h-ft^-R" Stefan-Boltzmann constant

|i kg/m-s-Pa perm Water-vapor permeability T s h Moisture storage time constant

Relative humidity

Indoor relative humidity

Hygroscopic memory

kg/kg lb/lb Indoor air humidity rafio kg/kg lb/lb Outdoor air humidity ratio

33
REFERENCES

ASHRAE. 1993. 1993 ASHRAE Handbook - Fundamentals. American Society of Heating,

Refrigerating and Air-Conditioning Engineers, Inc. Atlanta.

Bliss, R.W. 1961. "Atmospheric Radiation Near the Surface of the Ground." Solar Energy, 5, 103.

Burch, D.M., Thomas, W.C., and Fanney, A.H. 1992. "Water Vapor Permeability Measurements of

Common Building Materials." ASHRAE Transactions, 98(2).

Burch, D.M. and Desjarlais, A.O. 1995. "Water-Vapor Measurements of Lx)w-Slope Roofing

Materials." NISTIR 5681, National Institute of Standards and Technology, July.

Duffie, J. A. and Beckman, W.A. 1991. Solar Engineering of Thermal Processes. Second Edition.

John Wiley & Sons, Inc.

Hansen, K.K. 1986. "Sorption Isotherm - A Catalog." Technical Report 162/86. Technical

University of Denmark, Department of Civil Engineering, Building Materials Laboratory, Dec.

International Energy Agency. 1996. Annex 24. Heat, Air and Moisture Transfer through New and

Retrofitted Insulated Envelope Parts (HAMTIE). Task 3: Material Properties (Volume 3).

Palmiter, L. and Bond, T. 1991. "Interaction of Mechanical Systems and Natural Infiltration,"

Proceedings of AIVC Conference on Air Movement and Ventilation Control Within Buildings,

Ottawa, Ontario, Canada.

Pedersen, C.R. (Now Carsten Rode). 1990. "Combined Heat and Moisture Transfer in Building

Constructions." Report No. 214. Technical University of Denmark. September.

Richards, R.F., Burch, D.M. and Thomas, W.C. 1992. "Water Vapor Sorption Measurements of

Common Building Materials." ASHRAE Transactions, 98(2).

Sherman, M.H. and Grimsrud, D.T. 1980. "Infiltration-Pressurization Correlation: Simplified

Physical Modeling." ASHRAE Transaction, 86 (2):778.

TenWolde, A. 1994. "Ventilation, Humidity, and Condensation in Manufactured Houses during

Wmitir ASHRAE Transactions, Vol. 100, Part 1.

Tsongas, G.A. 1990. "The Northwest Wall Moisture Study: A Field Study of Excess Moisture in

Walls and Moisture Problems and Damage in New Northwest Homes." Report No. DOE/BP-91489-

1. U.S. DOE/Bonneville Power Administration; Portland, OR; June.

Tveit, A. 1966. "Measurement of Moisture Sorption and Moisture Permeability of Porous

Materials." Report No. UDC 532.685. Oslo: Norwegian Building Research Institute.

34
APPENDIX A. THEORY OF MOIST (RELEASE 3.0)

This section describes the theory for MOIST (Release 3.0). hi the previous version of the

MOIST model (Release 2.1), temperature was used as the potential for heat flow, and moisture

content was used as the potential for moisture flow. In the new model, temperature is used as the

potential for heat flow, water-vapor pressure is used as the potential for vapor transfer, and

capillary pressure is used as the potential for liquid flow.

Assumptions

Some of more important assumptions of the analysis are:

• heat and moisture transfer are one dimensional;

• the construction is airtight, and the transport of moisture by air movement

is neglected;

• the wetting of exterior surfaces by rain is neglected;

• snow accumulation on horizontal surfaces, and its effect on the solar absorptance

and thermal resistance is neglected; and

• the transport heat by liquid movement is neglected.

Other assumption are discussed below.

Basic Transport Equations

The basic transport equations are taken from Pedersen' (1990) and are briefly presented below.

Within each material layer of the construction, the moisture distribution is governed by the following

conservation of mass equation:

dy dy dy dy dt

The first term on the left side of eq (A-l) represents water-vapor diffusion, whereas the second term

represents capillary transfer. The right side of eq (1) represents moisture storage within the material.

The potential for transferring water vapor is the vapor pressure (pj with the permeability (|n) serving

as a transport coefficient. The potential for transferring liquid water is the capillary pressure (p,) with

the hydraulic conductivity (K) serving as the transport coefficient. The signs on the first two terms

are different because water vapor flows in the opposite direction of the gradient in water vapor

^ Carsten Pedersen has changed his name to Carsten Rode.

35
pressure, and capillary water flows in the same direction of the gradient in capillary pressure. Other

symbols contained in the above equation include the dry density of the material (p^), moisture

content (y), distance (y), and time (t). The sorption isotherm (i.e., the relationship between

equilibrium moisture content and moisture content) and the capillary pressure curve (i.e., the

relationship between capillary pressure and moisture content) were used as constitutive relations in

solving eq (1).

The hydraulic conductivity (K) in eq (A-1) is related to the liquid diffusivity (D^) by the relation:

dPi (A-2)

where the term in the denominator of the right side of the equation is the derivative of the capillary

pressure with respect to moisture content. When the temperature is above freezing, the liquid

diffusivity is calculated using eq (2) given in the main body. When the temperature is below

freezing, the liquid diffusivity is taken to be zero.

The temperature distribution is calculated from the following conservation of energy equation:

a „ aT, ^ d , 5Pv, , ,dT

The first term on the left side of eq (3) represents conduction, whereas the second term is the latent

heat transfer derived from any phase change associated with the movement of moisture. The right

side of eq (2) represents the storage of heat within the material and accumulated moisture. Symbols

in the above equation include temperature (T), the dry specific heat of the material (Cj), the specific

heat of water (c^), and latent heat of vaporization (h,y).

In eq (A-1), the water vapor permeability (|u) and the hydraulic conductivity (K) are strong functions

of moisture content. In eq (A-3), the thermal conductivity (k) is also a function of temperature and

moisture content (see eq (3) in the main body).

Adjacent Layer Boundary Conditions

The temperature, water-vapor pressure, and capillary pressure are continuous at adjacent material

layers.

Indoor Boundary Conditions

At the indoor boundary of the construction, the moisture transfer through an air film and paint layer

(or wallpaper) at the indoor surface is equated to the diffusion transfer into the solid material surface,

or:

36
dp

Me,i(Pv,i - Pv) = -M^^ (A-4)

where the quantities are evaluated at the indoor boundary. Here, an effective conductance (M^

defined by:

L + _L (A-5)

has been introduced. The effect of a thin paint layer is taken into account as a surface conductance

(Mpi) in series with the convective mass transfer coefficient (Mf j) associated with the air film.

At the same boundary, the heat transferred through the air film, ignoring the thermal resistance of

the paint layer, is equated to the heat conducted into the indoor boundary, giving:

dy

where all quantities are evaluated at the indoor surface.

Outdoor Boundary Conditions

At the outdoor boundary of the construction, a similar set of equations are applied to compute the

moisture transfer. The heat conducted into the outdoor boundary and the absorbed solar radiation

is set equal to the heat loss to the outdoor air by convection and radiation, or:

where all quantities are evaluated at the outdoor surface. Here \^ is the radiative heat transfer

coefficient defined by the relation:

K,o

- Eo{T^ + T^^){T^ ^ tX) (A-8)

37
where E is the emittance factor which includes the surface emissivity and the view factor from the

outdoor surface to the sky, is the surface temperature, and T^^y is the sky temperature. The solar

radiation (Hjo,) incident onto exterior surfaces having arbitrary tilt and orientation was predicted

using algorithms given in Duffie and Beckman (1991). The sky temperature was calculated using

an equation developed by Bliss (1961).

Variable Indoor Humidity Model

When the interior of a residential building can be modeled as single zone with a uniform temperature

and relative humidity, the MOIST program has an option to permit the indoor relative humidity to

vary during the winter. The variable indoor relative humidity feature is only applicable to

residential buildings and should not be applied to non-residential applications. When this option

is selected, the indoor conditions are determined in the following fashion.

Space Heating Operation. When the daily average outdoor temperature is less than or equal to the

balance point temperature for space heating, the house operates in a space heating mode. The indoor

temperature is taken to be equal to the heating set point temperature. The indoor relative humidity

is permitted to vary and is calculated from an indoor humidity model by TenWolde (1994).

The rate of moisture production by the occupants is equal to the rate of moisture removed by natural

and/or forced ventilation plus the rate of moisture storage at interior surfaces and furnishings, or:

Using the psychometric relationship between humidity ratio (o)) and relative humidity ((j)) as a

constitutive relation, the above equation may be solved for the indoor relative humidity.

The hygroscopic memory {'^^^ is computed from the scanning function:

E Z(n)(|)i(n)

1 n=N-4T . . ... ^^^^ ^ (A-10)

n=N-4i:

In the above equation, the hygroscopic memory at the current time step N is found from a pasthistory relative humidity time series. The time series is evaluated over four time constants (t). The

exponential weighting factors Z(n) are defined as:

Z(n) = e

-(N-")^" (A-11)

38
TenWolde (1994) has conducted experiments and determined the sorption constant per unit floor area (k) and the time constant (x) for manufactured and site-built houses.

The natural ventilation rate for the house is predicted by the single-zone Lawrence Berkeley

Laboratory (LBL) infiltration model developed by Sherman and Grimsrud (1980) and described by

ASHRAE (1993) which is given by:

r IT - T I +C v^ (A-12)

When mechanical ventilation „, is present, the total ventilation rate t (A-9) is determined

by the relation (Palmiter and Bond 1991):

If V,^ < 2V,„ , V,, = V,„ . 0.5 V,^

If Km >- 2V,, , V, , = V.

It should be noted that in eq (A- 13), is the actual mechanical ventilation rate produced by the

ventilation equipment installed in the house, as opposed to the rated value. The actual value is

typically about half of the rated value (Tsongas 1990).

In the hourly calculations, the dew-point temperature of the indoor air is compared with the

temperature of the inside glass surface to determine if window condensation occurs. When

condensation occurs, the vapor pressure of the indoor air is taken to be equal to the saturation

pressure at the inside glass surface. The indoor relative humidity is calculated from the indoor

temperature and vapor pressure using psychometric relationships.

Space Cooling Operation. When the daily average outdoor temperature is greater than or equal to

the balance point temperature for space cooling, the house operates in a space cooling mode. The

indoor temperature and relative humidity are maintained at constant specified values.

Window Opened Operation. When the daily average outdoor temperature is greater than the balance

point for space heating and less than the balance point for space cooling, then neither space heating

nor space cooling are required. It is assumed that the windows are opened, and the indoor

temperature and relative humidity are equal to the outdoor values. When the space cooling

equipment is turned off, it was assumed that the occupant will open the windows, and the building

again operates in a floating mode.

Solution Procedure

A FORTRAN 77 computer program, called the MOIST 3.0, was prepared to solve the above system

of equations. Finite-difference equations were developed to represent the basic moisture and heat

transport equations (eqs (A-1) and (A-3)).

39

APPENDIX B. DESCRIPTION OF FILES IN MOIST DIRECTORY

The following files should be in the MOIST directory after the program is installed.

MOIST.EXE: This is the Visual Basic program module that performs the Window environment

input/output processing for the MOIST program.

ANALYZE.EXE: This is the FORTRAN program module that performs the heat and moisture

transfer analysis.

MSTRST.EXE: This is the Visual Basic program module that resets the input data to the original

state as installed.

Run-Time Data Files

The following files are input data files for the MOIST program:

File Name File Description

PDATA.DAT Property Data of the Materials

DEFSET.DAT General Input Parameters

CONST.DAT Building Construction Data

PEROD.DAT Analysis Intervals

INDOOR.DAT Indoor Parameters for Calculating Variable Indoor RH

OPTS.DAT Analysis Options

UNITS.DAT Selected System of Units

CHECK.DAT Diagnostic Parameters

A corresponding set of eight files with a BCK extension are the original MOIST files which are used

to reset the input data to the original state as installed.

41
ASHRAE WYEC Weather Files:

File Name File Description

WYMADWI Madison, WI weather data

WYBOSMA Boston, MA weather data

WYATLGA Atlanta, GA weather data

WYLKCLA Lake Charles, LA weather data

WYPOROR Portland, OR weather data

WYWASDC Washington, DC weather data

RESULT FILES

The following files are output files generated by the MOIST program.

File Name File Description

RESULTS.BND boundary conditions for a simulation

RESULTS.MC instantaneous layer moisture contents

RESULTS.SMC instantaneous surface moisture contents of construction layers

RESULTS.SRH instantaneous surface relative humidities of construction layers

RESULTS.ARH weekly-average surface relative humidities of construction layers

RESULTS.OUT summary of all input data for a MOIST project

RESULTS.AQ boundary heat fluxes

RESULTS.MP boundary moisture fluxes

RESULTS.AR time-avg. thermal resistance of construction

RESULTS.AST time-avg. surface temperatures of construction layers

A corresponding set of nine files with a file name RESULT with the above extensions are the

original MOIST result files as installed.

42
FILE MANAGEMENT

These files contain information on saved MOIST projects. File *.ABT contains run identification

information on saved models, File *.MDL contains input data for saved models, and File *.RST

contains analysis results for saved models. Here the "*" refers to the run name assigned by the user. These saved project files may be retrieved during a MOIST session using the Open Project...

command button.

Ifyou would like to archive savedfiles, then copy the above threefilesfor save projects onto afioppy

disk drive.

43

APPENDIX C. USING NON-WYEC WEATHER DATA

This appendix describes how to use the MOIST program with non-WYEC weather data.

First you will need to convert your weather data file into WYEC format. Here you will need to give

your reformatted weather data file a unique seven-character file name that differs from the standard

names given to WYEC files. The first two letters of the file name must be "WY." A WYEC

weather data file contains 8760 hours (or 365 days) of hourly weather data. Each hour is represented

by a row of ASCI data. Beginning with hour 0 of January 1, the hourly weather data must be placed

in the following format:

Item Field Position Field Description

1 6-8 Dry Bulb Temperature, °F

2 9-11 Wet Bulb Temperature, °F

3 12-14 Dew Point Temperature, °F

4 15-17 Wind Direction

5 18-20 Wind Speed, knots

6 21-24 Station Pressure X 100, inHg

7 56-59 Solar Radiation X 10, Langleys

8 74-75 Month

9 76-77 Day

10 78-79 Hour (0 to 23)

11 80 Snow Indicator

In the above table, "field position" refers to the location of the data item in a horizontal field of data.

Items 2, 4, 6, and 1 1 are not used by the MOIST program and values of zero should be used in the

designated fields. Note that the station pressure is multiplied by 100. A station pressure of 29.92

inHg is entered as 2992. Similarly, the solar radiation is multiplied by 10, and a solar radiation of

10.1 Langleys is entered as 101.

Next, you will need to place this file in the MOIST directory. From the Input Parameters Dialog

Box, you may now select this weather data file and enter the latitude, longitude, and time zone for

the weather data location. When the program runs, it will used your new weather data file.

45



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