Hygrothermal Analysis 

of Wood-Frame Wall 

Assemblies in a 

Mixed-Humid Climate

Samuel V. Glass

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–675

So

al

r ra

diation

W

ni

d-driven

rain

Moisture transfer

Heat transfer
April 2013

Glass, Samuel V. 2013. Hygrothermal analysis of wood-frame wall assemblies in a mixed-humid climate. Research Paper FPL-RP-675. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 25 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726–2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

The USDA prohibits discrimination in all its programs and activities on the 

basis of race, color, national origin, age, disability, and where applicable, 

sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or a part 

of an individual’s income is derived from any public assistance program. 

(Not all prohibited bases apply to all programs.) Persons with disabilities 

who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET 

Center at (202) 720–2600 (voice and TDD). To file a complaint of discrimination, write to USDA, Director, Office of Civil Rights, 1400 Independence 

Avenue, S.W., Washington, D.C. 20250–9410, or call (800) 795–3272 

(voice) or (202) 720–6382 (TDD). USDA is an equal opportunity provider 

and employer.

Abstract

This study uses a one-dimensional hygrothermal model 

to investigate the moisture performance of 10 residential 

wood-frame wall assemblies in a representative mixedhumid climate location of Baltimore, Maryland (climate 

zone 4A). All the assemblies include oriented strandboard 

(OSB) sheathing and vinyl siding. The walls differ in stud 

cavity thickness, level of cavity insulation, presence and 

type of exterior insulation, and class of interior vapor retarder. The performance of the wall assemblies is compared 

in four respects. First, seasonal trends in moisture, humidity, 

and temperature conditions are investigated, and the potential for wintertime moisture accumulation in OSB sheathing 

from vapor diffusion is assessed. Second, the rate at which 

OSB sheathing is able to dry from a high initial moisture 

content is compared for each wall assembly at four different 

times of year. Both the vapor diffusion and drying performance studies include sensitivity analyses to gauge the 

extent to which simulation results depend on various input 

parameters. Third, the response of OSB moisture content to 

wind-driven rain intrusion is examined. Finally, the contribution of air exfiltration to wintertime moisture accumulation in OSB sheathing is evaluated.

Keywords: Hygrothermal analysis, moisture performance, 

simulation, durability, building envelope, wood-frame, 

oriented strandboard, wind-driven rain, air leakage

Acknowledgments

This study was supported by the NAHB Research Center 

through the U.S. Department of Energy Building America 

Program and by the USDA Forest Products Laboratory 

(FPL). This report was improved by critical comments 

from Joe Wiehagen and Vladimir Kochkin of the NAHB 

Research Center, C.R. Boardman of FPL, and Anton 

TenWolde, formerly of FPL.

Contents

1. Introduction.................................................................... 1

 1.1 Moisture Sources and Moisture Transfer................. 1

 1.2 Moisture-Related Properties of Wood and Wood 

 Products.......................................................................... 2

 1.3 Moisture Performance Thresholds........................... 2

 1.4 Methods of Hygrothermal Analysis......................... 3

2. Goals and Objectives ..................................................... 4

3. Modeling Approach and Input Parameters .................... 4

 3.1 Wall Assemblies....................................................... 5

 3.2 Material Properties................................................... 5

 3.3 Wind-Driven Rain.................................................... 6

 3.4 Air Exfiltration ......................................................... 7

 3.5 Wall Orientation....................................................... 8

 3.6 Initial Conditions ..................................................... 8

 3.7 Calculation Period.................................................... 8

 3.8 Outdoor Climate....................................................... 8

 3.9 Indoor Environment................................................. 8

 3.10 Sensitivity Analysis...............................................10

4. Results and Discussion .................................................10

 4.1 Vapor Diffusion Analysis: Seasonal OSB 

 Moisture Content, Temperature, and Relative 

 Humidity .......................................................................10

 4.2 Drying Performance of OSB...................................15

 4.3 Effect of Wind-Driven Rain on OSB 

 Moisture Content ..........................................................17

 4.4 Effect of Air Exfiltration on OSB Moisture 

 Content..........................................................................19

5. Summary.......................................................................20

 5.1 Moisture Accumulation from Vapor 

 Diffusion .......................................................................20

 5.2 OSB Drying Performance .......................................21

 5.3 Wind-Driven Rain Intrusion ...................................21

 5.4 Air Exfiltration ........................................................22

 5.5 Wall Assembly Performance Comparison ..............22

References.........................................................................22
Hygrothermal Analysis of Wood-Frame 

Wall Assemblies in a Mixed-Humid Climate

Samuel V. Glass, Research Physical Scientist

Forest Products Laboratory, Madison, Wisconsin

1. Introduction

Moisture performance is a key consideration in building 

envelope design. The occurrence of moisture problems 

resulting from poor design, construction, or unexpected 

interactions of new building materials can lead to a host of 

undesirable consequences: wood decay, mold growth, poor 

indoor air quality, infestation and degradation by insects, 

corrosion of metals, loss of thermal resistance in wet insulation, damage to materials and finishes from expansion or 

contraction, and loss of strength in building materials to the 

point of structural failure. Building repairs and litigation 

incur considerable costs.

Designers have many options in contemporary wood-frame 

wall construction when it comes to selecting materials such 

as exterior cladding, insulation, and vapor retarders. Building codes have recently required higher insulation levels, 

and in some cases continuous exterior rigid insulation may 

be an option or a requirement, depending on climate zone. 

Moisture performance is a multi-faceted issue; desirable 

performance includes avoiding moisture accumulation 

from bulk water intrusion, air leakage, and vapor diffusion, as well as having some degree of “forgiveness” such 

that assemblies have the ability to dry out if they get wet 

(either during construction or during their service life). The 

construction industry is currently not in agreement when it 

comes to the implications of exterior rigid foam insulation. 

For example, although exterior insulation can warm sensitive materials such as wood structural sheathing during cold 

weather and reduce the risk of condensation (Straube 2011), 

many practitioners question the risk involved in limiting the 

drying potential of the wall with vapor-impermeable foam 

(Gibson 2010). This issue involves many considerations, 

and moisture performance can depend strongly on the 

climate and the type of cladding, for example. This study 

attempts to clarify the various aspects of moisture performance by comparing 10 different residential wood-frame 

wall assemblies using hygrothermal modeling.

Hygrothermal analysis is a tool for evaluating the temperature and moisture conditions that might occur within a 

building envelope assembly over time. Such analysis can 

improve the understanding of how the building envelope 

responds to the interior and exterior environment and can 

identify potential moisture performance problems (such 

as those mentioned above). An important concept in hygrothermal analysis is that of “load.” This term is used in 

hygrothermal analysis in the sense of a burden or demand 

on the building; the response of the building to the loads 

can be analyzed, and the performance can be judged to be 

acceptable or unacceptable (TenWolde 2011). Hygrothermal loads are analogous to loads in structural analysis (e.g., 

snow load, dead load, wind load) and to heating and cooling loads (sensible and latent loads) in mechanical system 

design. Hygrothermal loads include initial moisture levels in 

building materials; indoor temperature and humidity levels; 

outdoor conditions such as temperature, humidity, wind, 

rain, and solar radiation; and air pressure differences across 

the building envelope.

This section provides a brief background on moisture 

sources, moisture transfer mechanisms, moisture-related 

properties of wood, performance thresholds, and methods 

of hygrothermal analysis.

1.1 Moisture Sources and Moisture Transfer

Moisture can come from a variety of sources interior or exterior to the building; it can also come from wet materials in 

newly constructed buildings, such as fresh concrete, green 

lumber, and wet-applied insulations (Christian 2009). Indoor 

moisture sources include people, pets, plants, combustion, 

cooking, dishwashing, showering, bathing, cleaning, washing and drying of clothing, damp foundations, mechanical 

humidification, swimming pools, saunas, fountains, etc. 

Outdoor sources include rain, fog, dew, snow, humid air, 

and soil moisture. Leaks caused by the failure of building 

materials can occur inside or outside a building.

Moisture migration can occur through a number of pathways. For building physics applications, the most important 

are the following (Kumaran 2009; Karagiozis 2001):

•	 Liquid water can be transported in a number of ways: 

gravitational flow, liquid diffusion driven by a difference 

in moisture content, and capillary flow driven by suction 

pressure. An example of the latter is water absorption in 

porous claddings wetted by wind-driven rain, such as masonry, stucco, or wood.

•	 Water vapor can be transported by the flow of air (convection). This includes air leakage through unintended 

gaps in the building envelope, such as at electrical outlets 

and around windows, doors, light fixtures, and pipes. This 

also includes convective looping within wall cavities with 

low-density fibrous insulation.
Research Paper FPL–RP–675

2

•	 Water vapor can migrate by diffusion, driven by a difference in the partial pressure of water vapor (hereafter 

called “vapor pressure”). The material parameter associated with vapor diffusion is known as vapor permeability.

Wetting can occur through any of these mechanisms. Drying 

can occur by air flow and vapor diffusion. Liquid transport 

can also redistribute moisture from a wet region over a 

larger volume.

1.2 Moisture-Related Properties of Wood 

and Wood Products

All the wall assemblies analyzed in this study are of woodframe construction and include oriented strandboard (OSB) 

sheathing. A brief overview of some important moisture 

relations is provided here; for further information, see Carll 

and Wiedenhoeft (2009) and Glass and Zelinka (2010).

Moisture content (MC) is defined here as the ratio of the 

mass of water in a given volume of dry material to the mass 

of the same volume of material in an oven-dry condition, 

expressed either as a decimal or as a percentage. A number 

of properties of wood depend on moisture content, such 

as dimensional shrinkage and swelling, strength, moisture 

transfer, and thermal properties. Water can exist in wood 

in the liquid or vapor phase in the cell lumina (cavities) or 

as bound water in the adsorbed phase within the cell walls. 

Conceptually, the fiber saturation point is defined as the 

moisture content at which cell walls are completely saturated (maximum amount of bound water) and no liquid water 

exists in the cell lumina; the fiber saturation point of wood 

averages approximately 30% MC but can vary by species 

and within individual pieces of wood. Adhered wood products such as OSB are generally less hygroscopic than solid 

wood (Carll and Wiedenhoeft 2009) and thus have lower 

fiber saturation points. This is a result of the manufacturing 

process, which involves high temperature and pressure, and 

a result of the fact that OSB includes non-hygroscopic resins 

and wax.

Below the fiber saturation point, moisture content of wood 

depends on the relative humidity (RH) and temperature of 

the surrounding air. Equilibrium moisture content (EMC) 

at a given temperature and RH is defined as that moisture 

content at which wood is neither gaining nor losing moisture. The relationship between EMC and relative humidity 

at constant temperature is referred to as a sorption isotherm. 

The history of a wood specimen also affects its EMC; this 

is called sorption hysteresis. The EMC at a particular RH is 

higher when equilibrium is reached from a prior wet condition (desorption) than when reached from a prior dry condition (adsorption). However, this phenomenon is commonly 

neglected in many hygrothermal analyses. For this study, 

an average of the adsorption and desorption curves is used. 

Figure 1 shows an average sorption isotherm for solid wood 

and for OSB at room temperature.

The term “condensation” is commonly used in the construction industry and the building science community in a broad 

sense (see the discussion regarding the dew point method in 

Section 1.4). Strictly speaking, condensation is the change 

in phase from vapor to liquid. The change from the vapor 

phase to the adsorbed phase (bound water) is known as sorption. Capillary condensation may occur in small pores in 

wood at very high relative humidity levels. The general term 

“moisture accumulation” is preferred to describe an increase 

in moisture content either from sorption or capillary condensation (in either the hygroscopic or over-hygroscopic range).

1.3 Moisture Performance Thresholds

A detailed overview of failure criteria for building materials 

is given by Viitanen and Salonvaara (2001). Potential issues 

relevant to wall structural sheathing are wood decay, mold 

growth, corrosion of metal fasteners, expansion/contraction damage, and loss of structural capacity. It is commonly 

held that the moisture content of wood needs to exceed the 

fiber saturation point for decay fungi to initiate propagation, 

while at levels below 20% MC their propagation is completely inhibited. The growth of decay fungi in wood also 

requires favorable temperature (approximately 5 to 40 °C). 

The traditional guideline for protection of wood and wood 

products from decay has been to keep the moisture content 

below 20% (Carll and Highley 1999). Although molds require a temperature range similar to that of decay fungi, they 

can propagate on surfaces without free water, provided the 

surface RH remains elevated. For protection 

of hygroscopic surfaces against mold propagation, 

0

5

10

15

20

25

30

20 40 60 80 100

E

quilibrium

m

oisture content (%)

Relative humidity (%)

Solid wood

Oriented strandboard

0

Figure 1. Sorption isotherms for solid wood and oriented 

strandboard (OSB) at room temperature. The curve for solid 

wood is based on the Wood Handbook (Glass and Zelinka 

2010). The curve for OSB is based on a fit to data compiled 

from Richards et al. (1992); Kumaran (2001); Kumaran et al. 

(2002); Ojanen et al. (2006); and Hartley et al. (2007).
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

3

International Energy Agency (IEA) Annex 14 recommended 

that the surface relative humidity be kept below 80% RH 

on a monthly mean basis (Hens 1990). A similar criterion 

for minimizing mold growth was adopted by ASHRAE 

Standard 160 (ASHRAE 2009a): a 30-day running average 

surface RH < 80% when the 30-day running average surface 

temperature is between 5 and 40 °C. This RH level corresponds to an equilibrium MC of about 16% for solid wood 

and about 13% for OSB (see Fig. 1). Corrosion of metal 

fasteners can occur when moisture content exceeds 18% to 

20% (Dennis et al. 1995). Expansion/contraction damage 

depends on the magnitude of the change in moisture content 

and the sensitivity of the particular wood product to such 

changes. Structural capacity generally decreases as moisture 

content increases. When wood products are used in conditions where moisture contents exceed 19% for solid wood or 

16% for engineered wood products (including OSB), structural design standards require that wet service factors be applied because of reduced strength and stiffness. Performance 

thresholds for OSB are further discussed by Drumheller and 

Carll (2010).

This information is provided as a frame of reference. The 

purpose of this analysis is to compare the moisture performance of different wall constructions and to examine 

how their performance depends on certain environmental 

and material parameters. As discussed by Viitanen and Salonvaara (2001), failure predictions generally include a high 

degree of uncertainty because material properties may vary 

widely, boundary conditions cannot always be known with 

satisfactory accuracy, and degradation mechanisms have 

their own statistical variability that is then coupled with the 

uncertainty in the simulated temperature and moisture conditions. Pass/fail criteria are therefore not assigned in this 

study. However, the various performance metrics evaluated 

in this study do indeed allow for relative comparison of the 

different wood-frame wall assemblies.

1.4 Methods of Hygrothermal Analysis

Heat, air, and moisture (HAM) analysis methods can range 

widely in terms of the physical phenomena that they include 

(ASHRAE 2009b; Straube and Burnett 2001). On one end 

of the spectrum are simple steady-state models that include 

only heat conduction and vapor diffusion with constant material properties; on the other end are comprehensive computer models that include transient heat, vapor, liquid, and 

air transfer in as many as three dimensions, with variable 

material properties and detailed descriptions of phenomena 

such as airflow and wind-driven rain.

The traditional dew point method and its limitations are 

described in the ASHRAE Handbook of Fundamentals 

(ASHRAE 2009b) and TenWolde and Bomberg (2009). For 

a given indoor and outdoor temperature (typically monthly 

or seasonal mean values), the temperatures at each material 

interface within an assembly are calculated based on 

steady-state heat flow using thermal resistance values of 

each material. The corresponding saturation vapor pressures 

can then be calculated. Vapor pressures are calculated at 

each material interface based on steady-state vapor diffusion, indoor and outdoor vapor pressures, and vapor permeance values of each material. If the vapor pressure exceeds 

the saturation vapor pressure at any location, then this condition is called “condensation.”

The dew point method has many significant limitations. 

Moisture storage in hygroscopic materials is neglected, and 

all moisture transfer mechanisms other than vapor diffusion 

are excluded (air movement, liquid water flow, wind-driven 

rain, solar effects). The dependence of vapor permeance 

values on relative humidity is neglected. As noted by 

TenWolde and Bomberg (2009), the focus of this method is 

restricted to prevention of sustained surface condensation; 

many building failures, such as mold and mildew, buckling 

of siding, or paint failure, are not necessarily related to surface condensation. Conversely, some materials can tolerate 

limited “condensation,” depending on the temperature conditions and the drying capability.

Over the past three decades, many detailed computer models 

have been developed to simulate temperature and moisture 

conditions in building envelope assemblies over time. Hens 

(1996) describes the general principles of hygrothermal 

analysis and provides details on models used in IEA Annex 

24. Further information on some advanced hygrothermal 

models can be found in ASTM Manual 40 (Trechsel 2001). 

Recent work within IEA Annex 41 has led to the development of whole-building HAM models (Woloszyn and Rode 

2008).

This study focuses on comparing the moisture performance 

of various wood-frame wall assemblies. For this application, a transient one-dimensional model with an hourly time 

step is sufficient. Software used in this study is Wärme und 

Feuchte instationär (WUFI, Transient Heat and Moisture) 

developed by the Fraunhofer Institute for Building Physics 

(IBP 2011). This hygrothermal model can account for the 

following phenomena:

•	 Heat capacity and heat transfer, including effects of phase 

change

•	 Moisture storage and moisture transfer, including vapor 

diffusion and liquid flow

•	 Material properties as a function of moisture content and 

temperature

•	 Boundary conditions including indoor temperature and 

humidity, outdoor temperature, humidity, solar radiation, 

cloud cover, wind speed and direction, rainfall, and winddriven rain

•	 Wind-driven rain penetration at a specified material layer
Research Paper FPL–RP–675

4

•	 Moisture accumulation from air exfiltration at a specified 

material layer

Further information regarding the model can be found in 

Künzel (1995) and Trechsel (2001).

Air leakage through insulated building envelope assemblies 

is recognized as an important moisture transfer mechanism, 

especially for lightweight wood-frame cavities with lowdensity insulation (Straube and Burnett 2001, 2005; Glass 

and TenWolde 2007). Simulating air leakage in a realistic 

manner is difficult because the flow paths through building 

envelope assemblies are three-dimensional and difficult to 

define. Nevertheless, a number of studies have used oneand two-dimensional models to investigate moisture accumulation from air leakage (see, for example, Ojanen and 

Kumaran 1992, 1996; Burch and TenWolde 1993; Hagentoft 

and Harderup 1996; Kalamees and Kurnitski 2010). For 

one-dimensional models, air leakage is assumed to occur 

uniformly; however, measurements indicate that air leakage 

can be concentrated at specific sites, where moisture accumulation is likely to be higher (Tsongas and Nelson 1991). 

Nevertheless, a one-dimensional air exfiltration model is 

useful for comparing relative performance of different wall 

assemblies.

2. Goals and Objectives

This study investigates the moisture performance of 10 different residential wood-frame wall assemblies in a mixedhumid climate. The walls differ in stud cavity thickness, 

level of thermal insulation, presence and type of exterior insulation, and use of a Class II (0.1–1 perm) versus Class III 

(1–10 perm) interior vapor retarder. All walls include OSB 

sheathing, which serves as a common location for drawing 

comparisons. The goals of this study are to gain insight into 

how these wall assemblies respond to hygrothermal loads, to 

identify which components of the assembly have the greatest influence on moisture performance, and to identify potential moisture performance problems. The performance of 

the walls is compared in four important respects. First, seasonal trends in moisture, humidity, and temperature conditions are investigated, and the potential for wintertime moisture accumulation in OSB sheathing from vapor diffusion is 

assessed. Second, the rate at which OSB sheathing is able to 

dry from a high initial moisture content is compared. Drying 

capability is important because there is a good chance that 

wall assemblies will get wet at some time during their service life. Both the drying performance and seasonal trends 

include a sensitivity analysis to gauge the extent to which 

simulation results depend on varying the input parameters. 

Third, the effect of wind-driven rain intrusion on OSB moisture content is examined. Finally, the extent to which air 

exfiltration contributes to wintertime moisture accumulation 

in OSB is investigated.

The objectives of the study are as follows:

•	 Rank the various wall constructions in terms of minimizing the potential for moisture accumulation in the OSB 

sheathing by vapor diffusion and by air leakage

•	 Determine whether wintertime moisture accumulation 

differs in walls with 140-mm- (5.5-in.-) thick cavity insulation relative to walls with 89-mm- (3.5-in-) thick cavity 

insulation

•	 Determine the extent to which wintertime moisture accumulation in OSB sheathing depends on the vapor permeance of the interior vapor retarder

•	 Determine the effects of exterior insulation on moisture 

accumulation in OSB resulting from vapor diffusion and 

air leakage 

•	 Rank the wall assemblies in terms of the drying capability 

of the OSB sheathing

•	 Determine critical parameters that affect drying rates

•	 Determine whether rigid insulation placed to the exterior 

of the OSB reduces the OSB drying rate

•	 Determine whether the presence of a kraft vapor retarder 

reduces the OSB drying rate

•	 Rank the wall assemblies in terms of their response to 

wind-driven rain penetration

3. Modeling Approach and Input 

Parameters

Simulations were run using WUFI® Pro 5.1 software for 

one-dimensional transient heat and moisture transfer (IBP 

2011). Model input parameters include the following:

•	 Wall construction, geometry of components, and material 

properties of each component

•	 Moisture sources such as wind-driven rain or air 

exfiltration 

•	 Wind-driven rain exposure

•	 Wall orientation

•	 Surface heat and mass transfer coefficients

•	 Initial temperature and moisture content in each 

component

•	 Calculation period (start and end dates and time step)

•	 Numerical calculation parameters

•	 Outdoor climate

•	 Indoor environment

Default values were selected for surface heat and mass 

transfer coefficients and numerical calculation parameters. 
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

5

Convergence failures were eliminated by selecting a fine numerical grid and adaptive time step control. Other inputs are 

described below. Many of these refer to ASHRAE Standard 

160 (ASHRAE 2009a). A sensitivity analysis was conducted 

with three of the wall assemblies to determine the extent to 

which simulation results depend on certain input parameters.

3.1 Wall Assemblies

All wall assemblies were simulated as having been constructed with wood framing, either standard 38 by 89 mm 

(nominal 2 by 4 in.) or standard 38 by 140 mm (nominal 

2 by 6 in.). Wood framing was not modeled; the model is 

one dimensional and includes a section through the insulated cavity rather than the framing. All wall assemblies had 

an interior finish of 12.5-mm (1⁄2-in.) gypsum board, one 

coat of latex primer, and one coat of latex paint. The primer 

and paint were not modeled as separate layers; instead, an 

interior surface diffusion resistance was specified to correspond with the desired vapor permeance. Exterior structural 

sheathing in all wall assemblies was 11 mm (7/16 in.) OSB. 

In most cases, the water-resistive barrier (WRB) was a spunbonded polyolefin (SBPO) membrane; however, for walls 

with extruded polystyrene (XPS) insulation, this membrane 

was omitted (in some cases XPS with taped seams qualifies 

as a WRB; for the purpose of hygrothermal simulation, it 

makes no difference whether an SBPO membrane is present 

in walls with exterior XPS). In the wall with exterior mineral fiber insulation (MFI), the SBPO membrane was placed 

between the OSB and the MFI. All wall assemblies were 

clad with vinyl siding. Table 1 summarizes the differences 

between the various wall assemblies.

3.2 Material Properties

Nominal thermal resistances (R-values) are expressed in this 

report in units of h·ft2·°F·Btu–1. In the simulations, thermal 

conductivity values have a default temperature coefficient 

of 2 × 10–4 W·m–1·K–2; thermal conductivity values were 

equated to values calculated from thickness and nominal 

R-values at a temperature of 24 °C (75 °F).

The vapor permeance values of certain materials were expected to strongly influence the simulation results. The key 

materials are described below. Vapor permeance values are 

given in units of U.S. perms. One perm is equivalent to 

1 grain·ft –2·h–1·(in Hg) –1. At 23 °C where laboratory permeance measurements are typically performed, 1 perm = 

57.45 ng·m–2·s–1·Pa–1 (Thompson and Taylor 2008; note 

that the conversion from in Hg to Pa is temperature dependent). Simulations were performed in the SI unit system, 

and values were entered either as vapor diffusion resistance 

factor or as sd value (the thickness of a still air layer with 

permeance equivalent to that of the material of given thickness). For conversions, a value of 190 ng·m–1·s–1·Pa–1 was 

used for the vapor permeability of still air.

3.2.1 Latex Primer and Paint on Interior Gypsum 

Board

Literature vapor permeance values show considerable variation, indicating that this parameter is important to include in 

the model sensitivity analysis. At the low end, the ASHRAE 

Handbook of Fundamentals (ASHRAE 2009c) lists values 

between 6 and 9 perms. ASHRAE Research Project 1018 

(Kumaran et al. 2002) gives the permeance of gypsum board 

with one coat of primer and two coats of latex paint as a 

function of relative humidity (RH); values in the 40% to 

60% RH range, typical of indoor conditions, are between 

5 and 12 perms. Other studies have measured dry cup values 

of 16 to 40 perms for gypsum board with two coats of latex 

paint (Martin and Verschoor 1994; MHRA 2000; NAHBRC 

2010). 10 perms was selected as the default value, and in the 

sensitivity analysis, the vapor permeance of the primer/paint 

layer was varied with values of 5 and 20 perms.

3.2.2 Asphalt-Coated Kraft Paper Facing on Batt 

Insulation

Measured values generally show an increase in permeance 

with increasing RH. The vapor permeance for this study 

was a user-defined function of RH based on literature data, 

as shown in Figure 2. Three design curves are shown. The 

“default” curve follows data from Burch et al. (1992). 

Table 1. Wall assembly insulation details

Wall Abbreviation Framinga Insulation

1 R13 KFB 2 by 4 R-13 kraft-faced batt

2 R19 KFB 2 by 6 R-19 kraft-faced batt

3 R13 UFB 2 by 4 R-13 unfaced batt

4 R19 UFB 2 by 6 R-19 unfaced batt

5 R23 BIBS 2 by 6 R-23 blow-in blanket systemb

6 R13 KFB + R5 XPS 2 by 4 R-13 kraft-faced batt in cavity plus exterior R-5 extruded polystyrene

7 R19 KFB + R5 XPS 2 by 6 R-19 kraft-faced batt in cavity plus exterior R-5 extruded polystyrene

8 R13 UFB + R5 XPS 2 by 4 R-13 unfaced batt in cavity plus exterior R-5 extruded polystyrene

9 R13 UFB + R10 XPS 2 by 4 R-13 unfaced batt in cavity plus exterior R-10 extruded polystyrene

10 R13 KFB + R5 MFI 2 by 4 R-13 kraft-faced batt in cavity plus exterior R-5 mineral fiber insulation

a Framing dimensions are nominal values in inches. Note that framing is not modeled; the model is one dimensional and includes a

slice through the insulated cavity rather than the framing.

b The vapor diffusion resistance of the non-woven fabric in the blow-in blanket system is assumed to be negligible.
Research Paper FPL–RP–675

6

The “high” curve is an attempt to fit data at the higher end 

(ASHRAE 2009c, NAHBRC 2010). The “low” curve is an 

attempt to fit data at the lower end (ASHRAE 2009c; Gatland 2005).

3.2.3 Oriented Strandboard

The properties of OSB (with density of 650 kg·m–3) from 

the WUFI Generic North America Database were used in 

the simulations. The vapor permeance at a thickness of 

11 mm as a function of RH is shown as the default curve in 

Figure 3. This curve is reportedly taken from the measurements of Kumaran et al. (2002); it may be an average of the 

three different OSBs. It should be noted that WUFI includes 

simulation of liquid water transport at RH values above 80% 

in addition to vapor diffusion with RH-dependent vapor 

permeance. Figure 3 also shows other literature data (Dahl 

et al. 1996; Karagiozis and Wilkes 2004; Kumaran 2001; 

NAHBRC 2010; Ojanen et al. 2006; Timusk et al. 2009). 

These data were fit using a nonlinear least squares method; 

the best fit was found to agree well with the default curve. 

The “low” and “high” curves are the default curve scaled by 

factors of 1⁄2 and 2, respectively.

3.2.4 Spun-Bonded Polyolefin Membrane

The default vapor permeance was 50 perms, independent 

of RH (taken from the WUFI Generic North America Database). Other measured values range from 16 perms to 

approximately 120 perms (Kumaran 2001; Kumaran et al. 

2002; NAHBRC 2010). In the sensitivity analysis, the 

vapor permeance was varied by factors of 1⁄2 (25 perms) 

and 2 (100 perms).

3.2.5 Extruded Polystyrene Rigid Insulation

The default vapor permeance was 0.76 perms for 25 mm 

(1 in., R-5) thickness, independent of RH (taken from the 

WUFI Generic North America Database). Kumaran (1996, 

2001) lists values ranging from 0.65 perms to 1.4 perms. 

Kumaran et al. (2002) give a value of 0.84 perms. Manufacturer product literature gives a value of 1.1 perms. In the 

sensitivity analysis, a value of 1.4 perms was included in 

addition to the default value.

3.2.6 Rigid Mineral Fiber Insulation

The MFI product selected from the WUFI material database 

had a density of 87 kg·m–3 and a thermal conductivity of 

0.035 W·m–1·K–1 at 24 °C. A thickness of 31 mm 

(1.2 in.) was selected to achieve a thermal resistance of 

0.88 m2·K·W–1 (R-5). The vapor permeance at this thickness was 89 perms.

3.2.7 Vinyl Siding

An “equivalent vapor permeance” was used for vinyl siding. 

This is a method of modeling a cladding that is vapor impermeable but is back-ventilated by airflow. Values of 40 perms 

(BSC 2010b) and 70 perms (BSC 2010a) have been recommended. The default value used was 40 perms. In the sensitivity analysis, the vapor permeance was varied by factors 

of 1⁄2 (20 perms) and 2 (80 perms). The alternative, which is 

modeling an air space between the siding and house wrap, 

requires the selection of an air change rate. This air change 

rate is currently not well defined, and determining proper 

values is beyond the scope of this study.

3.3 Wind-Driven Rain

The default mode for simulations in this study was not to 

include wind-driven rain (WDR) intrusion. The ASHRAE 

Standard 160 calculation method prescribes that 1% of the 

wind-driven rain that reaches the wall is deposited on the 

exterior side of the WRB. However, this presents a problem 

when the WRB has no moisture storage capacity (as is the 

case for SBPO). An alternative approach is to introduce an 

additional layer in the wall assembly on the exterior side of 

the WRB to provide moisture storage capacity for the WDR 

that penetrates the cladding. This method has been used, for 

example, by Tariku et al. (2007). In this study, additional 

moisture storage layers in the assemblies were not included, 

and wind-driven rain was excluded from the simulations 

with the exception of Section 4.3.

The sensitivity of the wall assemblies to wind-driven rain 

intrusion directly into the OSB layer is examined in Section 

4.3. Note that placing the penetrating rain at this location is 

an excursion from ASHRAE Standard 160. The calculation 

method used the following equation:

 (1)

0

1

2

3

4

5

6

7

8

9

10

20 40 60 80 100

Vap

or permeance (perms)

Relative humidity (%)

ASHRAE (2009c)

Burch et al. (1992)

Gatland (2005)

NAHBRC (2010)

High

Default

Low

0

Figure 2. Data and design curves for water vapor permeance of asphalt-coated kraft paper as a function of relative 

humidity.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

7

where

rbv is rain deposition on vertical wall (kg·m–2·h–1)

FE rain exposure factor

FD rain deposition factor

FL empirical constant (0.2 kg·s·m–3·mm–1)

U hourly average wind speed at 10 m height (m·s–1)

q angle between wind direction and normal to the 

 wall

rh rainfall intensity, horizontal surface (mm·h–1)

Wind speed, wind direction, and horizontal rainfall were 

taken from the outdoor climate file. ASHRAE Standard 160 

rain exposure and deposition factors were selected: FE = 1.3 

is recommended for buildings up to 15 m (50 ft) on severe 

terrain; FD = 0.35 is recommended for buildings with steep 

slope roofs (typical for residential construction). Two cases 

were examined in which 0.5% and 1% of the wind-driven 

rain that reached the cladding was deposited uniformly in 

the OSB layer. Simulations were run for 3 to 5 years to ensure that the annual moisture trend was stable.

3.4 Air Exfiltration

Water vapor carried by exfiltrating air can be deposited on 

a cold surface within the wall cavity. This phenomenon was 

simulated by introducing a moisture source at the interior 

surface of the OSB sheathing, using the IBP Infiltration 

Model in WUFI (IBP 2011). This moisture source 

m (kg·m-2·h–1) was calculated as follows:

 

 (2)

where

q is air flow through the component (m3·m–2·h–1)

ci indoor water vapor concentration (kg·m–3)

csat,T water vapor concentration at saturation 

corresponding to the temperature at the deposition 

site (kg·m–3)

Equation (2) neglects thermal effects associated with air exfiltration and water vapor phase change. It also treats sorption as equivalent to condensation; it assumes that the location where water vapor is deposited is at 100% RH. Only 

wetting is modeled; convective drying is neglected.

0 10 20 30 40 50 60 70 80 90 100

Relative humidity (%)

0

5

10

15

20

25

30

35

Vap

or permeance (perms)

High

Default

Low

Dahl et al. (1996)

Karagiozis and Wilkes (2004)

Kumaran (2001)

Kumaran et al. (2002) OSB 1

Kumaran et al. (2002) OSB 2

Kumaran et al. (2002) OSB 3

NAHBRC (2010)

Ojanen et al. (2006)

Timusk et al. (2009) 689 kg/m3

Timusk et al. (2009) 626 kg/m3

Timusk et al. (2009) 554 kg/m3

Figure 3. Data and design curves for vapor permeance of OSB at 11-mm thickness as a function of relative humidity.
Research Paper FPL–RP–675

8

The air flow rate was calculated as the product of a leakage 

coefficient k (m3·m-2·h–1·Pa–1) and a pressure difference DP

(Pa):

 (3)

The leakage coefficient is associated with the level of airtightness of the construction. The IBP Infiltration Model 

in WUFI includes three default airtightness classes (A, B, 

C). For the simulations in this study, k values of 0.02 and 

0.04 m3·m–2·h–1·Pa–1 were selected, which are three and six 

times leakier, respectively, than the least airtight class (C) of 

the default classes. Pressure differences were assumed to be 

driven by stack effect only, and were calculated as follows:

 

 (4)

where

ρo is density of outdoor air (1.3 kg·m–3)

Ti indoor air temperature (K)

To outdoor air temperature (K)

g gravitational acceleration (9.81 m·s–2)

H height of the building (m)

Equation (4) calculates the pressure difference at the top 

of the wall at the top floor of the building and assumes that 

the reference pressure (neutral pressure plane) is at half the 

building height. A building height of 5 m (two-story residential building) was selected. For the month of January, the 

average pressure difference was 2.8 Pa. Values were generally between 1.5 Pa and 3.5 Pa during winter.

As stated previously, this model assumes that air leakage 

has no effect on temperature. This is reasonable for small 

airflows, but higher airflows entail greater heat flux and can 

raise the sheathing temperature, thereby reducing the rate of 

moisture accumulation. For the conditions investigated here, 

airflows were less than 0.2 m3·m–2·h–1 (0.06 L·m-2·s–1), 

which is well below the point at which air leakage lowers 

the rate of moisture accumulation (Ojanen and Kumaran 

1996). Therefore, the model assumptions are not unreasonable.

3.5 Wall Orientation

The north-facing orientation was selected as the default 

because it was expected to have the highest OSB moisture 

content in winter and the slowest drying rate. South-, 

east-, and west-facing orientations were also examined in 

the sensitivity analysis. In the analysis of summer inward 

vapor diffusion, the south-facing orientation was chosen.

3.6 Initial Conditions

The initial temperature in each component was set to 20 °C. 

For investigating drying rates from a high moisture content, 

the initial moisture content of OSB was set to two times 

the equilibrium moisture content at 80% RH (at 20 °C), in 

accordance with ASHRAE Standard 160. This corresponds 

to 25.6% MC. For investigations other than drying performance, the initial conditions are not relevant; the simulations were run for multiple years to attain stable annual 

trends.

3.7 Calculation Period

Simulations were generally started on October 1 and ended 

3 years later using a 1-h time step. In Section 4.2.1, simulations were started at different times of year to investigate the 

seasonal effect on drying performance.

3.8 Outdoor Climate

Simulations were run with both the “warm year” and “cold 

year” for Baltimore, Maryland. These correspond to the calendar years with third highest and third lowest annual average temperature out of 30 consecutive years (1961–1990). 

These are called “moisture design reference years” in 

ASHRAE Standard 160. Temperature and relative humidity 

conditions are depicted in Figures 4 and 5.

3.9 Indoor Environment

Indoor temperature and humidity conditions were sine 

curves as described below.

–20

–10

0

10

20

30

40

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Oct

N

o

v

Dec

Month

0

10

20

30

40

90

100

Relative humidity (%) Temperature (°C)

80

50

60

70

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Oct

N

o

v

Dec

Month

(a)

(b)

Figure 4. Hourly temperature (a) and relative humidity 

(b) conditions for the Baltimore warm year. Thick curves 

indicate 30-day running average values.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

9

3.9.1 Indoor Temperature

The indoor temperature curve (Fig. 6) has a mean value 

of 22.3 °C (72.1 °F), a minimum of 20.6 °C (69.1 °F) on 

January 15, and a maximum of 24.0 °C (75.2 °F) on July 

15. These values were based on recent measurements from 

a mixed-humid climate location, Knoxville, Tennessee 

(Antretter et al. 2010). These values also happen to be in 

relatively good agreement with the ASHRAE Standard 160 

indoor design temperature, which defines a heating set point 

of 21.1 °C (70 °F) and a cooling set point of 23.9 °C 

(75 °F).

3.9.2 Indoor Humidity

The default indoor humidity curve (Fig. 7), which represents 

higher-than-average humidity levels (approximating 

90th percentile values), has a minimum of 45% RH on 

February 15 and a maximum of 60% RH on August 15. 

For the sensitivity analysis, a second curve was used to represent typical (average) humidity levels. This curve has a 

minimum of 35% RH and a maximum of 55% RH.

The simulation results were expected to depend strongly 

on the winter indoor RH values. The values of 35% RH for 

“typical” and 45% RH for “default” were based on measurements in occupied houses in climate zone 4A (personal 

communication from J. Wiehagen, Energy Engineer, NAHB 

Research Center, February 2011). The following justification supports these selected values.

First, the typical winter minimum value (35% RH) is similar 

to measurements reporting a winter average of 40% RH for 

10 houses in another mixed-humid location, Knoxville, Tennessee (Antretter et al. 2010). Knoxville is a slightly warmer 

climate, so the trend is in the right direction (lower RH in 

the colder location).

Second, the default winter minimum value (45% RH) is 

between the design values calculated according to the 

ASHRAE Standard 160 simplified and intermediate methods. In the simplified method, the indoor RH (at 70 °F) is 

calculated as a function of daily average outdoor temperature. Using instead the 1971–2000 monthly average outdoor 

temperatures for December, January, and February in Baltimore, the winter design indoor RH averaged over these 

months is 51% RH.

–20

–10

0

10

20

30

40

Tem

perature (°C)

0

10

20

30

40

50

60

70

80

90

100

Relative humidity (%)

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Oct

N

o

v

Dec

Month

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Oct

N

o

v

Dec

Month

(a)

(b)

Figure 5. Hourly temperature (a) and relative humidity 

(b) conditions for the Baltimore cold year. Thick curves 

indicate 30-day running average values.

68

69

70

71

72

73

74

75

76

77

20.0

20.5

21.0

21.5

22.0

22.5

23.0

23.5

24.0

24.5

25.0

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Ind

o

or tem

perature (°F)

Ind

o

or tem

perature (°C)

Month

Figure 6. Indoor temperature conditions for simulations.

30

35

40

45

50

55

60

65

70

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Ind

o

or relative humidity (%)

Month

45%

35%

55%

60%

Default curve

Typical curve

Figure 7. Indoor relative humidity conditions for 

simulations.
Research Paper FPL–RP–675

10

The intermediate method calculates the indoor vapor pressure using a mass balance with inputs including outdoor 

vapor pressure, moisture generation rate, and ventilation rate 

(TenWolde and Walker 2001). The ASHRAE Standard 160 

committee has recently approved Addendum b, which revises the residential moisture generation rates in the standard, 

as recent analysis suggested that these rates were too high 

(Glass and TenWolde 2009; TenWolde 2011). For a 

house with four bedrooms, a moisture generation rate of 

11 kg·day–1 was selected (based on the value in the addendum versus the old value of 15 kg·day–1). The default value 

of 0.2 air changes per hour was used, in conjunction with 

a building volume of 500 m3 (17,700 ft3, approximately 

the volume of a house with an area of 2,200 ft2). Using 

1971–2000 monthly average outdoor vapor pressures for 

December, January, and February in Baltimore, the winter 

design indoor humidity value averaged over these months is 

44% RH.

Figure 8 compares indoor and outdoor vapor pressure conditions (calculated from temperature and RH). The outdoor 

vapor pressures are based on the Baltimore warm year. As 

expected, indoor vapor pressure generally exceeds outdoor 

vapor pressure during late fall, winter, and early spring. The 

trend is opposite during summer.

3.10 Sensitivity Analysis

To determine which input parameters are most critical and 

to gauge the extent to which simulation results depend on 

varying the input parameters, a systematic sensitivity analysis was conducted that included several different wall constructions. Only one parameter was altered at a time while 

all the others were held at default values. The default parameters and alternatives are given in Table 2.

4. Results and Discussion

4.1 Vapor Diffusion Analysis: Seasonal 

OSB Moisture Content, Temperature, 

and Relative Humidity

This section discusses the seasonal vapor diffusion performance of the wall assemblies with a focus on wintertime 

moisture accumulation in OSB sheathing. Summertime 

inward vapor diffusion is also considered. Wind-driven rain 

and air exfiltration are not included here; the effects of these 

phenomena are considered in Sections 4.3 and 4.4.

4.1.1 Wintertime OSB Moisture Content

Simulations for each wall assembly were run for 3 years. 

The OSB moisture content during the second and third years 

was nearly identical indicating stable annual cycles. Figures 

in this section show the third year of the simulation.

Figure 9 shows the daily average OSB moisture content (the 

ratio of mass of moisture to mass of dry material expressed 

as a percentage, averaged over the thickness of the OSB 

layer) over a year starting October 1 for Wall 3 with unfaced 

batt insulation (R13 UFB). The figure indicates that OSB 

reaches a higher moisture content when simulated with the 

Baltimore warm year than with the cold year. This trend is 

found for all 10 walls. This peculiar result happens because 

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Vap

or pressure (Pa)

Month

Indoor default

Indoor typical

Outdoor hourly

Outdoor 7-day

running average

Figure 8. Indoor and outdoor vapor pressure conditions for 

simulations (Baltimore warm year).

Table 2. Parameter values for sensitivity analysis

Wall assembly Parameter Default value Alternatives Reference

2. R19 KFB Kraft vapor permeance Default curve Low, high Figure 2

4. R19 UFB Primer + paint vapor permeance 10 perms 5 perms, 20 perms Section 3.2.1

OSBa vapor permeance Default curve × 1⁄2, × 2 Figure 3

SPBOb vapor permeance 50 perms 25 perms, 100 perms Section 3.2.4

Vinyl siding equivalent vapor 

permeance

40 perms 20 perms, 80 perms Section 3.2.7

Wall orientation North South, East, West Section 3.5

Indoor humidity Default curve Typical curve Figure 7

8. R13 UFB

+ R5 XPS

XPSc vapor permeance 0.76 perms 1.4 perms Section 3.2.5

a

Oriented strandboard.

b

Spun-bonded polyolefin membrane. c

Extruded polystyrene.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

11

Figure 10a compares seasonal trends in the four walls having a kraft vapor retarder. The annual variation in moisture 

content is small, with values reaching approximately 11% 

in winter and approximately 7% in summer. Wintertime 

MC values are slightly higher for walls with R-19 cavity 

insulation than their R-13 counterparts; this difference is 

more pronounced for the walls with exterior XPS insulation. 

The slight differences in OSB moisture content for walls of 

differing thickness stem from differences in temperature of 

OSB during winter, as discussed later.

In contrast, wall assemblies without a kraft vapor retarder 

reach much higher OSB moisture contents during winter, as 

shown in Figure 10b. Peak values range from about 13% in 

Wall 10 (R13 UFB + R5 MFI) to about 24% in Wall 3 (R13 

UFB). Walls 4 (R19 UFB) and 5 (R23 BIBS) are nearly 

identical. In this analysis that considers only vapor diffusion, a kraft vapor retarder has a greater effect on keeping 

OSB dry than exterior insulation. This finding could have 

been predicted from a simple dew point method calculation.

The walls with exterior insulation (without a kraft vapor 

retarder) accumulate less moisture in the OSB than the R13 

UFB reference wall, as a result of the OSB temperature being higher. Two effects are apparent. First, OSB moisture 

content is considerably lower with R10 XPS than with R5 

XPS, consistent with the OSB being warmer with R10 XPS; 

warmer OSB tends to stay drier. Second, OSB moisture content is considerably lower with R5 mineral fiber insulation 

than with R5 extruded polystyrene. This difference occurs 

because MFI is vapor permeable, allowing water vapor to 

pass through the OSB to the exterior, whereas XPS is vapor 

impermeable, impeding outward drying of the OSB.

4.1.2 Wintertime Temperature and Relative Humidity

Figures 11a and 11b compare temperatures at the interior 

surface of the OSB for walls with and without a kraft vapor 

retarder. 30-day running average values are depicted for 

ease of comparison (hourly or daily average values fluctuate to a large degree, making visual comparison difficult). 

The kraft vapor retarder essentially has no effect on OSB 

temperature. As expected, the OSB is warmer during winter 

in walls with exterior insulation. R-5 XPS and R-5 MFI are 

practically identical, and R-10 XPS keeps the OSB warmer 

than R-5 XPS (Figure 11b). Walls with R-13 cavity insulation have slightly warmer OSB than otherwise identical 

walls with R-19 cavity insulation.

Figure 12 illustrates the effect of the kraft vapor retarder 

on the relative humidity at the interior surface of the OSB. 

Wall 3 (R13 UFB) has a much higher RH than Wall 1 (R13 

KFB). Figures 13a and 13b show the relative humidity at the 

interior surface of the OSB for the same walls as depicted 

in Figures 11a and 11b, respectively (using 30-day running 

averages).

The relationship between wintertime relative humidity and 

temperature at the interior surface of the OSB is depicted 

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

O

S

B

m

oisture content (%)

Month

R13 UFB

Warm year

Cold year

Figure 9. OSB moisture content in Wall 3 (R13 UFB) for one 

year simulation using Baltimore warm and cold years.

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

(a) R13 KFB

R19 KFB

R13 KFB + R5 XPS

R19 KFB + R5 XPS

0

5

10

15

20

25

30

O

S

B

m

oisture content (%)

(b) R13 UFB

R19 UFB

R23 BIBS

R13 UFB + R5 XPS

R13 UFB + R10 XPS

R13 UFB + R5 MFI

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

O

S

B

m

oisture content (%)

Figure 10. OSB moisture content in walls with (a) and 

without (b) a kraft vapor retarder.

the 10th percentile warm and cold years are selected based 

on annual average temperature. The “warm” year actually 

has a colder winter than the “cold” year. The warm year is 

used in subsequent simulations.
Research Paper FPL–RP–675

12

in Figure 14, using January mean values. This figure indicates that surface RH generally decreases as temperature 

increases, and that RH is generally lower when a kraft vapor 

retarder is present. The trends in temperature and relative 

humidity are consistent with the trends in moisture content 

described above.

Further insight into the moisture dynamics of the different 

walls can be gained by looking at the relative humidity levels at various points through the OSB. Figure 15a shows 

30-day running average RH values at the interior surface, 

–5

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

(a)

R13 KFB

R19 KFB

R13 KFB + R5 XPS

R19 KFB + R5 XPS

–5

0

5

10

15

20

25

30

30-

day running average tem

perature

at O

S

B interior surface (°C)

(b)

R13 UFB

R19 UFB

R13 UFB + R5 XPS

R13 UFB + R10 XPS

R13 UFB + R5 MFI

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

30-

day running average tem

perature

at O

S

B interior surface (°C)

Figure 11. OSB interior surface temperatures (30-day running average values) in walls with (a) and without (b) a kraft 

vapor retarder.

0

10

20

30

40

50

60

70

80

90

100

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Relative humidity at

O

S

B interior surface (%)

Month

R13 KFB

R13 UFB

Figure 12. Effect of kraft vapor retarder on OSB interior 

surface relative humidity (hourly values).

0

10

20

30

40

50

60

70

80

90

100

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

(a)

R13 KFB

R19 KFB

R13 KFB + R5 XPS

R19 KFB + R5 XPS

0

10

20

30

40

50

60

70

80

90

100

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

(b)

R13 UFB

R19 UFB

R13 UFB + R5 XPS

R13 UFB + R10 XPS

R13 UFB + R5 MFI

30-

day running average relative 

humidity at O

S

B interior surface (%)

30-

day running average relative 

humidity at O

S

B interior surface (%)

Figure 13. OSB interior surface relative humidity (30-day 

running average values) in walls with (a) and without (b) a 

kraft vapor retarder.

0

10

20

30

40

50

60

70

80

90

100

–4 –2 0 2 4 6 8 10

January mean RH at OSB 

interior surface (%)

January mean temperature at OSB interior surface (°C)

Walls without kraft VR

Walls with kraft VR

Figure 14. Relative humidity versus temperature at interior 

surface of OSB (January mean values) for walls with and 

without a kraft vapor retarder (VR).
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

13

middle, and exterior surface of the OSB in wall 3 

(R13 UFB). The temperature gradient across the OSB is 

negligible, so the RH values are approximately proportional 

to vapor pressure values. The RH at the interior surface 

exceeds 90% for much of the winter; there is a strong gradient from the interior to the exterior surface of the OSB. 

The OSB middle and interior surface dry below 70% RH by 

May, and in June the gradient is reversed (higher RH at the 

exterior).

Figure 15b shows the effect of adding exterior XPS. First 

the RH at the OSB interior surface is lower than in Figure 

15a because of the temperature effect discussed above. Second, the RH at the exterior surface is elevated compared to 

Figure 15a. This occurs because the low-perm XPS reduces 

the rate at which moisture passes through the OSB when 

the vapor drive is outward. The OSB dries more uniformly 

(smaller RH gradient) than in Figure 15a and at a slower 

rate, from March into July.

When XPS is replaced with MFI (Figure 15c), the temperature effect still lowers the RH at the OSB interior surface 

compared with Figure 15a. In contrast to XPS, the high vapor permeance of MFI in conjunction with the temperature 

effect results in lower RH at the OSB exterior surface compared with Figure 15a. Furthermore, the OSB middle and 

interior surface dry more quickly, falling below 70% RH by 

April.

4.1.3 Sensitivity of Wintertime OSB Moisture Content 

to Varying Parameters

This section examines the effects of varying different 

parameters on wintertime moisture performance of OSB 

sheathing based on vapor diffusion. Parameters were varied 

as described in Section 3.10.

Table 3 summarizes the influence of the different parameters, using maximum wintertime OSB moisture content as 

a metric. Many of the observed trends are expected; trends 

related to indoor humidity levels and vapor permeance of 

the various components could have been predicted from the 

dew point method.

OSB moisture content in Wall 2 (R19 KFB) is sensitive to 

the vapor permeance of the kraft facing: as expected, higher 

permeance corresponds with higher wintertime moisture 

content.

The following observations are made regarding OSB moisture content in Wall 4 (R19 UFB):

•	 Wintertime MC is highly sensitive to the vapor permeance of the latex primer/paint on the interior gypsum 

board. Higher permeance corresponds with higher moisture content, as more water vapor diffuses into the OSB.

•	 Wintertime MC is somewhat sensitive to the vapor permeance of the OSB itself. Higher permeance corresponds 

with lower moisture content, as moisture more readily 

diffuses through the OSB.

•	 Wintertime MC is relatively insensitive to the vapor permeance of the spun bonded polyolefin membrane and to 

the equivalent vapor permeance of the vinyl siding, both 

of which are much more permeable than OSB.

•	 Wintertime MC is highly sensitive to wall orientation. 

The north-facing wall (default) accumulates the most 

moisture; the east- and west-facing walls are nearly identical; and the south-facing wall accumulates the least 

moisture because of solar exposure.

•	 Wintertime MC is highly sensitive to indoor humidity 

levels. The higher (default) indoor humidity curve (Fig. 7) 

results in a greater degree of moisture accumulation than 

the typical indoor humidity curve.

0

10

20

30

40

50

60

70

80

90

100

Month

(a) R13 UFB

0

10

20

30

40

50

60

70

80

90

100

Oct

N

o

v

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Month

(b) R13 UFB + R5 XPS

0

10

20

30

40

50

60

70

80

90

100

Oct

N

o

v

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

30-day running average

relative humidity (%)

Month

(c) R13 UFB + R5 MFI

Interior surface

Middle

Exterior surface

Oct

N

o

v

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

30-day running average

relative humidity (%)

30-day running average

relative humidity (%)

Figure 15. Relative humidity at interior surface, middle, and 

exterior surface of OSB (30-day running average values) for 

(a) Wall 3, R13 UFB; (b) Wall 8, R13 UFB + R5 XPS; and 

(c) Wall 10, R13 UFB + R5 MFI.
Research Paper FPL–RP–675

14

OSB moisture content in Wall 8 (R13 UFB + R5 XPS) is 

slightly sensitive to the vapor permeance of the exterior 

extruded polystyrene insulation. Higher permeance corresponds with lower wintertime MC, as moisture diffuses 

more readily through the OSB and XPS. The vapor permeance of XPS is in a range similar to that of OSB (in contrast 

to SBPO for example), so it does make a difference.

Ranking the sensitivity of the maximum wintertime OSB 

moisture content, it is most sensitive to indoor humidity 

levels. Next, this metric is sensitive to kraft vapor permeance, wall orientation, and primer/paint vapor permeance 

to approximately the same extent. The maximum MC is 

somewhat sensitive to OSB vapor permeance and less so to 

XPS vapor permeance. It is rather insensitive to vinyl siding 

equivalent vapor permeance and SBPO vapor permeance.

4.1.4 Summertime Inward Vapor Diffusion

Inward vapor diffusion occurs during summer when outdoor 

vapor pressure exceeds indoor vapor pressure (Fig. 8). The 

inward vapor drive can be magnified when moisture stored 

in absorptive claddings, such as masonry, stucco, or adhered 

stone veneer, is warmed by solar radiation. This effect has 

been studied extensively with field testing, laboratory measurements, and modeling (Derome 2010). In this study, the 

cladding is vinyl siding, which is nonabsorptive and therefore does not amplify the inward vapor drive.

The location of interest in the wall assemblies is the exterior 

surface (cavity side) of the kraft vapor retarder because this 

layer limits inward drying. Less permeable vapor retarders, 

such as polyethylene, may experience condensation from 

summer inward vapor diffusion. Condensation would not 

be expected on kraft paper because it is hygroscopic and 

because its vapor permeance increases considerably at high 

relative humidity (Fig. 2). However, sustained high RH 

could lead to mold growth. RH levels at the exterior surface 

of the kraft vapor retarder were therefore examined. Simulations were run for south-facing walls using the Baltimore 

warm year.

Figure 16 compares hourly RH values in Wall 1 (R13 KFB) 

using default and low kraft vapor permeance functions. As 

expected, RH values are higher when the vapor permeance 

Table 3. Summary of wintertime vapor diffusion sensitivity analysis 

Wall assembly Parameter Value 

Maximum OSB 

moisture content 

(kg∙kg–1)

Change from 

default

(%)

R19 KFB Default values 0.111 — 

Kraft vapor permeance 

Low 0.104 – 6 

High 0.136 + 22 

R19 UFB Default values 0.223 — 

Primer/paint vapor 

permeance 

5 perms 0.175 – 22 

20 perms 0.268 + 20 

OSB vapor

permeance 

×0.5 0.240 + 8 

×2 0.199 – 11 

SBPO vapor

permeance 

25 perms 0.227 + 2 

100 perms 0.221 – 1 

Vinyl siding 

equivalent vapor 

permeance 

20 perms 0.228 + 2 

80 perms 0.221 – 1 

Wall orientation South 0.173 – 22 

East 0.209 – 6 

West 0.205 – 8 

Indoor humidity Typical curve 0.164 – 26 

R13 UFB XPS vapor Default (0.76 perms) 0.184 — 

+ R5 XPS Permeance 1.4 perms 0.174 – 5 

0

10

20

30

40

50

60

70

80

90

100

1-May 29-May 26-Jun 24-Jul 21-Aug 18-Sep

Relative humidity at kraft exterior surface (%)

Date

Default kraft permeance

Low kraft permeance

Figure 16. Effect of kraft vapor permeance on relative humidity levels at the cavity side of the kraft vapor retarder 

(Wall 1, R13 KFB). Hourly values and 7-day running average 

values are shown for each case.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

15

is lower. Hourly values in some cases reach 93% RH, but 

7-day running average values never exceed 80% RH. 

The maximum 30-day running average values are given in 

Table 4 for the walls with kraft vapor retarders. All values 

are well below the 80% threshold for mold growth (Section 

1.3). Differences between R-13 and R-19 walls are minimal, 

but walls with exterior XPS insulation have somewhat lower 

RH than those without XPS because the low-perm XPS impedes inward vapor diffusion.

4.2 Drying Performance of OSB

4.2.1 Effect of Simulation Starting Date

Figure 17 shows the daily average OSB moisture content 

over a year starting October 1 for Wall 1 (R13 KFB). The 

initial moisture content is approximately 25%. The wall 

faces north, and the default input parameters given previously are used in simulations. There is little difference in 

performance between the warm and cold years, so from here 

on different wall constructions are compared using only the 

warm year.

Figure 18 depicts OSB moisture content for the same wall 

(R13 KFB) with simulations starting October 1, January 1, 

April 1, and July 1. The drying rate is fastest in July and 

slowest in January, as expected; drying normally occurs 

faster at higher temperatures.

Table 5 summarizes the drying performance of all 10 wall 

assemblies. It lists the number of days it takes the OSB to 

dry below 16% MC (an arbitrary threshold for comparing 

relative performance). The wall assemblies can be grouped 

into four different classes:

Table 4. Summertime relative humidity at exterior 

surface of kraft vapor retarder

Maximum 30-day running average 

relative humidity (%)

Wall assembly

Default kraft vapor 

permeance

Low kraft vapor 

permeance

1. R13 KFB 68.7 73.1

2. R19 KFB 68.8 72.9

6. R13 KFB + R5 XPS 63.6 64.7

7. R19 KFB + R5 XPS 64.5 65.1

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

O

S

B

m

oisture content (%)

Month

Warm year

Cold year

Figure 17. OSB moisture content in Wall 1 (R13 KFB) for 

one year simulation starting October 1 using Baltimore 

warm and cold years.

5

10

15

20

25

30

0

30 60 90 120 150 180 210 240

O

S

B

m

oisture content (%)

Day

Starting Oct 1

Jan 1

Apr 1

Jul 1

0

Figure 18. Effect of different starting dates on OSB drying 

performance in Wall 1 (R13 KFB).

Table 5. Number of days for OSB to dry below 16% MC from 

different starting dates

Simulation starting date

Wall assembly October 1 January 1 April 1 July 1

1. R13 KFB 22 64 13 5

2. R19 KFB 21 62 13 5

3. R13 UFB 28 98 14 5

4. R19 UFB 25 90 13 5

5. R23 BIBS 28 91 13 5

6. R13 KFB + R5 XPS 203 136 59 29

7. R19 KFB + R5 XPS 205 137 56 20

8. R13 UFB + R5 XPS 202 125 42 18

9. R13 UFB + R10 XPS 167 111 43 22

10. R13 UFB + R5 MFI 22 66 14 9
Research Paper FPL–RP–675

16

1. Walls 1 and 2 (R13 KFB and R19 KFB) are practically 

identical in drying performance.

2. Walls 3–5 (R13 UFB, R19 UFB, and R23 BIBS) are 

slower to dry in fall and winter than Group 1, but are 

practically identical to Group 1 in spring and summer. 

These walls dry more slowly in fall and winter because 

of the interior humidity load, and they accumulate 

more moisture during cold weather (see Section 4.1). 

Figure 19 shows the wall with R13 UFB as an example 

of this group. Figure 20 depicts vapor pressure conditions 

indoors, outdoors, and at the interior and exterior surfaces 

of the OSB in the same wall starting October 1. This 

figure indicates that the exterior OSB surface dries more 

rapidly than the interior surface and that the vapor pressure gradient across the OSB is from interior to exterior.

3. Walls 6–9 with exterior XPS insulation dry much more 

slowly in all seasons than walls in Groups 1 and 2. 

Figure 21 shows Wall 6 (R13 KFB + R5 XPS) as an 

example of this group. The low-perm XPS greatly impedes outward drying of the OSB. In spring and summer, 

drying is generally faster in walls without a kraft vapor 

retarder (compare R13 UFB + R5 XPS with R13 KFB + 

R5 XPS). Drying is faster for R10 XPS than for R5 XPS 

in fall and winter because the temperature of the OSB is 

higher. However, this trend is reversed in summer; drying is faster for R5 XPS than for R10 XPS because R10 

keeps the OSB cooler in summer and reduces heat flow 

to the OSB.

4. Wall 10 (R13 UFB + R5 MFI) is similar to Groups 1 and 

2. The vapor permeable exterior insulation appears to 

have differing effects in cold weather and warm weather. 

This wall dries faster in fall and winter than Group 2 because the R5 MFI warms the OSB. However, in summer 

it dries slightly slower than Groups 1 and 2 because the 

exterior insulation keeps the OSB cooler and reduces heat 

flow to the OSB.

Comparisons from here on are drawn based on October 1 as 

the starting date. Figure 22a compares wall assemblies without exterior insulation (Walls 1–5). The differences in initial 

drying rate are minimal. The divergence in OSB moisture 

content in late November is evident between walls with and 

without a kraft vapor retarder. This reflects the wintertime 

trends discussed in Section 4.1.

Figure 22b compares OSB drying performance for walls 

with XPS or MFI exterior insulation. In all walls with R-5 

XPS, drying below 20% MC does not occur until spring 

(note the different x-axis between Figures 22a and b). Increasing the R-value of XPS from 5 to 10 (with R-13 UFB) 

improves the drying rate. The most dramatic difference 

is between XPS and MFI (green and orange curves in 

Fig. 22b); permeable exterior insulation allows the OSB to 

dry much more rapidly. The same trend of faster drying with 

exterior MFI than with exterior XPS was found by Smegal 

5

10

15

20

25

30

0 O

30 60 90 120 150 180 210 240

S

B

m

oisture content (%)

Day

Starting Oct 1 Jan 1 Apr 1 Jul 1

0

Figure 19. Effect of different starting dates on OSB drying 

performance in Wall 3 (R13 UFB).

0

500

1,000

1,500

2,000

2,500

1-Oct

15-Oct29-Oct

12-N

o

v

26-N

o

v

10-Dec

24-Dec

7-Jan

21-Jan

7-

day running average vap

or pressure (Pa)

Date

Indoor

OSB interior surface

OSB middle

OSB exterior surface

Outdoor

Figure 20. Vapor pressure conditions (7-day running average values) at indoor and outdoor boundaries and at interior, middle, and exterior of OSB in Wall 3 (R13 UFB).

0

5

10

15

20

25

30

30 60 90 120 150 180 210 240 270 300 330 360

O

S

B

m

oisture content (%)

Day

Starting Oct 1

Jan 1

Apr 1

Jul 1

0

Figure 21. Effect of different starting dates on OSB moisture content in Wall 6 (R13 KFB + R5 XPS).
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

17

and Straube (2011) for simulations of plywood sheathing in 

2 by 6 wood-frame walls in the climate of Portland, Oregon 

(zone 4C).

4.2.2 Sensitivity of Drying Performance to Varying 

Parameters

This section investigates the effects of various parameters 

on the drying performance of OSB sheathing. Simulations 

were started on October 1 using the Baltimore warm year, 

an initial OSB moisture content of 25.6%, and the default 

parameters with variations given in Section 3.10. 

Table 6 summarizes the influence of varying the different 

parameters on OSB drying performance. Drying performance is characterized here in terms of moisture content 

during the first two months, prior to wintertime moisture 

accumulation. Table 6 lists moisture content values at 

7 weeks (November 19), which in most cases is the 

minimum in moisture content after initial drying, prior to 

wintertime moisture accumulation.

The OSB drying rate in Wall 2 (R19 KFB) is insensitive to 

the vapor permeance of the asphalt-coated kraft facing. This 

implies that drying of the OSB is primarily in the outward 

direction. It also happens to be the case that for most of October and November outdoor vapor pressure is lower than 

indoor vapor pressure (Fig. 8).

The following observations are made regarding how the 

OSB drying rate in Wall 4 (R19 UFB) varies with selected 

parameters:

•	 Drying rate is insensitive to the vapor permeance of the 

latex primer/paint on the interior gypsum board.

•	 Drying rate is highly sensitive to the vapor permeance of 

the OSB itself. Higher permeance corresponds with faster 

drying rate, as moisture diffuses more rapidly out of the 

OSB.

•	 Drying rate is slightly sensitive to the vapor permeance of 

the spun bonded polyolefin membrane and to the equivalent vapor permeance of the vinyl siding. Higher permeance corresponds with faster drying rate.

•	 Drying rate is highly sensitive to wall orientation. The 

north-facing wall dries most slowly; the east- and westfacing walls are nearly identical; and the south-facing 

wall dries most rapidly.

•	 Drying rate is somewhat sensitive to indoor humidity 

levels. Higher (default) indoor humidity levels result in 

slower drying.

Drying of Wall 8 (R13 UFB + R5 XPS) is moderately sensitive to the vapor permeance of the exterior extruded polystyrene insulation. Higher permeance corresponds with faster 

drying rate, as moisture diffuses more rapidly out of the 

OSB through the XPS.

Ranking the sensitivity of the OSB drying rate using the 

metric of Table 6, it is most sensitive to wall orientation 

followed by OSB vapor permeance. Next in order of importance is indoor humidity level. Then vinyl siding equivalent 

vapor permeance, SBPO vapor permeance, and XPS vapor 

permeance have approximately the same influence. The drying performance metric is rather insensitive to primer/paint 

vapor permeance and kraft vapor permeance.

4.3 Effect of Wind-Driven Rain on OSB 

Moisture Content

Simulations are compared here for north-facing walls 

using the Baltimore warm year. Over the course of a year, 

142 kg·m–2 of wind-driven rain strikes the north facing wall. 

The simulations look at cases where 0.5% and 1% of this 

incident wind-driven rain is deposited uniformly within the 

OSB layer. The results discussed below are particular to the 

weather file, wall orientation, and modeling assumptions. 

0

5

10

15

20

25

30

1-Oct

15-Oct29-Oct

12-N

o

v

26-N

o

v

10-Dec

24-Dec

7-Jan

21-Jan

Date

(a)

R13 KFB

R19 KFB

R13 UFB

R19 UFB

R23 BIBS

0

5

10

15

20

25

30

1-Oct

29-Oct

26-N

o

v

24-Dec

21-Jan

18-Feb

18-Mar

15-A

pr

13-May

O

S

B

m

oisture content (%)

Date

(b)

R13 KFB + R5 XPS

R19 KFB + R5 XPS

R13 UFB + R5 XPS

R13 UFB + R10 XPS

R13 UFB + R5 MFI O

S

B

m

oisture content (%)

Figure 22. OSB drying performance in walls without (a) and 

with (b) exterior insulation.
Research Paper FPL–RP–675

18

As stated previously, simulations were run for at least three 

years. Stable annual trends were found by the third year in 

all cases.

The moisture content of OSB in Wall 1 (R13 KFB) and the 

amount of incident wind-driven rain are shown in Figure 23. 

The major rain events occur in fall and winter. The steps in 

OSB moisture content with rain events are clearly evident. 

Incident winddriven rain (kg m–2day–1

)

O

S

B

m

oisture content (%)

0

5

10

15

20

25

30

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

Incident WDR

No WDR

0.5% into OSB

1% into OSB

Figure 23. Effect of wind-driven rain intrusion on OSB moisture content in Wall 1 (R13 KFB). Daily values of incident 

wind-driven rain (right axis) were calculated by summing 

hourly values.

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Se

O

p

S

B

m

oisture content (%)

Month

R19 KFB

R19 UFB

R13 UFB + R5 XPS

R13 UFB + R5 MFI

Figure 24. Effect of 1% wind-driven rain intrusion on OSB 

moisture content in various wall assemblies.

For a rain event of a given intensity, OSB moisture content 

increases suddenly and then decreases more rapidly during 

warm weather than during cold weather because drying capability is greater at higher temperature (Section 4.2).

Figure 24 compares OSB moisture content in four selected 

wall assemblies with 1% wind-driven rain intrusion. Values 

for Walls 1 and 2 (R13 KFB, Figure 23, and R19 KFB, Figure 24) are practically identical. In Wall 4 (R19 UFB), wintertime vapor diffusion is dominant and has a greater 

effect on OSB moisture content than wind-driven rain. 

Walls 3 (R13 UFB) and 5 (R23 BIBS) behave similarly (not 

shown). Wall 8 (R13 UFB + R5 XPS) also behaves similarly 

except that it dries more slowly in the spring, as would be 

Table 6. Summary of drying performance sensitivity analysis 

Wall assembly Parameter Value 

OSB moisture 

content 

(kg∙kg-1)

after 7 weeks 

Change

from

default

(%)

R19 KFB Default values 0.130 — 

Kraft vapor 

permeance 

Low 0.129 – 0.6 

High 0.131 + 0.6 

R19 UFB Default values 0.133 — 

Primer + paint 

vapor 

permeance 

5 perms 0.135 + 2 

20 perms 0.131 – 1 

OSB vapor

permeance 

×1⁄2 0.154 + 16 

×2 0.120 – 9 

SBPO vapor 

permeance 

25 perms 0.139 + 5 

100 perms 0.130 – 2 

Vinyl siding 

equivalent

vapor

permeance 

20 perms 0.140 + 6 

80 perms 0.129 – 3 

Wall orientation South 0.107 – 19 

East 0.123 – 7 

West 0.123 – 7 

Indoor humidity Typical curve 0.119 – 10 

R13 UFB XPS vapor 

permeance 

Default (0.76 

perms) 

0.199 — 

+ R5 XPS 1.4 perms 0.188 –5 
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

19

expected from Section 4.2. Wall 10 (R13 UFB + R5 MFI) 

behaves similarly to Wall 2 (R19 KFB); both walls avoid 

excessive moisture accumulation from vapor diffusion and 

are able to accommodate this quantity of wind-driven rain 

because of their drying capability.

Table 7 summarizes the effect of 1% wind-driven rain intrusion on OSB moisture content in all 10 wall assemblies. It 

lists the maximum OSB moisture content in winter and the 

annual mean excess moisture content, which is the difference in OSB moisture content with 1% WDR intrusion relative to the case without WDR intrusion, averaged over one 

year. The trends in mean excess moisture content are similar 

to the drying performance trends discussed above in Section 

4.2. Wall 10 (R13 UFB + R5 MFI) has the greatest capability to survive wind-driven rain intrusion, followed by Walls 

1 and 2 (R13 KFB and R19 KFB). Walls 6 and 7 (R13 KFB 

+ R5 XPS and R19 KFB + R5 XPS) have the highest mean 

MC excess (Table 7) and the slowest drying times (Table 

5). Comparing Walls 8 and 9 (R13 UFB + R5 XPS and R13 

UFB + R10 XPS), R10 XPS performs better than R5 XPS in 

that it lowers the wintertime maximum MC and lowers the 

mean annual excess MC, as expected from the trends in 

Sections 4.1 and 4.2.

Ueno (2010) conducted a similar hygrothermal analysis of 

the effect of a superinsulation retrofit on the capacity of a 

wall to dry after wind-driven rain penetration. In a Massachusetts climate (zone 5A), the original wall construction 

(nominal 2 by 4) could survive 0.5% penetration of incident 

wind-driven rain into the wood sheathing. The north-facing 

wall sheathing reached a maximum of 23% MC in winter 

and dried to below 10% MC in summer (repeatable annual 

cycle over three years). However, under the same rain penetration conditions, a retrofit wall with 100 mm (4 in.) of 

impermeable foil-faced rigid polyisocyanurate insulation 

exterior to the wood sheathing reduced its ability to dry; 

wood moisture content exceeded 30% by the third winter 

in the north-facing wall. A similar result was found when 

the same thickness of extruded polystyrene (XPS) was 

substituted for polyisocyanurate. The same thickness of 

expanded polystyrene (EPS), however, resulted in lower 

winter moisture contents than the original wall under the 

same rain penetration conditions because EPS is more vapor 

permeable than XPS and polyisocyanurate. Although differences in climate and wall construction exist between this 

report and that of Ueno (2010), the trend with respect to 

vapor permeance of the exterior insulation is similar.

4.4 Effect of Air Exfiltration on OSB Moisture 

Content

Simulations are compared here for north-facing walls using 

the Baltimore warm year. These simulations look at air 

exfiltration with leakage coefficients of 0.02 and 0.04 

m3·m–2·h–1·Pa–1, hereafter called “moderate exfiltration” 

and “excessive exfiltration,” respectively. As discussed in 

Section 3.4, these airflows are still relatively small and do 

not violate the model assumption that air leakage does not 

significantly affect the sheathing temperature. As stated 

previously, simulations were run for at least 3 years. Stable 

annual trends were found by the third year in all cases.

Figure 25 compares the effect of air exfiltration on the 

moisture content of OSB for three different wall assemblies. 

Wall 1 (R13 KFB) undergoes a considerable increase in 

wintertime moisture accumulation when air exfiltration is 

included; Wall 6 (R13 KFB + R5 XPS) sees much less of 

an effect; and exfiltration has barely any effect on Wall 9 

(R13 UFB + R10 XPS). The different effects of air exfiltration in these wall assemblies clearly result from the exterior 

insulation warming the OSB during winter, as discussed in 

Section 4.1.2.

Table 8 summarizes the effect of air exfiltration on OSB 

moisture content in all 10 wall assemblies. It gives the maximum OSB moisture content in winter as well as the amount 

of moisture deposited in OSB from air exfiltration. Equation 2 implies that the amount of moisture deposited in 

OSB depends on the rate of air leakage, the indoor humidity level, and the temperature of the OSB interior surface. 

The effect of the rate of air exfiltration is clear from Table 

8. Other modeling studies (e.g., Ojanen and Kumaran 1992) 

have shown that moisture accumulation from air exfiltration 

is more severe when indoor humidity levels are higher.

Two temperature-related trends are highlighted here. First, 

walls with higher levels of cavity insulation accumulate 

slightly more moisture in the OSB sheathing than otherwise 

identical walls with lower levels of cavity insulation. This 

can be seen most clearly by contrasting Wall 6 (R13 KFB + 

R5 XPS) with Wall 7 (R19 KFB + R5 XPS). The trend also 

holds for Wall 1 (R13 KFB) vs. Wall 2 (R19 KFB) and for 

Wall 3 (R13 UFB) vs. Wall 4 (R19 UFB) vs. Wall 5 (R23 

BIBS). Second, increasing the R-value of exterior insulation 

decreases the amount of moisture deposited by exfiltration. 

This trend can be seen by contrasting Wall 3 (no exterior 

insulation), Wall 8 (R5 XPS), and Wall 9 (R10 XPS).

Table 7. Effect of 1% wind-driven rain intrusion on 

OSB moisture content

Wall assembly

Maximum OSB 

moisture content 

(%)

Mean annual 

excess OSB 

moisture content 

(%)

1. R13 KFB 16.4 1.74

2. R19 KFB 16.6 1.78

3. R13 UFB 28.6 2.48

4. R19 UFB 26.4 2.05

5. R23 BIBS 26.6 2.07

6. R13 KFB + R5 XPS 20.3 5.69

7. R19 KFB + R5 XPS 21.7 6.23

8. R13 UFB + R5 XPS 26.1 4.18

9. R13 UFB + R10 XPS 20.5 3.23

10. R13 UFB + R5 MFI 16.1 1.25
Research Paper FPL–RP–675

20

5. Summary

5.1 Moisture Accumulation from Vapor 

Diffusion

All wall assemblies simulated in this study show a trend of 

higher OSB moisture content in winter and lower MC in 

summer. The critical parameters that affect wintertime MC 

are indoor humidity levels and vapor permeance of the interior of the wall assembly. Walls with kraft-faced batts have 

lower wintertime OSB moisture content (typically 10% to 

11% MC) than walls with unfaced cavity insulation (typically 13% to 24% MC). Exterior insulation significantly lowers 

wintertime OSB moisture content in walls with unfaced 

cavity insulation.

In walls without exterior insulation, an interior vapor retarder appears to be necessary to avoid excessive wintertime 

moisture accumulation in OSB in north-facing walls under 

design indoor humidity conditions. Although a number of 

criteria are available for defining performance thresholds, 

this study does not attempt to address pass/fail criteria (see 

Section 1.3), and therefore does not set a threshold for 

permeance of the interior vapor retarder. Nonetheless it is 

noteworthy that OSB in Wall 2 (R19 KFB) simulated with 

“high” kraft permeance (3–4 perms) reaches a maximum of 

14% MC, and that OSB in Wall 4 (R19 UFB) with primer 

and paint on interior gypsum board simulated at 5 perms 

reaches a maximum of 18% MC. When the latter wall is 

simulated with 10- and 20-perm interior finishes, wintertime 

MC rises to 23% and 28%, respectively.

Wall thickness and the R-value of cavity insulation have 

little effect on seasonal vapor diffusion trends in OSB 

moisture content.

Exterior insulation (XPS simulated at R5 and R10 and MFI 

at R5) dampens the seasonal trend in OSB moisture content. The maximum MC in winter and the minimum MC 

in summer occur later in time compared with otherwise 

identical walls without XPS or MFI. In addition, the wintertime MC is lower for walls with exterior insulation than 

Table 8. Effect of different levels of air exfiltration on moisture accumulation in OSB

Maximum OSB moisture content 

(%)

Amount of moisture deposited 

in OSB (kg m–2 yr–1)

Wall assembly No 

exfiltration

Moderate 

exfiltration

Excessive 

exfiltration

Moderate 

exfiltration

Excessive 

exfiltration

1. R13 KFB 10.9 14.4 18.4 0.44 0.87

2. R19 KFB 11.1 14.7 18.9 0.45 0.91

3. R13 UFB 23.6 27.7 32.5 0.43 0.86

4. R19 UFB 22.3 26.2 31.0 0.45 0.90

5. R23 BIBS 22.6 26.5 31.3 0.46 0.91

6. R13 KFB + R5 XPS 10.4 12.0 13.7 0.16 0.31

7. R19 KFB + R5 XPS 11.0 13.2 15.6 0.22 0.44

8. R13 UFB + R5 XPS 18.4 19.9 21.5 0.15 0.29

9. R13 UFB + R10 XPS 14.1 14.3 14.5 0.03 0.06

10. R13 UFB + R5 MFI 13.2 14.5 16.0 0.16 0.32

0

5

10

15

20

25

30

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

(a) R13 KFB

No exfiltration

Moderate

Excessive

0

5

10

15

20

25

30

O

S

B

m

oisture content (%)

(b) R13 KFB + R5 XPS

0

5

10

15

20

25

30 (c) R13 UFB + R10 XPS

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

Oct

N

o

v

Dec

Jan

Feb

Mar

A

pr

May

Jun

Jul

Aug

Sep

Month

O

S

B

m

oisture content (%)

O

S

B

m

oisture content (%)

Figure 25. Effect of air exfiltration on OSB moisture content 

in (a) Wall 1, R13 KFB; (b) Wall 6, R13 KFB + R5 XPS; and 

(c) Wall 9, R13 UFB + R10 XPS.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

21

otherwise identical walls without exterior insulation. OSB 

moisture content is considerably lower with R10 XPS than 

with R5 XPS; the greater the amount of exterior insulation, 

the warmer the OSB, and therefore the lower the relative 

humidity and moisture content. In addition, OSB moisture 

content is considerably lower with R5 mineral fiber insulation than with R5 extruded polystyrene. This difference 

occurs because MFI is vapor permeable, allowing water vapor to pass through the OSB to the exterior, whereas XPS is 

semi-impermeable.

Summertime inward vapor diffusion does not result in excessive cavity RH values, even when simulated with a low 

permeance kraft vapor retarder. However, all the walls were 

simulated with non-absorptive vinyl siding. Further analysis 

of inward vapor diffusion in walls with absorptive claddings 

in mixed-humid climates would be worthwhile.

5.2 OSB Drying Performance

The critical variables that affect OSB drying performance 

are the following:

•	 Time of year: drying occurs most rapidly in summer 

and most slowly in winter.

•	 Wall orientation: south-facing walls dry fastest, followed 

by east- and west-facing walls, with north-facing walls 

drying slowest.

•	 Vapor permeance of exterior wall components: walls 

with exterior XPS insulation dry much more slowly in all 

seasons than walls with exterior MFI insulation or walls 

without exterior insulation; the sensitivity analysis further 

indicates that the higher the vapor permeance of the OSB 

itself, the higher the drying rate; to a lesser extent, the 

higher the vapor permeance of the vinyl siding and the 

SBPO membrane, the higher the drying rate.

•	 Indoor humidity levels: drying is faster with lower indoor 

humidity levels.

OSB with a high initial moisture content dries primarily to 

the exterior for simulations starting on October 1. This is 

evident from the sensitivity analysis, which shows that wall 

orientation and vapor permeance of OSB, SBPO membrane, 

and vinyl siding substantially affect drying rates. In addition, the sensitivity analysis shows that OSB drying rates 

depend minimally on the permeance of the kraft vapor retarder or the interior paint.

For simulations starting at other times of the year, outward 

drying of OSB also contributes significantly to the overall 

drying rate in walls with permeable exterior components. 

Walls with exterior XPS insulation dry at a much slower rate 

than walls with MFI or walls without exterior insulation. 

OSB in walls with XPS does dry to the interior in this climate (except during the four coldest months of the year), but 

the rate of inward drying is slow. Even in July, when inward 

drying would be expected to be most significant, walls with 

exterior XPS take about three to six times longer to dry than 

comparable walls without XPS. This is due partly to the low 

permeance of XPS and partly to a thermal effect. The thermal effect is evident in that the wall with permeable mineral 

fiber insulation takes about twice as long to dry in July than 

the comparable wall without MFI; at other times of year the 

drying rates for these walls are similar to each other. The 

exterior insulation cools the OSB and reduces heat flow to 

the OSB in July (when outdoors is warmer than indoors), 

thereby lowering the drying rate. The same trend exists for 

walls with different levels of XPS insulation: in summer 

drying is faster for R5 XPS than for R10 XPS. This trend 

reverses in fall and winter: drying is faster for R10 XPS than 

for R5 XPS because the OSB temperature is higher with 

R10 XPS.

5.3 Wind-Driven Rain Intrusion

The results discussed here are particular to several factors: 

the wall orientation was north-facing; the weather file had 

the major rain events during fall and winter; and the simulations assumed the intruding wind-driven rain went directly 

into the OSB layer. Under these assumptions, the ability 

of wall assemblies to dry in response to wind-driven rain 

penetration essentially reflects their drying capability from 

Table 9. Comparison of moisture performance

Wall assembly

Avoidance of 

moisture 

accumulation from 

vapor diffusion

Avoidance of 

moisture 

accumulation from 

air exfiltration

Drying 

capability

Ability to 

survive winddriven rain 

penetration

1. R13 KFB Excellent Poor Good Good

2. R19 KFB Excellent Poor Good Good

3. R13 UFB Poor Poor Good Good

4. R19 UFB Poor Poor Good Good

5. R23 BIBS Poor Poor Good Good

6. R13 KFB + R5 XPS Excellent Good Poor Poor

7. R19 KFB + R5 XPS Excellent Fair Poor Poor

8. R13 UFB + R5 XPS Fair Good Poor Fair

9. R13 UFB + R10 XPS Good Excellent Poor Fair

10. R13 UFB + R5 MFI Good Good Good Excellent
Research Paper FPL–RP–675

22

an initially high moisture content (see above). In response 

to a rain event of a given intensity, OSB moisture content 

remains elevated longer during cold weather than during 

warm weather because drying capability is greater at higher 

temperature. The annual mean excess moisture content was 

used as a metric, which is the difference in OSB moisture 

content with 1% WDR intrusion relative to the case without 

WDR intrusion, averaged over one year. In general, walls 

without exterior XPS insulation have a greater capability to 

dry after wind-driven rain intrusion than those with XPS. 

The best wall according to this metric was R13 UFB + R5 

MFI; the worst wall was R19 KFB + R5 XPS. R13 UFB + 

R5 XPS performed better than R13 KFB + R5 XPS, and 

R13 UFB + R10 XPS performed better than R13 UFB + R5 

XPS.

5.4 Air Exfiltration

The potential for wintertime moisture accumulation in 

OSB sheathing from air exfiltration relates to the rate of air 

leakage, the interior humidity levels, and the OSB interior 

surface temperature. Walls with higher levels of cavity insulation accumulate more moisture in the OSB than walls with 

lower levels of cavity insulation. Exterior insulation warms 

the OSB and decreases the amount of moisture deposited by 

air leakage.

5.5 Wall Assembly Performance Comparison

Table 9 summarizes hygrothermal performance in qualitative terms based on the metrics discussed above for vapor 

diffusion, air exfiltration, drying capability at various times 

of the year, and wind-driven rain intrusion. This provides 

a relative indication of the potential moisture performance 

risks associated with each assembly. However, it is not a 

quantitative risk assessment.

From this comparison, the following concluding remarks are 

made, as limited by the simulation assumptions discussed 

above:

•	 Walls with kraft vapor retarders and no exterior insulation perform well in all categories except risk of moisture 

accumulation from air exfiltration; however, these walls 

also have good drying capability, so the moisture accumulated from air leakage during cold weather dries rapidly in 

warmer weather.

•	 Walls with no kraft vapor retarder and no exterior insulation are prone to moisture accumulation from vapor diffusion and air leakage, though their drying capability is 

good.

•	 Low-perm extruded polystyrene exterior insulation impedes drying of OSB, and walls that include XPS have 

limited ability to dry out in the event of wind-driven rain 

penetration, particularly during cold weather. The exterior insulation helps avoid moisture accumulation from 

air exfiltration (and from vapor diffusion when a kraft 

vapor retarder is lacking). The extent to which moisture 

accumulation is avoided improves as the ratio of exterior 

insulation to cavity insulation increases.

•	 Permeable rigid mineral fiber insulation does not impede 

the drying of OSB and has the same benefit as XPS in 

reducing moisture accumulation from air leakage and vapor diffusion. According to these metrics, the overall best 

performing wall assembly simulated in this study was 

Wall 10 (R13 UFB + R5 MFI). This assembly has good 

performance in all categories and has the highest capability to dry in response to wind-driven rain intrusion.

References

Antretter, F.; Holm, A.; Karagiozis, A.; Glass, S. 2010. 

Interior temperature and relative humidity distributions 

in mixed-humid and cold climates as building simulation 

boundary conditions. In: Proceedings: Thermal Performance 

of the Exterior Envelopes of Whole Buildings XI International Conference, Paper 133. Atlanta: American Society of 

Heating, Refrigerating and Air-Conditioning Engineers, Inc.

ASHRAE. 2009a. Criteria for moisture-control design 

analysis in buildings. ANSI/ASHRAE Standard 160-2009. 

Atlanta: American Society of Heating, Refrigerating and 

Air-Conditioning Engineers, Inc. 14 p.

ASHRAE. 2009b. Heat, air, and moisture control in building 

assemblies-Fundamentals. In: 2009 ASHRAE HandbookFundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Chapter 25.

ASHRAE. 2009c. Heat, air, and moisture control in building 

assemblies-Material properties. In: 2009 ASHRAE Handbook-Fundamentals. Atlanta: American Society of Heating, 

Refrigerating and Air-Conditioning Engineers, Inc. 

Chapter 26.

BSC. 2010a. Building materials property table [Information Sheet Info-500]. Building Science Corporation. http://

www.buildingscience.com/documents/information-sheets/

building-materials-property-table. (15 January 2013).

BSC. 2010b. Vapor permeance of some building materials 

[Information Sheet Info-312]. Building Science Corporation. http://www.buildingscience.com/documents/information-sheets/info-312-vapor-permeance-some-materials. 

(15 January 2013).

Burch, D.M; TenWolde, A. 1993. A computer analysis of 

moisture accumulation in the walls of manufactured housing. ASHRAE Transactions. 99(2):977–990.

Burch, D.M.; Thomas, W.C.; Fanney, A.H. 1992. Water vapor permeability measurements of common building materials. ASHRAE Transactions. 98(2):486–494.

Carll, C.G.; Highley, T.L. 1999. Decay of wood and woodbased products above ground in buildings. Journal of Testing and Evaluation. 27(2):150–158.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

23

Carll, C.; Wiedenhoeft, A.C. 2009. Moisture-related properties of wood and the effects of moisture on wood and wood 

products. In: Trechsel, H.R.; Bomberg, M.T., eds., Moisture 

control in buildings: the key factor in mold prevention, 2nd 

ed. West Conshohocken, PA: ASTM International: 54–79. 

Chapter 4.

Christian, J.E. 2009. Moisture sources. In: Trechsel, H.R.; 

Bomberg, M.T., eds., Moisture control in buildings: the key 

factor in mold prevention, 2nd ed. West Conshohocken, PA: 

ASTM International: 103–109. Chapter 7.

Dahl, S.D.; Kuehn, T.H.; Ramsey, J.W.; Yang, C.-H. 1996. 

Moisture storage and non-isothermal transport properties 

of common building materials. HVAC&R Research. 

2(1):42–58.

Drumheller, S.C.; Carll, C. 2010. Effect of cladding systems 

on moisture performance of wood-framed walls in a mixedhumid climate. In: Proceedings: Thermal Performance of 

the Exterior Envelopes of Whole Buildings XI International 

Conference, Paper 49. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Dennis, J.K.; Zou, C.; Short, N.R. 1995. Corrosion behavior 

of zinc and zinc alloy coated steel in preservative treated 

timber. Transactions of the Institute of Metal Finishing. 

73(3):96–101.

Derome, D. 2010. The nature, significance, and control of 

solar-driven diffusion in wall systems. Final report, Research Project RP-1235. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Gatland, S. 2005. Comparison of water vapor permeance 

data of common interior building materials in North American wall systems. In: Proceedings of the 10th Canadian 

Conference on Building Science and Technology, Ottawa, 

ON, pp. 182–194.

Gibson, S. 2010. Can exterior foam insulation cause mold 

and moisture problems? Green Building Advisor. http://

www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/

can-exterior-foam-insulation-cause-mold-and-moistureproblems. (15 January 2013).

Glass, S.V.; TenWolde, A. 2007. Review of in-service 

moisture and temperature conditions in wood-frame buildings. General Technical Report FPL-GTR-174. Madison, 

WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 53 p.

Glass, S.V.; TenWolde, A. 2009. Review of moisture balance models for residential indoor humidity. In: Proceedings 

of the 12th Canadian Conference on Building Science and 

Technology, Vol. 1:231–245. Montréal: Québec Building 

Envelope Council.

Glass, S.V.; Zelinka, S.L. 2010. Moisture relations and 

physical properties of wood. In: Ross, R.J., ed., Wood handbook-Wood as an engineering material. General Technical 

Report FPL–GTR–190. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 

Chapter 4.

Hagentoft, C.-E.; Harderup, E. 1996. Moisture conditions 

in a north facing wall with cellulose loose fill insulation: 

Constructions with and without vapor retarder and air leakage. Journal of Thermal Insulation and Building Envelopes. 

19:228–243.

Hartley, I.D.; Wang, S.; Zhang, Y. 2007. Water vapor sorption isotherm modeling of commercial oriented strand panel 

based on species groups and resin type. Building and Environment. 42:3655–3659.

Hens, H. 1990. Guidelines and practice. International Energy Agency Annex XIV: Condensation and Energy, Vol. 2. 

Leuven, Belgium: K.U. Leuven, Laboratory for Building 

Physics.

Hens, H. 1996. Final Report, Vol. 1, Task 1: Modelling. 

International Energy Agency Annex 24-Heat, Air and 

Moisture Transfer Through New and Retrofitted Insulated 

Envelope Parts (HAMTIE). Leuven, Belgium: Katholieke 

Universiteit Leuven, Departement Burgerlijke Bouwkunde, 

Laboratorium Bouwfysica.

IBP. 2011. WUFI® Pro version 5.1. Holzkirchen, Germany: 

Fraunhofer Institute for Building Physics. http://www.wufi.

de/index_e.html. (15 January 2013).

Kalamees, T.; Kurnitski, J. 2010. Moisture convection performance of external walls and roofs. Journal of Building 

Physics. 33(3):225–247.

Karagiozis, A.N. 2001. Advanced numerical models for hygrothermal research. In: Trechsel, H.R., ed., Moisture analysis and condensation control in building envelopes. ASTM 

MNL40. West Conshohocken, PA: American Society for 

Testing and Materials: 90–106. Chapter 6.

Karagiozis, A.; Wilkes, K. 2004. Hygrothermal properties 

of selected materials. Report 2, ASHRAE Research Project 

1091, Development of design strategies for rainscreen and 

sheathing membrane performance in wood frame walls. Atlanta: American Society of Heating, Refrigerating and AirConditioning Engineers, Inc.

Kumaran, M.K. 1996. Material properties. Final Report, 

Vol. 3, Task 3. International Energy Agency Annex 24-Heat, 

Air and Moisture Transfer Through New and Retrofitted 

Insulated Envelope Parts (HAMTIE). Leuven, Belgium: 

Katholieke Universiteit Leuven, Departement Burgerlijke 

Bouwkunde, Laboratorium Bouwfysica.

Kumaran, M.K. 2001. Hygrothermal properties of building materials. In: Trechsel, H.R., ed., Moisture analysis and 

condensation control in building envelopes. ASTM MNL40. 

West Conshohocken, PA: American Society for Testing and 

Materials: 29–65. Chapter 3.
Research Paper FPL–RP–675

24

Kumaran, M.K. 2009. Fundamentals of transport and storage of moisture in building materials and components. In: 

Trechsel, H.R.; Bomberg, M.T., eds., Moisture control in 

buildings: the key factor in mold prevention, 2nd ed. West 

Conshohocken, PA: ASTM International: 1–15. Chapter 1.

Kumaran, M.K., J.C. Lackey, N. Normandin, F. Tariku, 

and D. van Reenen. 2002. A thermal and moisture transport 

property database for common building and insulation materials. Final Report, ASHRAE Research Project RP-1018. 

Atlanta: American Society of Heating, Refrigerating and 

Air-Conditioning Engineers, Inc.

Künzel, H.M. 1995. Simultaneous heat and moisture transport in building components: one- and two-dimensional 

calculation using simple parameters. Stuttgart, Germany: 

Fraunhofer IRB Verlag.

Martin, P.C.; Verschoor, J.D. 1994. Investigation of water 

vapor migration and moisture storage in an insulated wall 

structure. Final Report, ASHRAE Research Project 496-RP. 

Atlanta: American Society of Heating, Refrigerating and 

Air-Conditioning Engineers, Inc.

MHRA. 2000. Measured permeance values for selected 

interior wall assemblies. New York: Manufactured Housing 

Research Alliance. http://www.research-alliance.org/pages/

perm.htm. (15 January 2013).

Ojanen, T.; Ahonen, J.; Simonson, C.J. 2006. Moisture performance characteristics of OSB and spruce plywood exterior sheathing products. In: Fazio, P.; Ge, H.; Rao, J.; Desmarais, G., eds., Research in building physics and building 

engineering: Proceedings of the third international building 

physics conference. London: Taylor & Francis: 97–105.

Ojanen, T.; Kumaran, M.K. 1992. Air exfiltration and moisture accumulation in residential wall cavities. In: Proceedings: Thermal Performance of the Exterior Envelopes of 

Buildings V International Conference. Atlanta: American 

Society of Heating, Refrigerating and Air-Conditioning 

Engineers, Inc.: 491–500.

Ojanen, T.; Kumaran, K. 1996. Effect of exfiltration on 

the hygrothermal behaviour of a residential wall assembly. 

Journal of Thermal Insulation and Building Envelopes. 

19:215–227.

NAHB Research Center. 2010. Moisture performance of 

wood-based sheathing on exterior walls clad with absorptive 

materials. Report prepared for U.S. Forest Products Laboratory and U.S. Department of Housing and Urban Development. http://www.toolbase.org/PDF/CaseStudies/MoisturePerformanceWoodBasedSheathing.pdf . (15 January 2013).

Richards, R.F.; Burch, D.M.; Thomas, W.C. 1992. Water 

vapor sorption measurements of common building materials. ASHRAE Transactions. 98(2):117–127.

Smegal, J.; Straube, J. 2011. Hygrothermal analysis of exterior rockwool insulation. Research Report 1104. Somerville, 

MA: Building Science Press.

Straube, J. 2011. Controlling cold-weather condensation 

using insulation. Building Science Digest 163. Somerville, 

MA: Building Science Press. 8 p.

Straube, J.; Burnett, E. 2001. Overview of hygrothermal 

(HAM) analysis methods. In: Trechsel, H.R., ed., Moisture 

analysis and condensation control in building envelopes. 

West Conshohocken, PA: American Society for Testing and 

Materials: 81–89. Chapter 5.

Straube, J.F.; Burnett, E.F.P. 2005. Building science for 

building enclosures. Westford, MA: Building Science Press. 

542 p.

Tariku, F.; Cornick, S.M.; Lacasse, M.A. 2007. Simulation 

of wind-driven rain penetration effects on the performance 

of a stucco-clad wall. In: Proceedings: Thermal Performance 

of the Exterior Envelopes of Whole Buildings X International Conference, Paper 198. Atlanta: American Society of 

Heating, Refrigerating and Air-Conditioning Engineers, Inc.

TenWolde, A. 2011. A review of ASHRAE Standard 

160-Criteria for moisture control design analysis in buildings. Journal of Testing and Evaluation. 39(1). doi:10.1520/

JTE102896.

TenWolde, A.; Bomberg, M.T. 2009. Design tools. In: 

Trechsel, H.R.; Bomberg, M.T., eds., Moisture control in 

buildings: the key factor in mold prevention, 2nd ed. West 

Conshohocken, PA: ASTM International: 128–138. 

Chapter 10.

TenWolde, A.; Walker, I.S. 2001. Interior moisture design 

loads for residences. In: Proceedings: Performance of Exterior Envelopes of Whole Buildings VIII International Conference, Paper 33. Atlanta: American Society of Heating, 

Refrigerating and Air-Conditioning Engineers, Inc.

Thompson, A.; Taylor, B.N. 2008. Guide for the use of the 

International System of Units (SI). NIST Special Publication 

811. Gaithersburg, MD: National Institute of Standards and 

Technology.

Timusk, P.C.; Pressnail, K.D.; Cooper, P.A. 2009. The effects of board density, resin content and component layers 

on the permeability properties of mill-fabricated oriented 

strandboard. In: Proceedings of the 12th Canadian Conference on Building Science and Technology, Vol. 1:325–334. 

Montréal: Québec Building Envelope Council.

Trechsel, H.R., ed. 2001. Moisture analysis and condensation control in building envelopes. ASTM MNL40. West 

Conshohocken, PA: American Society for Testing and 

Materials.
Hygrothermal Analysis of Wood-Frame Wall Assemblies in a Mixed-Humid Climate

25

Tsongas, G.A.; Nelson, G.D. 1991. A field test for correlation of air leakage and high moisture content sites in tightly 

built walls. ASHRAE Transactions. 97(1):1–8.

Ueno, K. 2010. Residential exterior wall superinsulation 

retrofit details and analysis. In: Proceedings: Thermal Performance of the Exterior Envelopes of Whole Buildings XI 

International Conference, Paper 200. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning 

Engineers, Inc.

Viitanen, H.; Salonvaara, M. 2001. Failure criteria. In: 

Trechsel, H.R., ed., Moisture analysis and condensation 

control in building envelopes. ASTM MNL40. West Conshohocken, PA: American Society for Testing and Materials: 

66–80. Chapter 4.

Wiehagen, J. 2011. Personal communication. Energy Engineer, NAHB Research Center, 400 Prince Georges Blvd., 

Upper Marlboro, MD 20774.

Woloszyn, M.; Rode, C. 2008. IEA Annex 41: Whole building heat, air, moisture response, Subtask 1: Modelling principles and common exercises. International Energy Agency, 

Executive Committee on Energy Conservation in Buildings 

and Community Systems.