Field Moisture Performance of 

Wood-Framed Walls with Exterior 

Insulation in a Cold Climate

C.R. Boardman

Samuel V. Glass

Robert Munson

Borjen Yeh

Kingston Chow

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Paper

FPL–RP–698

March

2019
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Abstract

Continuous exterior insulation is becoming more common 

in North America in above-grade exterior walls in both 

retrofit applications and new construction. It is used to 

improve the overall thermal performance of wall assemblies. 

The drying capability of wall assemblies with exterior 

insulation and an interior vapor retarder in cold climates 

is not well characterized. The moisture performance of 

wood-framed wall assemblies with and without exterior 

insulation was monitored during a 2-year period in the cold 

climate of Madison, Wisconsin, USA, under low and high 

interior humidity conditions and with intentional wetting 

of the wood structural panel sheathing. Moisture content 

and temperature of standard 38- by 140-mm wood framing 

and 11-mm-thick oriented strandboard (OSB) sheathing 

were measured in eight different wall assemblies, each with 

north and south orientation, in a conditioned test structure. 

Either a kraft paper or a polyethylene vapor retarder was 

used on the interior in combination with fiberglass cavity 

insulation. Exterior insulation was mineral wool, expanded 

polystyrene, or extruded polystyrene. The OSB sheathing 

was wetted in a controlled manner at three different times 

of year to investigate drying response. Wintertime moisture 

accumulation in OSB under the tested conditions was not a 

concern except in the wall with no exterior insulation and 

interior kraft vapor retarder, although rapid drying occurred 

in springtime. Exterior insulation had a predictable effect 

March 2019

Boardman, C.R.; Glass, Samuel V.; Munson, Robert; Yeh, Borjen; Chow, 

Kingston. 2019. Field moisture performance of wood-framed walls 

with exterior insulation in a cold climate. Research Paper FPL-RP-698. 

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

Products Laboratory. 40 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.

Contents

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

2 Methods............................................................................3

3 Results..............................................................................8

4 Discussion......................................................................34

5 Conclusions....................................................................35

Acknowledgments..............................................................35

Appendix A—Apparent Surface Relative 

Humidity Calculations.......................................................35

Appendix B—Moisture Content Decay Function..............36

References..........................................................................38

on wall cavity temperature. All 16 test walls were able to 

dry out quickly enough to keep moisture content below 

dangerous levels when challenged by modest water injection 

onto the interior OSB surface. The observed decrease 

in OSB moisture content after controlled wetting events 

was generally more rapid during warm weather than cold 

weather, more rapid with exterior insulation than without 

during cold weather, more rapid with vapor-open exterior 

insulation than vapor-tight exterior insulation during cold 

weather, and more rapid with interior kraft vapor retarder 

than with polyethylene.

Keywords: moisture performance, hygrothermal 

performance, durability, drying potential, exterior insulation, 

continuous insulation, building envelope, vapor retarder
1 Introduction

The building envelope is a key component affecting overall 

building energy usage, and building codes have recently 

mandated higher insulation levels. Adding continuous 

exterior insulation has become a common strategy to 

improve overall thermal performance in North American 

above-grade exterior wall assemblies in both retrofit 

applications and new construction. This construction method 

is particularly relevant for wood-framed construction in cold 

climate zones. Although performance compliance paths in 

the International Energy Conservation Code (IECC) (ICC 

2015) are often flexible in the design of exterior walls, the 

prescriptive compliance path requires wood-framed walls 

located in cold climate zones (IECC Climate Zone 6 or 

higher) to incorporate continuous insulation at a minimum 

level of 0.88 m2·K/W (5 h·ft2·°F/Btu) or R-5. This is often 

implemented with a split-insulated wall design that includes 

both cavity insulation and continuous exterior insulation 

(Fig. 1).

Long-term moisture performance of exterior wall 

assemblies is a key consideration for contemporary energyefficient structures. Improper design can lead to problems 

with moisture accumulation and subsequent degradation 

of materials. Although considerable research has been 

conducted on exterior insulation in wood-framed walls, 

further work is needed to provide a quantitative basis to 

minimize the risk from moisture performance and associated 

durability issues.

Moisture control strategies for exterior wall assemblies 

address the various sources of moisture interior and exterior 

of the building and the ways in which moisture migrates 

(TenWolde and Rose 1996). Exterior water management 

is critical to avoid bulk water intrusion. A continuous air 

barrier system minimizes moisture accumulation caused by 

uncontrolled air leakage. Vapor diffusion control strategies 

vary according to climate and properties of the materials in 

the assembly (Lstiburek 2004). In addition, wall assemblies 

Field Moisture Performance of WoodFramed Walls with Exterior Insulation in a 

Cold Climate

C.R. Boardman, General Engineer

Samuel V. Glass, Research Physical Scientist

Robert Munson, Engineering Technician

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin, USA

Borjen Yeh, Director, Technical Services Division

Kingston Chow, Building Scientist, Technical Services Division

APA – The Engineered Wood Association, Tacoma, Washington, USA

should have the ability to dry out if they get wet (either 

during construction or during their service life). Drying 

potential is often a concern for wall assemblies that are 

insulated and air sealed to levels required by current model 

energy codes (Lstiburek 2013a, 2013b). Drying potential 

depends not only on the configuration of the wall assembly 

but also on the climate. Drying capability of wall assemblies 

with exterior insulation and an interior vapor retarder in cold 

climates is not well characterized.

Continuous exterior insulation raises the temperature of 

wood structural members in exterior walls during cold 

weather, relative to walls without exterior insulation, 

thereby decreasing the potential for wintertime moisture 

accumulation (Tsongas 1991, Straube 2011). This thermal 

effect decreases the vulnerability of the wall to water vapor 

migration from the interior carried either by air leakage 

or vapor diffusion. Different types of exterior insulation 

Figure 1. Example of split-insulated wall with 

insulation both in the stud cavity and exterior of 

the sheathing system (OSB, oriented strandboard).

Flat polyolefin

OSB

Stud wall with

fiberglass batt

Kraft

paper

Extruded 

polystyrene

Vinyl siding
Research Paper FPL–RP–698

2

materials vary in vapor permeability; mineral wool (MW), 

for example, is highly vapor-open whereas rigid foam 

insulation is typically vapor-tight. Vapor-tight exterior 

insulation may limit the outward drying potential of wall 

systems (Lstiburek 2013a, 2013b), although a small amount 

of air leakage between vapor-tight exterior insulation and 

the wood structural members may enhance outward drying 

potential (Lstiburek 2016).

A joint research project was initiated in 2014 by 

APA – The Engineered Wood Association and the USDA 

Forest Service, Forest Products Laboratory, to study the 

hygrothermal performance of split-insulated wall assemblies 

using a combination of different interior vapor retarders and 

continuous exterior insulation in the cold climate location 

of Madison, Wisconsin, USA. The project included field 

monitoring, laboratory measurements, and hygrothermal 

modeling. This report summarizes the field monitoring 

study. Also in this report, the background literature is 

reviewed and the study objectives are presented.

1.1 Previous Research

A considerable amount of field research has been conducted 

on moisture performance of wood-framed walls in cold 

climates (Glass and TenWolde 2007). A recent literature 

review by Trainor and Smegal (2017) focused specifically 

on various types of highly insulated walls (including walls 

with exterior insulation and double-stud walls) in North 

American cold climate locations. Several of the trends noted 

in this review are explored below.

Surveys and wall inspection studies generally have found no 

moisture problems with the use of exterior insulation. Wang 

(1981) inspected more than 70 houses during winter months, 

most of which were located in the Midwest and Northeast 

regions of the United States. No appreciable difference in 

moisture content (MC) was found between walls sheathed 

with extruded polystyrene, wood fiberboard, and plywood. 

In contrast, a survey of houses in the Northwest United 

States reported by Tsongas (1991) found that walls with 

exterior insulation had significantly lower wall moisture 

levels than walls without exterior insulation. In that study, 

some of the houses were in coastal Washington and the 

Seattle metropolitan area (marine climate) and some were 

in Montana (cold climate). Additional Canadian studies in 

various locations found that walls with extruded polystyrene 

sheathing did not have evidence of moisture accumulation 

in wood framing or evidence of interstitial condensation 

(Kane and Titley 1987, Proskiw 1995).

Multiple studies have investigated moisture performance of 

walls installed in conditioned test structures. Such studies 

have typically applied moisture loads to stress the walls, 

such as high interior relative humidity (RH), pressurization 

of the interior space to force exfiltration through wall 

assemblies, or intentional injection of a controlled amount 

of water at a known location. Additionally, some studies 

have included intentional deficiencies in the interior air 

barrier.

Many studies have concluded that adding exterior insulation 

does not increase the moisture-related durability risk 

(Sherwood 1983, Maref and others 2011, Smegal and others 

2013, Fox and others 2014, Trainor and others 2016). An 

exception was noted by Trainor and Smegal (2017) for 

a subarctic climate (Fairbanks, Alaska; Climate Zone 8), 

for which the combination of intentional deficiencies in 

the interior polyethylene air–vapor barrier, high interior 

RH, and pressurization of the interior space resulted in 

wintertime moisture accumulation (Craven and GarberSlaght 2012, 2014). Drying in spring and summer was 

significantly impeded for walls with low-permeance exterior 

insulation, thus increasing the moisture risk compared with 

walls with no exterior insulation. However, increasing the 

amount of exterior insulation such that 68% of the total 

thermal resistance was exterior of the plywood sheathing 

resulted in the lowest moisture risks. Exterior insulated 

walls without interior polyethylene had an appreciably faster 

drying rate in spring and summer than those with interior 

polyethylene.

In addition to low-permeance exterior foam insulation, 

several studies have monitored wall assemblies with highpermeance exterior MW insulation. Results indicate that 

walls with vapor-open exterior insulation allowed drying 

to the outside at a faster rate than walls with exterior foam 

insulation (Maref and others 2011, Fox and others 2014, 

Trainor and others 2016).

Conversely, several studies have found that low-permeance 

exterior insulation can decrease the inward flow of water 

vapor into the wall assembly from absorptive claddings 

such as brick veneer (Straube and Burnett 1998) and 

adhered stone veneer (Smegal and Grin 2015). In walls with 

these types of claddings (without low-permeance exterior 

insulation or another feature to decrease the inward vapor 

flow), an interior polyethylene vapor retarder has been 

shown to impede the inward vapor flow during summer 

conditions and to act as a condensing surface, resulting in 

condensed water running down and accumulating in the 

bottom plate (Wilkinson and others 2007).

The previously mentioned studies investigating moisture 

performance of walls with exterior insulation in cold 

climates have looked at a variety of wall configurations. 

Walls with 38- by 89-mm (nominal 2 by 4) framing 

have been studied with exterior insulation ranging from 

0.88 m2·K/W (R-5) (Sherwood 1983, Smegal and Grin 

2015) to as high as 4.1 m2·K/W (R-23) (Craven and GarberSlaght 2012, 2014). Walls with 38- by 140-mm (nominal 

2 by 6) framing have been monitored with 1.3 m2·K/W 

(R-7.5) (Smegal and others 2013) or 1.8 m2·K/W (R-10) 

exterior insulation (Maref and others 2011, Fox and others 

2014, Trainor and others 2016). None of the previously 

mentioned studies has investigated 38- by 140-mm walls 
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

3

with 0.88 m2·K/W (R-5) exterior insulation, which meets 

the IECC prescriptive compliance path in cold climate zones 

(IECC Climate Zone 6 or higher). The objective of this 

study was to fill this gap.

1.2 Objectives

The overall goal of the project was to measure the moisture 

performance of 38- by 140-mm wood-framed wall 

assemblies with and without exterior insulation in a cold 

climate location. Specific objectives were to characterize 

wall assembly moisture and temperature conditions under 

ambient environmental conditions with low and high 

interior moisture load conditions and to characterize wall 

assembly drying rates after intentional wetting of the wood 

structural panel sheathing. The primary variables included in 

the study were the following:

• Type of exterior insulation: none, MW, expanded 

polystyrene, or extruded polystyrene

• Type of interior vapor retarder: polyethylene sheet or 

asphalt-coated kraft paper

• Interior moisture load levels: wintertime RH of 42% 

(first year) and 34% (second year)

2 Methods

2.1 Description of Test Structure

The test structure was an existing building located at the 

Valley View test site of the USDA Forest Service, Forest 

Products Laboratory. This location just west of Madison, 

Wisconsin, USA, has been used for many outdoor 

experiments (Duff 1968, Sherwood 1983, TenWolde and 

others 1995, Carll and others 2013) to represent a cold 

climate, specifically IECC Zone 6 (Baechler and others 

2010). The 17.2- by 4.9-m building has a treated wood 

post and beam foundation, which raises it off the ground. 

The insulated floor was not changed, but the 38- by 140-

mm stud walls of the original building were refurbished to 

create test sections (Fig. 2), and the ceiling was insulated 

with cellulose fiber to facilitate conditioning of the interior. 

The long dimension of the building runs east to west with 

a central room and a wing on each end. The central room, 

which houses the data acquisition system and heating and 

cooling equipment, has a door on the north side. Existing 

siding, building paper, insulation, vapor retarder, and 

gypsum board were removed from the north and south walls 

of the wings, and these walls were subsequently all sheathed 

with 11-mm oriented strandboard (OSB) purchased at a 

local building supply store. Each 1.2-m-wide by 2.2-m-high 

test section, consisting of three cavities with studs 406 mm 

(16 in.) on center, was separated from adjacent wall sections 

using a plastic composite trim board on the outside (to keep 

the OSB panels from touching) and by adding an additional 

38- by 140-mm stud separated from the existing studs using 

an impermeable self-adhering membrane. Test sections were 

thus isolated to minimize any moisture transfer between 

adjacent wall sections. Figure 3 shows a plan view with 

16 test sections. The outside walls were covered with 

spun-bonded polyolefin house wrap (Fig. 4) and exterior 

insulation (depending on the assembly, as subsequently 

described), and vinyl siding was installed (Fig. 5). The 

140-mm stud cavities were filled with fiberglass batt 

insulation that had nominal thermal resistance of 

3.7 m2·K/W (R-21). The interior was finished with 

12.7-mm gypsum board, latex primer, and latex paint.

Figure 2. Northeast corner of test structure undergoing partial reconstruction.
Research Paper FPL–RP–698

4

1.2 m 1.2 m

Oriented strandboard

38 by 140 mm lumber

Drywall

North

1N 2N 3N 4N

1S 2S 3S 4S

5N 6N 7N 8N

5S 6S 7S 8S

1.2 m

406 mm 406 mm 406 mm 406 mm 406 mm 406 mm

1.2 m

Figure 3. Plan view of test structure showing 16 test sections.

Figure 4. Northwest corner of test structure before installation of exterior insulation. The right-most 

section is not an instrumented section.

Figure 5. Completed south side of the test structure.
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

5

The interior of the building was temperature controlled with 

an electric furnace during winter (20 °C heating set point) 

and an air conditioner during summer (25.6 °C cooling set 

point). Humidity levels were maintained in the winter using 

a humidistat (42% RH first winter and 34% RH second 

winter) and associated atomizer in each of the wings. 

There was no separate dehumidification in summer beyond 

that provided by operation of the air conditioner under 

thermostat control. Fans circulated air within each wing 

and between wings to promote uniform temperature and 

humidity conditions within the building.

2.2 Wall Configurations

Each of the eight different test sections had an identical 

configuration for north and south (aside from orientation). 

As previously mentioned, all wall sections included vinyl 

siding, house wrap, OSB sheathing, fiberglass cavity 

insulation, and interior gypsum drywall. All wall sections 

had gasketing installed between the framing and drywall for 

airtightness. Figure 6 shows the insulated walls before the 

drywall installation in the northwest section. Test sections 

had varied combinations of interior vapor retarder and 

exterior insulation (outboard of the OSB and house wrap) to 

implement a split-insulated wall design similar to Figure 1.

Figure 6. Northwest interior corner of test structure with wall cavities insulated before drywall installation 

(Cl, continuous insulation; MW, mineral wool).

Walls 1 and 2 (numbered west to east) were the base case 

without any exterior insulation and differed only by the 

internal vapor retarder, which was the asphalt-coated kraft 

paper facing of the fiberglass insulation in Wall 1 and a 

0.15-mm polyethylene sheet in Wall 2.

Walls 3 and 4 repeat this pattern but included 38 mm of 

MW exterior insulation to add 1.1 m2·K/W (R-6) to the 

configuration.

Wall 5 used 38 mm of expanded polystyrene (EPS) paired 

with the kraft interior to provide 1.1 m2·K/W (R-6).

Walls 6 and 7 were both 25.4 mm of extruded polystyrene 

(XPS) paired with kraft and polyethylene interior vapor 

retarders, respectively, to provide 0.88 m2·K/W (R-5).

Wall 8 was again 25.4 mm of XPS and kraft interior, but 

the house wrap was not the flat spunbond polyolefin fiber 

used in all the other walls. Instead, Wall 8 used a similar 

product structured to create a vertically grooved surface that 

promotes water drainage, which resulted in a small air gap 

between the sheathing and the XPS.

Table 1 shows all eight different configurations.

1N, no CL, kraft 2N, no CL, poly 3N, MW, kraft 4N, MW, poly
Research Paper FPL–RP–698

6

2.3 Instrumentation

The test hut walls were instrumented with a focus on 

the MC of the OSB sheathing. Each test section had an 

identical set of sensors, which were always installed in the 

middle cavity (of the three that make up a section). That 

central cavity was caulked between the OSB and framing 

to decrease air infiltration. Wiring to the sensors went 

through the framing to an adjacent cavity in the section and 

was also caulked at that framing penetration. Each section 

included six paired sensors that combined a moisture pin 

pair (measuring resistance to calculate MC) and thermistor 

(measuring resistance to calculate temperature). The 

moisture pin pair was not the traditional driven nail. Instead, 

the connection to the OSB for the resistance measurement 

was accomplished using 18-8 stainless steel sheet metal 

screws (slotted hex washer head, #6 x 12.7 mm), which 

then contacted the point moisture measurement (PMM) 

sensor (SMT Research Ltd., Vancouver, British Columbia, 

Canada) that also housed the thermistor (MF58104F3950, 

Cantherm, Montreal, Quebec, Canada). Such a moisture pin 

system has been studied in OSB by Boardman and others 

(2017) and provides good long term contact with the OSB. 

Leads from the moisture pin and thermistor connected 

to a wireless 8-channel data acquisition (DAQ) unit that 

read and stored the resistance measurements (A3, SMT 

Research Ltd.). Each wireless DAQ unit communicated to a 

central hub running display and storage software (Building 

Intelligence Gateway, SMT Research Ltd.) on a laptop 

computer. Data were collected once per hour. The DAQ unit 

read resistance with accuracy of ±5% up to a maximum of 

1 GΩ. When combined with the thermistor error, this led 

to an uncertainty in temperature readings of at most 1.2 °C. 

When combined with the uncertainty of the OSB resistance 

to MC correlation, this led to an uncertainty in MC of at 

most 1.3%. However, very low MC (below around 8%) had 

large errors because the recorded resistance has a maximum 

value of 1 GΩ.

Two of the PMM sensors measured structural lumber, 

whereas four were in the OSB (Fig. 7). PMM4 was in the 

bottom plate capturing MC (MC4) and temperature (T4). 

PMM2 was in the 38- by 140-mm structural lumber near 

the middle of the cavity (1.14 m up from the bottom plate). 

PMM1 was in OSB 100 mm down from top plate, whereas 

PMM3 was 100 mm up from bottom plate. PMM5 and 

PMM6 were in the field of a shop towel used for water 

injections, which will be subsequently discussed. PMM5 

was 1.17 m up from bottom plate, whereas PMM6 was 

100 mm below PMM5. All the MC measurements in OSB 

were calculated from the resistance measurements using the 

formula in Boardman and others (2017), which describes 

the efforts to improve correlation of MC with resistance and 

temperature data in OSB. The A3 DAQ units were mounted 

in an adjacent cavity.

From the OSB MC and temperature measurements, an 

“apparent surface RH” value was calculated using a generic 

OSB sorption isotherm relation based on data sources 

outlined in Boardman and others (2017) and described 

in Appendix A. In addition to the combined MC and 

temperature measurements, two other sensors recorded RH 

and temperature in the cavity. One was a Humirel HTS2010 

sensor that connected to the SMT A3 system. It was placed 

just below MC1 as shown in Figure 8 to record conditions 

near the OSB capturing RH (RH7) and temperature (T7). 

This humidity sensor had accuracy of ±3% to ±5% RH. The 

other RH sensor was placed near MC2 to capture overall 

conditions in the cavity at midheight (RH8, T8). That 

system was an independent data logger (HOBO U23-002 

Pro v2 External Temperature/RH Logger, Onset Computer 

Corporation, Bourne, Massachusetts, USA), which also 

logged once per hour. The HOBO humidity sensor also 

had accuracy between ±3% to ±5% RH, with temperature 

accuracy of 0.21 °C.

Both indoor and outdoor RH and temperature were 

measured using HMP233 sensors (Vaisala, Vantaa, 

Finland), which output 0–5 volt signals read by an Adam 

6017 analog input module (Advantech Co., Ltd., Taipei, 

Taiwan) with custom software recording once per hour. 

The Vaisala units had accuracy of ±1% RH and ±0.2 °C. 

Local wind conditions were measured on top of a 7.6-m 

fiberglass tower using a Gill WindSonic (Gill Instruments 

Table 1—Wall configuration summary

Wall Interior vapor retarder House wrap Exterior insulation Labela

1 Kraft paper Flat polyolefin None No CI, kraft

2 Polyethylene Flat polyolefin None No CI, poly

3 Kraft paper Flat polyolefin 38-mm mineral wool MW, kraft

4 Polyethylene Flat polyolefin 38-mm mineral wool MW, poly

5 Kraft paper Flat polyolefin 38-mm expanded polystyrene EPS, kraft

6 Kraft paper Flat polyolefin 25.4-mm extruded polystyrene XPS, kraft

7 Polyethylene Flat polyolefin 25.4-mm extruded polystyrene XPS, poly

8 Kraft paper Crinkled polyolefin 25.4-mm extruded polystyrene XPS, kraft, crinkled

a

CI, continuous insulation; MW, mineral wool; EPS, expanded polystyrene; XPS, extruded polystyrene.
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

7

Figure 7. Sensor placement in each test section (OSB, oriented strandboard; 

RH, relative humidity).

Figure 8. Instrumentation at top of cavity recording moisture content, 

relative humidity, and temperature.

MC1,T1 In OSB, 100 mm down from top

Humirel RH sensor just below MC1

Moisture pins in the field of the wetting 

system. MC5 was 1.17 m up from bottom 

plate, and MC6 was 100 mm below MC5.

MC3,T3

RH7,T7

MC5,T5

MC6,T6

RH8,T8

MC4,T4

MC2,T2

In OSB, 100 mm up from bottom

In bottom plate lumber

In stud lumber, 1.14 m up from bottom

Hobo RH sensor in cavity at midheight

Wetting system
Research Paper FPL–RP–698

8

Limited, New Milton, UK) with data captured every 2 s and 

averaged to yield hourly average wind speed and direction. 

Local rainfall was measured using a tipping bucket (Series 

525 rainfall sensor, Texas Electronics, Dallas, Texas, 

USA) with each tip recorded on the EL-USB-5 counter 

(Lascar Electronics Ltd., Salisbury, England). Both the 

rain and wind sensors were located in an open area more 

than 7.6 m from the test facility. Solar radiation was not 

measured directly on site but was taken from a Madison, 

Wisconsin, station maintained by the National Oceanic 

and Atmospheric Administraton’s Earth System Research 

Laboratory through the SOLRAD network (https://www.

esrl.noaa.gov/gmd/grad/solrad/index.html).

2.4 Water Injections

To provide a drying challenge to the wall systems, each 

cavity was subjected to an identical water injection 

schedule at three different times during the 2-year study. 

The wetting occurred inside the center cavity using a 

3.1-mm inner diameter vinyl tube that allowed water to 

flow from the indoor entry point near the drywall surface 

to the OSB inside surface behind the fiberglass insulation 

where the water was held in place by a shop towel (Van 

Straaten 2003). Three small holes in the tube allowed a 

trickle of water to enter the paper shop towel stapled to 

the OSB surface. This wetting system has been found to 

yield consistently reproducible wetting of a localized OSB 

area and to promote OSB water absorption. Each injection 

had a volume of 40 mL, which wet the shop towel without 

running down the OSB sheathing. Figure 9 shows the tube 

and towel system installed with MC5 and MC6 in the field 

of the shop towel. The shop towel was cut out around the 

sensors, which allowed the screws for MC measurement to 

go directly into the OSB without touching the shop towel.

The first series of injections occurred in late summer, 

starting August 13, 2015, with one injection per day for 

3 days (total of 120 mL). The second series of injections 

occurred in late fall, starting November 6, 2015, and lasted 

5 days (total of 200 mL). The last series of injections 

occurred the following spring, starting May 20, 2016, and 

lasted 4 days (total of 160 mL).

3 Results

3.1 Interior and Exterior Environmental 

Conditions

Outdoor temperatures were similar to 1981–2010 

averages recorded at the KMSN weather station (Madison, 

Wisconsin, airport) for the 2-year study, but the relative 

humidities were higher than average. Figure 10 shows a 

weekly plot of recorded outdoor temperature along with the 

30-year normal. Figure 11 shows a weekly plot of recorded 

outdoor humidity along with the 30-year normal. The 

number of heating degree days (65 °F basis) recorded at the 

KMSN weather station averaged 7,333 for the 1981–2010 

period, whereas somewhat lower than normal values were 

recorded for the study years with 6,667 (91% normal) in 

2015 and 6,417 (88% normal) in 2016. Calculated heating 

degree days using recorded temperatures at the test site 

were slightly (less than 1%) higher than the KMSN weather 

station values.

Similarly, precipitation was above average, particularly 

in the second year. Figure 12 shows the cumulative 

precipitation by month.

Indoor temperatures were moderated by the heating and 

air conditioning systems to typical interior conditions. 

Figure 13 shows the indoor weekly temperatures compared 

Figure 9. Water injection system.
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

9

with outdoor for the full 2 years, whereas Figure 14 shows 

the same data at an hourly level for the summer to illustrate 

exterior conditions cycling above interior during parts of the 

day.

Similarly, indoor RH levels were moderated by the 

humidifier during the winter and air conditioning during the 

summer. Figure 15 shows the weekly indoor and outdoor 

RH averages.

Inspection of Figure 15 reveals that the winter RH set point 

was higher the first winter (near 42% RH) than the second 

winter (near 34% RH). Figure 16 shows the difference 

between indoor and outdoor vapor pressure during the 

whole period. The indoor vapor pressure was higher than 

outdoors in winter but less than outdoors in summer. The 

equation used for saturation vapor pressure (Pws) needed 

to calculate indoor vapor pressure is slightly modified from 

Buck (1981) with output in pascals given temperature t

in °C:

18.729   257.87

227.3

/

Pws 613.65 

t t t

e

                           (1)

–20

–15

–10

–5

0

5

10

15

20

25

30

Temperature (°C)

Normal

Outdoor

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

Figure 10. Weekly average outdoor temperatures compared with 30-year normal.

Figure 11. Weekly average outdoor relative humidity compared with 30-year normal.

40

50

60

70

80

90

100

Relative humidity (%)

Normal

Outdoor

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

10

Figure 12. Cumulative precipitation compared with 30-year normal.

Figure 13. Weekly average indoor and outdoor temperatures.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Cumulative rain (mm)

Normal

Outdoor

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

–20

–15

–10

–5

0

5

10

15

20

25

30

Temperature (°C)

Indoor

Outdoor

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

11

Figure 14. Hourly indoor and outdoor temperatures during summer.

Figure 15. Weekly average indoor and outdoor relative humidity.

5

10

15

20

25

30

35

Temperature (°C)

Indoor

Outdoor

Jun 2015 Jul 2015 Aug 2015 Sep 2015

20

30

40

50

60

70

80

90

100

Relative humidity (%)

Indoor

Outdoor

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

12

Figure 16. Indoor–outdoor vapor pressure difference.

3.2 Wall Assembly Temperatures

Temperatures of the OSB inside each cavity unsurprisingly 

tracked outdoor temperatures. Figure 17 shows the weekly 

average of T1, 100 mm down from the top plate, for all 

north walls. Inspection of Figure 17 shows that, during 

winter, the OSB was colder when there was no exterior 

insulation.

Most temperature sensors also picked up the expected 

vertical stratification inside the wall cavity. Figure 18 

illustrates this in Wall 2 north (no continuous insulation 

(CI), poly) by plotting T1 (near top plate) with T6 (lower in 

the cavity). However, the difference was small and usually 

below the uncertainty in temperature measurement. Also 

against the expected trend, Wall 5 north (EPS, kraft) showed 

T1 at a lower temperature than T6 when averaged across the 

whole 2-year period. Similarly, the south walls were slightly 

warmer than the north walls as shown in Figure 19, which 

plots T1 for Wall 2 (no CI, poly) north and south. Again, the 

difference was usually below the uncertainty in temperature 

measurement, but in this case, all walls showed south 

warmer than north.

Finally, for a quantitative comparison of the effects of the 

exterior insulation, Table 2 reports the average temperature 

difference between T7 (Humirel sensor close to inside 

surface of OSB at top) and outdoors during the winter 

months, December 2015 through February 2016. Each wall 

type reported includes the average of both north and south 

walls.

–1000

–800

–600

–400

–200

0

200

400

600

800

1000

Vapor pressure difference (Pa)

Vin-Vout

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

Table 2—Average temperature difference 

between oriented strandboard and outdoors 

for north and south walls during winter

Wall Labela

Temperature 

difference 

(°C)

1 No CI, kraft 3

2 No CI, poly 4

3 MW, kraft 8

4 MW, poly 9

5 EPS, kraft 7

6 XPS, kraft 7

7 XPS, poly 8

8 XPS, kraft, crinkled 7

a

CI, continuous insulation; MW, mineral wool; EPS, 

expanded polystyrene; XPS, extruded polystyrene.
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

13

Figure 17. North wall temperatures in oriented strandboard near top plate (Cl, continuous insulation; MW, mineral wool; 

EPS, expanded polystyrene; XPS, extruded polystyrene).

–20

–15

–10

–5

0

5

10

15

20

25

30

Temperature (°C)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Outdoor

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

14

Figure 18. North, no Cl, poly wall showing temperature stratification (CI, continuous insulation).

Figure 19. T1, no CI, poly wall showing south (S) warmer than north (N) (CI, continuous insulation).

–10

–5

0

5

10

15

20

25

Temperature (°C)

T1

T6

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

–10

–5

0

5

10

15

20

25

Temperature (°C)

S

N

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

15

Figure 20. Vapor pressure in two north wall cavities compared with indoor and outdoor conditions 

(Cl, continuous insulation).

3.3 Vapor Pressures

Vapor pressures inside each cavity typically fell between 

the indoor and outdoor boundary conditions. Figure 20 

illustrates this by plotting the weekly average vapor pressure 

calculated using Equation (1) from the RH and temperature 

readings of the HOBO sensor (midheight in the cavity at 

mid-depth) for the two base case north walls along with 

indoor and outdoor conditions. Generally, the cavity with 

kraft vapor retarder was closer to indoor conditions. All 

the north wall cavity weekly average vapor pressures are 

plotted in Figure 21, which shows that the cavities with 

polyethylene vapor barrier had higher vapor pressure than 

cavities with kraft in summer and generally had lower vapor 

pressure than kraft cavities during winter. Figures 20 and 21 

also show the timing of the water injections with the red bar 

at the bottom.

Figure 22 is a plot of daily average vapor pressures in two 

north wall cavities after the first injection. During August 

and September 2015, the outdoor vapor pressure was 

generally greater than indoor vapor pressure and the cavity 

vapor pressures were typically in between, indicating an 

inward vapor drive. It is unclear if the increase in cavity 

vapor pressure after the water injection was a result of 

the injection or the rise in outdoor vapor pressure. There 

were brief windows after the injection in which the cavity 

vapor pressures rose above both indoor and outdoor vapor 

pressure, indicating that drying was occurring in both 

directions. This effect was greater and lasted longer when 

there was exterior insulation. Although the difference 

between MW and no-CI walls was not larger than the 

measurement error in Figure 22, it was often significant in 

Figure 23, which plots the daily average vapor pressures for 

the second injection. In Figure 23, there are longer periods 

during which the wall was drying in both directions. This 

injection occurred during colder weather, and the outdoor 

vapor pressure fluctuated above and below indoor vapor 

pressure during November and December.

Also shown in Figure 23 is the vapor pressure at the surface 

of the OSB compared with the HOBO sensor for one cavity, 

the north MW, kraft. In general, these tracked closely, but 

right after the injection, the OSB surface conditions were 

wetter than the overall cavity. The surface vapor pressure 

was calculated using the M5 moisture pin (in field of 

wetting system) and the apparent surface RH from an OSB 

sorption isotherm (Appendix A). That surface RH was then 

combined with the surface temperature to calculate surface 

vapor pressure. The same trends can be seen during the third 

injection shown in Figure 24, which occurred during mild 

spring weather.

For a different way to compare the eight wall types, the 

following calculation was made with hourly values to show 

the average vapor pressure difference between each cavity 

and outdoors. The cavity vapor pressure was the average of 

the Humirel sensor (RH7, T7) near the OSB and the HOBO 

sensor midcavity. North and south values were averaged 

for each wall. These values were normalized by subtracting 

the outdoor vapor pressure. Finally, the hourly values were 

averaged across time into three bins: 5 weeks before an 

500

1000

1500

2000

2500

Vapor pressure (Pa)

Indoor

Outdoor

No CI, kraft

No CI, poly

 Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

16

injection, 1 week after injection, and weeks 2–5 following 

injection. These averages for each injection are shown in 

Table 3 for all eight walls.

Prior to the first injection, all wall cavities were at lower 

vapor pressure than outdoors. This injection coincided 

with an increase in cavity vapor pressure in the first week, 

followed by a decrease in weeks 2–5. The rise in cavity 

vapor pressure was most pronounced for the walls with 

interior polyethylene (Walls 2, 4, and 7), and these walls 

also had considerably higher cavity vapor pressures than the 

other walls prior to the injections, which is consistent with 

the interior polyethylene impeding inward drying. Prior to 

the second injection, all wall cavities were slightly above 

outdoor vapor pressure. The largest increase in cavity vapor 

pressure in the week after this injection was in Walls 4 

and 7, which also remained higher than the other walls in 

weeks 2–5. Following the third injection, all wall cavities 

had a downward trend in vapor pressure relative to outdoors 

except for the walls with interior polyethylene (Walls 2, 4, 

and 7).

Figure 21. Vapor pressure for all north cavities (CI, continuous insulation; MW, mineral wool; EPS, expanded 

polystyrene; XPS, extruded polystyrene).

500

1000

1500

2000

2500

Vapor pressure (Pa)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

17

Figure 22. Daily average vapor pressure after Injection 1 (Cl, continuous insulation; MW, mineral wool).

500

1000

1500

2000

2500

Vapor pressure (Pa)

Indoor

Outdoor

No CI, kraft

MW, kraft

Water injection

Aug 19, 2015 Sep 02, 2015 Sep 16, 2015 Sep 30, 2015 Oct 14, 2015

Table 3—Average vapor pressure difference between cavity and outdoors

Vapor pressure difference (Pa)

Injection 1 (August 2015) Injection 2 (November 2015) Injection 3 (May 2016)

Wall Labela 5 weeks pre

1 week 

post

2–5 

weeks 

post

5 weeks 

pre

1 week 

post

2–5 

weeks 

post

5 weeks 

pre

1 week 

post

2–5 

weeks 

post

1 No CI, k –282 –230 –243 52 123 36 137 –19 –172

2 No CI, p –151 –12 –88 38 157 64 114 118 –37

3 MW, k –334 –236 –266 56 166 81 64 –46 –218

4 MW, p –226 2 –106 47 242 161 25 115 –87

5 EPS, k –335 –245 –280 52 150 72 94 –23 –202

6 XPS, k –362 –297 –313 51 136 71 127 –16 –199

7 XPS, p –232 –61 –154 62 212 125 169 195 42

8 XPS, k, cr –359 –273 –312 49 148 78 130 15 –187

a

CI, continuous insulation; MW, mineral wool; EPS, extruded polystyrene; XPS, expanded polystyrene; k, kraft; p, polyethylene; cr, crinkled.
Research Paper FPL–RP–698

18

Figure 23. Daily average vapor pressure after Injection 2 (Cl, continuous insulation; MW, mineral wool; 

VP, vapor pressure).

200

400

600

800

1000

1200

1400

1600

Vapor pressure (Pa)

Indoor

Outdoor

No CI, kraft

MW, kraft

Surface VP from MW

Water injection

Nov 11, 2015 Nov 25, 2015 Dec 09, 2015 Dec 23, 2015 Jan 06, 2016 Jan 20, 2016
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

19

Figure 24. Vapor pressures after Injection 3 (Cl, continuous insulation; MW, mineral wool).

3.4 Wood Moisture Contents

3.4.1 General Trends

The most basic trend in OSB MC was the rise in MC during 

the winter for the kraft-faced cavities caused by moisture 

diffusion from the interior humidification. This is illustrated 

by comparing weekly average MC values near the top plate 

for all the north walls in Figure 25. Only Walls 1 (no CI, 

kraft) and 8 (XPS, kraft, crinkled) were above 20% MC, 

and this was true only briefly during winter. During the 

winter, the kraft-faced cavities had higher MC than their 

corresponding cavities with polyethylene vapor retarders. 

The OSB near the top plate did not have a significant 

response to the water injections (shown as red bars in 

Figure 25).

In contrast to the OSB sheathing, the wood framing 

components did not exhibit a seasonal trend in MC but were 

slightly affected by the water injections, which is shown in 

the graphs again as a red bar indicating the timing of the 

injections. Figure 26 plots MC values in the 38- by 140-mm 

studs near midheight.

Figure 27 shows a larger effect from the water injections 

plotting the MC values in the field of the shop towel wetting 

system.

These same two effects can also be seen in the RH of 

the cavities. Figure 28 plots RH8, the HOBO sensor in 

midcavity, for Walls 3 (MW, draft) and 4 (MW, poly) 

showing the generally lower RH of Wall 4 (poly), but also 

the spikes caused by water injection, which can increase the 

RH of Wall 4 above Wall 3 until it dries out again. Further 

water injection results and drying potential will be presented 

in the following section.

Similar trends occurred in the south walls. Figure 29 

illustrates this by comparing MC values in OSB at top 

of Wall 1 (no CI, kraft) for both north and south. Again in 

kraft-faced cavities such as Wall 1, there was significant 

moisture accumulation in the winter, higher in the first 

winter during which the indoor humidity set point was 

higher. The higher MC of the north walls might be expected 

because of the slightly higher temperature of the south walls 

(as a result of solar radiation), but this effect was small 

and not consistent in all results. More consistent was the 

result that walls with exterior insulation had lower MC than 

the base walls. Figure 30 illustrates this for all walls with 

polyethylene vapor retarders again plotting MC values near 

the top of the OSB. Figure 31 plots the MC of all south 

walls near the top of the OSB.

500

1000

1500

2000

2500

Vapor pressure (Pa)

Indoor

Outdoor

No CI, kraft

MW, kraft

Water injection

May 18, 2016 May 25, 2016 Jun 01, 2016 Jun 08, 2016 Jun 15, 2016 Jun 22, 2016 Jun 29, 2016 Jul 06, 2016
Research Paper FPL–RP–698

20

Figure 25. Moisture content in oriented strandboard near top in north walls (CI, continuous insulation; 

MW, mineral wool; EPS, expanded polystyrene; XPS, extruded polystyrene).

5

10

15

20

25

30

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

21

Figure 26. Moisture content in studs at midheight in north walls (CI, continuous insulation; MW, mineral wool; 

EPS, expanded polystyrene; XPS, extruded polystyrene).

5

10

15

20

25

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

Figure 27. Moisture content at water injection site in north walls (CI, continuous insulation; MW, mineral wool; 

EPS, expanded polystyrene; XPS, extruded polystyrene).

5

10

15

20

25

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, 

crinkled

Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

22

Figure 28. Relative humidity from mineral wool (MW) walls, kraft and poly, with effects of water injection.

Figure 29. Moisture content in oriented strandboard near top in no continuous insulation, 

kraft wall comparing north (N) and south (S) elevations.

30

35

40

45

50

55

60

65

70

75

80

Relative humidity (%)

MW, poly

MW, kraft

Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

5

10

15

20

25

30

Moisture content (%)

S

N

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

23

Figure 30. Moisture content in oriented strandboard near top for all walls with polyethylene vapor retarder 

(CI, continuous insulation; MW, mineral wool; XPS, extruded polystyrene).

Figure 31. Moisture content in oriented strandboard near top in south walls (CI, continuous insulation; 

MW, mineral wool; EPS, expanded polystyrene; XPS, extruded polystyrene).

5

10

15

20

25

30

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Water injection

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017

5

10

15

20

Moisture content (%)

N 2 No CI, poly

S 2 No CI, poly

N 4 MW, poly

S 4 MW, poly

N 7 XPS, poly

S 7 XPS, poly

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

24

3.4.2 Water Injection and Drying Potential

So far, results have been presented primarily as weekly 

averages, but for the discussion of drying after water 

injection, daily averages will be plotted around the times 

of the injection. Figure 32 presents the first drying period 

showing MC values in OSB in the field of the wetting for all 

north walls, whereas Figure 33 shows the same MC values 

for the south walls. In both cases, the kraft-faced cavities 

dried faster because they could dry inward.

The second and third drying periods are similar and will be 

discussed in more detail subsequently. Before reviewing 

these drying data, two other effects are illustrated in 

Figures 34 and 35, which plot the MC values near the 

bottom plate (near but outside the field of injection) during 

and after the second injection. This moisture pin (MC3) is 

not in the field of the paper towel and hence does not see the 

strong spike from water injection, but it does show both the 

lateral movement of water away from the field of the paper 

towel and the winter moisture accumulation in the OSB 

from the kraft vapor retarder. The modest rise in moisture 

from MC3 for all north walls on November 11 in Figure 34 

is water migrating away from the shop towel. The rise in 

moisture from MC3 for some walls starting in January 2016 

is the winter accumulation caused by indoor humidity.

Figure 32. Moisture content in oriented strandboard in field of wetting: daily averages around Injection 1 (Aug. 2015) 

for north walls (CI, continuous insulation; MW, mineral wool; EPS, expanded polystyrene; XPS, extruded polystyrene).

8

10

12

14

16

18

20

22

24

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Aug 19, 2015 Sep 02, 2015 Sep 16, 2015 Sep 30, 2015 Oct 14, 2015
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

25

Further analysis of drying will use a value, labeled M5net, 

which normalizes all the MC5 values (in the field of wetting 

system) by subtracting the value of MC on August 8, 2015, 

in each wall from all the other values of MC for that wall. 

This forces each wall to start at an M5net value of zero just 

before the first injection thus allowing easy comparison of 

the changes in MC across walls. In addition, the focus will 

be primarily on the walls with polyethylene, which forces 

outward drying only. Figures 36 and 37 present M5net for 

the first injection for north and south walls, respectively. 

Figures 38 and 39 are the same for the second injection, and 

Figures 40 and 41 show the third injection.

A number of observations can be confirmed by inspection 

of Figures 36 through 41. First, the walls had not completed 

drying before the second injection in late fall of 2015. 

Second, all walls except those with XPS had returned to 

near initial conditions before the third injection. Third, in 

the winter, the walls with MW were able to dry quickly.

Figure 33. Moisture content of oriented strandboard in field of wetting: daily averages around Injection 1 (Aug. 2015) 

for south walls (CI, continuous insulation; MW, mineral wool; EPS, expanded polystyrene; XPS, extruded polystyrene).

8

10

12

14

16

18

20

22

24

26

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Aug 19, 2015 Sep 02, 2015 Sep 16, 2015 Sep 30, 2015 Oct 14, 2015
Research Paper FPL–RP–698

26

Figure 34. Moisture content in oriented strandboard near bottom plate: daily averages around Injection 2 

(Nov. 2015) for north walls (CI, continuous insulation; MW, mineral wool; EPS, expanded polystyrene; 

XPS, extruded polystyrene).

8

10

12

14

16

18

20

22

24

26

Moisture content (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Nov 11, 2015 Nov 25, 2015 Dec 09, 2015 Dec 23, 2015 Jan 06, 2016 Jan 20, 2016
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

27

Figure 35. Moisture content in oriented strandboard near bottom plate: daily averages during second winter 

for north walls (MW, mineral wool; EPS, expanded polystyrene; XPS, extruded polystyrene).

Figure 36. Net moisture content around the time of Injection 1 (Aug. 2015) for north walls (CI, continuous insulation; 

MW, mineral wool; XPS, extruded polystyrene).

8

10

12

14

16

18

20

22

24

26

Moisture content (%)

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Jan 06, 2016 Jan 20, 2016 Feb 03, 2016 Feb 17, 2016 Mar 02, 2016 Mar 16, 2016 Mar 30, 2016

0

2

4

6

8

10

12

14

16

M5net (%)

No CI, poly

MW, poly

XPS, poly

Aug 19, 2015 Sep 02, 2015 Sep 16, 2015 Sep 30, 2015 Oct 14, 2015
Research Paper FPL–RP–698

28

Figure 37. Net moisture content around the time of Injection 1 (Aug. 2015) for south walls (CI, continuous insulation; 

MW, mineral wool; XPS, extruded polystyrene).

Figure 38. Net moisture content around the time of Injection 2 (Nov. 2015) for north walls (CI, continuous insulation; 

MW, mineral wool; XPS, extruded polystyrene).

0

2

4

6

8

10

12

14

16

18

M5net (%)

No CI, poly

MW, poly

XPS, poly

Aug 19, 2015 Sep 02, 2015 Sep 16, 2015 Sep 30, 2015 Oct 14, 2015

0

2

4

6

8

10

12

14

16

18

M5net (%)

No CI, poly

MW, poly

XPS, poly

Nov 11, 2015 Nov 25, 2015 Dec 09, 2015 Dec 23, 2015 Jan 06, 2016 Jan 20, 2016
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

29

Figure 39. Net moisture content around time of Injection 2 (Nov. 2015) for south walls (CI, continuous insulation; 

MW, mineral wool; XPS, extruded polystyrene).

Figure 40. Net moisture content around time of Injection 3 (May 2016) for north walls (CI, continuous insulation; 

MW, mineral wool; XPS, extruded polystyrene).

0

2

4

6

8

10

12

14

16

18

M5net (%)

No CI, poly

MW, poly

XPS, poly

Nov 11, 2015 Nov 25, 2015 Dec 09, 2015 Dec 23, 2015 Jan 06, 2016 Jan 20, 2016

0

2

4

6

8

10

12

14

16

18

M5net (%)

No CI, poly

MW, poly

XPS, poly

May 18, 2016 May 25, 2016 Jun 01, 2016 Jun 08, 2016 Jun 15, 2016 Jun 22, 2016 Jun 29, 2016 Jul 06, 2016
Research Paper FPL–RP–698

30

Figure 41. Net moisture content around the time of Injection 3 (May 2016) for south walls (CI, continuous insulation; 

MW, mineral wool; XPS, extruded polystyrene).

3.4.3  Quantification of Drying Potential

To quantify the drying behavior, each series of MC data in 

Figures 36 through 41 was fit with a near-exponential decay 

curve with a variable scale factor (K):

(2)

Details of the fit method are provided in Appendix B, 

whereas the basic near-exponential decay function is 

detailed in Whitehead and others (2009). A smaller time 

constant (τ) indicates more rapid drying, but the shape of the 

curve is also influenced by c, which controls the deviation 

from pure exponential decay. To compare how τ varies for 

all walls, c was fixed at 0.756 and τ was plotted in the bar 

graph of Figure 42.

Overall, drying was faster during winter for walls with 

interior kraft paper compared with those with polyethylene, 

and the MW exterior insulation showed faster drying during 

the winter than the base walls and walls with other exterior 

insulation materials.

Further comparison will focus first on the walls with 

polyethylene, which only dried outward. The same decay 

function was fit but c was allowed to rise to 1.33, which 

reflects slower drying. Τ is compared for these walls in 

Table 4.

8

pgpg p gp

e top of OSB.

ater Injection and Drying Potential

s have been presented primarily as weekly averages, but for the discussion of drying after water 

ly averages will be plotted around the times of the injection. Figure 32 presents the first drying period 

 values in OSB in the field of the wetting for all north walls, whereas Figure 33 shows the same MC 

e south walls. In both cases, the kraft-faced cavities dried faster because they could dry inward.

and third drying periods are similar and will be discussed in more detail subsequently. Before reviewing 

ata, two other effects are illustrated in Figures 34 and 35, which plot the MC values near the bottom 

ut outside the field of injection) during and after the second injection. This moisture pin (MC3) is not in 

he paper towel and hence does not see the strong spike from water injection, but it does show both the 

ment of water away from the field of the paper towel and the winter moisture accumulation in the OSB 

ft vapor retarder. The modest rise in moisture from MC3 for all north walls on November 11th in Figure 

migrating away from the shop towel. The rise in moisture from MC3 for some walls starting in January 

winter accumulation caused by indoor humidity.

ysis of drying will use a value, labeled M5net, which normalizes all the MC5 values (in the field of 

m) by subtracting the value of MC on August 8, 2015, in each wall from all the other values of MC for 

is forces each wall to start at an M5net value of zero just before the first injection thus allowing easy

of the changes in MC across walls. In addition, the focus will be primarily on the walls with 

, which forces outward drying only. Figures 36 and 37 present M5net for the first injection for north and 

respectively. Figures 38 and 39 are the same for the second injection, and Figures 40 and 41 show the 

n.

 observations can be confirmed by inspection of Figures 36 through 41. First, the walls had not 

ying before the second injection in late fall of 2015. Second, all walls except those with XPS had 

ear initial conditions before the third injection. Third, in the winter, the walls with MW were able to dry 

uantification of Drying Potential

he drying behavior, each series of MC data in Figures 36 through 41 was fit with a near-exponential 

with a variable scale factor (K):

( )

1

1

1

c

ft K

c

t

τ

   

=  

      +      

(2)

Again, the MW walls improved drying potential during 

winter. Very similar trends can also be seen in the kraftfaced walls. Applying the decay function fit yields a c of 

0.372, reflecting the faster response possible because of 

drying in both directions. The associated τ values for these 

kraft-faced walls are shown in Table 5.

Finally, the same trends can also be seen using a simpler 

measure of drying potential. Table 6 shows the percentage 

of the initial maximum net MC still left in each polyethylene 

wall 5 weeks after the injections stopped. Again, smaller 

numbers indicate faster drying. Only the polyethylene walls 

are reported in Table 6, similar to Table 4.

Table 4—Drying time constants (days) for walls with 

interior polyethylene vapor retardersa

Injection Wall No CI MW XPS

1, Aug–Sept N 17 24 23

1, Aug–Sept S 7 7 11

2, Nov–Dec N 103 35 70

2, Nov–Dec S 64 19 52

3, May–June N 5 10 14

3, May–June S 5 6 15

a

N, north; S, south; CI, continuous insulation; MW, mineral wool; 

XPS, extruded polystyrene.

2016 

0

2

4

6

8

10

12

14

16

18

M5net (%)

No CI, poly

MW, poly

XPS, poly

May 18, 2016 May 25, 2016 Jun 01, 2016 Jun 08, 2016 Jun 15, 2016 Jun 22, 2016 Jun 29, 2016 Jul 06, 2016

τ
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

31

Figure 42. Drying time constants for all walls (CI, continuous insulation; MW, mineral wool; 

EPS, expanded polystyrene; XPS, extruded polystyrene).

100

90

80

70

60

50

40

30

20

10

0

Time constant (days) Time constant (days)

Dry1 N

Aug–Sept 2015

Dry2 N

Nov–Dec 2015

Dry3 N

May–June 2015

1 – No Cl, kraft

2 – No Cl, poly

3 – MW, kraft

4 – MW, poly

5 – EPS, kraft

6 – XPS, kraft

7 – XPS, poly

8 – XPS, kraft crinkled

100

90

80

70

60

50

40

30

20

10

0

Dry1 S

Aug–Sept 2015

Dry2 S

Nov–Dec 2015

Dry3 S

May–June 2015

1 – No Cl, kraft

2 – No Cl, poly

3 – MW, kraft

4 – MW, poly

5 – EPS, kraft

6 – XPS, kraft

7 – XPS, poly

8 – XPS, kraft crinkled
Research Paper FPL–RP–698

32

Table 5—Drying constants (days) for kraft-faced wallsa

Injection Wall No CI MW EPS XPS XPS-cr

1, Aug–Sept N 10 4 26 23 4

1, Aug–Sept S 10 5 11 11 12

2, Nov–Dec N 55 19 33 62 22

2, Nov–Dec S 34 13 29 29 28

3, May–June N 5 5 6 10 7

3, May–June S 6 4 7 7 9

a

N, north; S, south; CI, continuous insulation; MW, mineral wool; EPS, expanded polystyrene; 

XPS, extruded polystyrene; XPS-cr, extruded polystyrene crinkled.

Table 6—Percentage of initial peak net MC after 

5 weeks drying in walls with polyethylene vapor 

retardersa

Injection Wall No CI MW XPS

1, Aug–Sept N 35 41 38

1, Aug–Sept S 22 23 28

2, Nov–Dec N 65 55 57

2, Nov–Dec S 58 39 54

3, May–June N 5 22 34

3, May–June S 12 14 34

a

N, north; S, south; CI, continuous insulation; MW, mineral wool; 

XPS, extruded polystyrene.

Figure 43. Surface relative humidity (RH) compared with measured RH near top of base case north walls 

(CI, continuous insulation).

40

50

60

70

80

90

100

Relative humidity (%)

No CI, kraft

No CI, poly

Surface RH – No CI, kraft

Surface RH – No CI, poly

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

33

3.5 OSB “Surface” Relative Humidity Levels

In this section, the OSB sorption isotherm detailed in 

Appendix A is again used to calculate apparent surface RH. 

Note that the moisture pins extended through the depth of 

the OSB, recording the lowest resistance path at any depth, 

and hence may not always represent the surface RH but 

instead the highest RH in the OSB. Here we present the 

apparent surface RH calculated from MC1 in OSB near the 

top of each wall. Figure 43 plots that surface RH along with 

the measured RH near that location for the two north base 

walls.

Generally, the surface RH tracked the measured cavity 

RH but did not drop as much during the summer, and 

clearly kraft-faced walls had higher RH than walls with 

polyethylene barriers during winter. The expected result 

that kraft-faced walls had higher RH during winter is also 

illustrated in Figure 44, which plots surface RH for all north 

walls. In addition, inspection of Figure 44 shows that adding 

exterior insulation lowers surface RH.

That effect can be seen again in Figure 45, which compares 

the differences between north and south walls for the 

base case and the MW case. Furthermore, the south walls 

sometimes had slightly higher surface RH.

Figure 44. Surface relative humidity for all north walls (CI, continuous insulation; MW, mineral wool; EPS, expanded 

polystyrene; XPS, extruded polystyrene).

40

50

60

70

80

90

100

Surface relative humidity (%)

No CI, kraft

No CI, poly

MW, kraft

MW, poly

EPS, kraft

XPS, kraft

XPS, poly

XPS, kraft, crinkled

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Research Paper FPL–RP–698

34

Figure 45. Surface relative humidity north (N) compared with south (S) for base and mineral wool (MW) walls 

(CI, continuous insulation).

4 Discussion

Moisture accumulation was observed in OSB sheathing 

during winter to the largest extent in Walls 1N and 1S with 

no exterior insulation and interior kraft vapor retarder. 

Moisture contents were above 20% in both winters; they 

reached higher levels in the first winter than in the second 

winter as a result of the higher interior humidity conditions. 

These walls dried rapidly in springtime. Sherwood (1983) 

and Parsons and others (2016) also measured wintertime 

sheathing MC above 20% in 38- by 140-mm walls with 

kraft vapor retarders and no exterior insulation, although 

Rose and McCaa (1998) measured considerably lower peak 

MC in similar wall assemblies.

Traditional guidance for protection of wood from decay 

has been to keep the MC below 20% (Carll and Highley 

1999). In this case, the risk of wood decay was probably 

quite low because the periods of high MC coincided with 

cold temperatures and the walls dried rapidly in warmer 

weather. The risk of mold growth on OSB, which depended 

on the surface temperature and moisture conditions, will be 

addressed in a separate report.

In the remaining wall assemblies, the OSB MC remained 

in a safe range during the 2-year investigation despite 

being challenged by high interior humidity in winter. It 

was expected that the walls with kraft vapor retarders and 

exterior insulation would have lower wintertime OSB MC 

than the corresponding walls without exterior insulation as a 

result of the exterior insulation warming the OSB sheathing.

All wall assemblies were able to accommodate controlled 

wettings multiple times during the study. The water 

injections were not severe but were enough to stress the 

wall around the injection site, similar to a localized water 

leak. The “drying” that occurred in M5net after injections 

was a combination of water redistribution away from the 

moisture pin and water leaving the OSB. The moisture 

pin data alone cannot help us determine the proportion 

between redistribution and actual drying. Further research 

is underway to help understand the effect of redistribution 

on MC readings. The observed decrease in OSB MC after 

controlled wetting events depended on weather conditions, 

type of exterior insulation, and type of interior vapor 

retarder. The temperature of the OSB had a large effect on 

the drying rate (i.e., rate of decrease in M5 in the field of 

the wetting towel). In all walls, the drying rate was much 

slower during the cold months. During warm periods, the 

XPS exterior insulation slowed down but did not prevent 

outward drying. However, during winter, the XPS exterior 

insulation sped up drying somewhat relative to the case 

without exterior insulation because the OSB was warmer. 

This effect was more dramatic in the vapor-open MW, 

which significantly improved drying during winter. The 

faster drying of walls with vapor-open MW than comparable 

walls with XPS is consistent with prior work by Maref and 

others (2011), Fox and others (2014), and Trainor and others 

(2016). Given that both MW and XPS warm the OSB during 

winter and redistribution would be expected to be similar, 

some water must be actually leaving the OSB because the 

wall with MW dried more quickly than the wall with XPS. 

In addition, walls with kraft vapor retarders generally dried 

more quickly than corresponding walls with polyethylene.

40

50

60

70

80

90

100

Surface relative humidity (%)

S – No CI, poly

N – No CI, poly

N – MW, poly

S – MW, poly

Jan 2015 Apr 2015 Jul 2015 Oct 2015 Jan 2016 Apr 2016 Jul 2016 Oct 2016 Jan 2017
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

35

5 Conclusions

The results of this study support the conclusion that adding 

exterior insulation does not increase the risk of moisturerelated deterioration in wood-framed wall assemblies. For 

2 years, we monitored the performance of a wood-framed 

structure in a cold climate built with 38- by 140-mm lumber 

and 11-mm-thick OSB sheathing. We tested two interior 

vapor retarders (kraft-faced fiberglass cavity insulation 

and polyethylene sheeting) in combination with commonly 

used exterior insulation materials, including XPS, EPS, and 

MW. Adding 25.4 mm of XPS to a wall with an interior 

vapor barrier did not prevent the wall from drying under 

modest load. Although under some conditions, a low 

permeance exterior material does slow drying, exterior 

insulation also keeps the wall warmer during cold weather, 

thereby enhancing moisture movement. The drying benefit 

of exterior insulation during cold weather is enhanced by 

using vapor-permeable exterior insulation such as MW. 

Vapor-open exterior insulation allowed our test walls to dry 

more quickly during winter that our base walls insulated 

only with fiberglass in the cavity. We also found that the 

kraft facing of fiberglass insulation did not prevent moisture 

accumulation in the exterior OSB during winter. The 

magnitude of this effect depended on interior RH conditions. 

In very few cases did the OSB MC rise to dangerous levels, 

but concurrent low temperatures decreased decay potential 

and the walls always dried out quickly in the spring when 

warm weather arrived.

Acknowledgments

This manuscript was improved by comments from Marshall 

Begel. The authors gratefully acknowledge the help of many 

FPL colleagues during the course of this investigation. Also, 

the support of APA – The Engineered Wood Association was 

instrumental in furthering this work. This work was funded 

by the USDA Forest Service, Forest Products Laboratory, 

and APA – The Engineered Wood Association through Joint 

Venture Agreement 14-JV-11111136-054.

Appendix A—Apparent Surface 

Relative Humidity Calculations

When the apparent surface RH of OSB is calculated from an 

MC reading, either to compare with other RH measurements 

or to calculate vapor pressure, an OSB sorption isotherm 

is needed to relate RH and MC. There is a large range of 

values in the literature for OSB sorption isotherms. Further 

experimental work is warranted. For this report, we used the 

same sources documented in Boardman and others (2017), 

averaging the adsorption and desorption values to generate 

a generic OSB sorption curve suitable for quick calculation 

of RH given OSB MC. Because we had no temperaturedependence data for OSB, we used the temperature 

dependence of an existing correlation for wood discussed as 

“H4” in Glass and others (2014). We then fit that formula to 

our room temperature data for OSB, only optimizing the fit 

parameters that were not temperature related. The generic 

formula is given in Equation (A-1) with m as the decimal 

MC (0–1), h as the decimal RH (0–1), and T as absolute 

temperature (K).

When the apparent surface RH of OSB is calculated from an MC reading, either to compare with o

measurements or to calculate vapor pressure, an OSB sorption isotherm is needed to relate RH and 

large range of values in the literature for OSB sorption isotherms. Further experimental work is w

report, we used the same sources documented in Boardman and others (2017), averaging the adsor

desorption values to generate a generic OSB sorption curve suitable for quick calculation of RH g

Because we had no temperature-dependence data for OSB, we used the temperature dependence o

correlation for wood discussed as “H4” in Glass and others (2014). We then fit that formula to our 

data for OSB, only optimizing the fit parameters that were not temperature related. The generic fo

Equation (A-1) with m as the decimal MC (0-1), h as the decimal RH (0-1), and T as absolute tem

1 ln 1( )

D CT B

c

T m AT h

T

    =− −        

(A-1) 

where A, B, C, and D are fitting parameters and critical 

temperature Tc = 647.1 K. Our fit was constrained to yield 

less error at the high RH region. The inverted formula 

to calculate h given m is shown with final values in 

Equation (A-2) and plotted in Figure 46.

where A, B, C, and D are fitting parameters and critical temperature, Tc, = 647.1 K. Our fit was 

less error at the high RH region. The inverted formula to calculate h given m is shown with fina

(A-2) and plotted in Figure 46.

0.451 2.43 1520 17.3 1 exp 1 647.1

T T h m

T

− −   −   = − −        

The time response of the MC sensors is somewhat delayed relative to the surface RH, because 

are damped by the storage capacity of the OSB sheathing. Furthermore, the moisture pins read 

anywhere throughout the depth of the OSB. For these reasons, we refer to calculated values as 

RH values.

H1 Appendix B—Moisture Content Decay Function

To help quantify the drying behavior of the OSB walls, the M5net values were fit to a near-expo

function. Background and details on the function can be found in Whitehead and others (2009)

presented in Equation (B-1) and plotted in Figure 47 with a scale factor (K) of 12.

( )

1

1

1

c

ft K

c

t

τ

   

=  

      +      

where t is time (days), c is a dimensionless shape parameter, and τ is a decay time constant (day

which must be greater than 0, determines how near this function is to pure exponential decay, w

0 (in which case Eq. (B-1) would not apply). For the fits to polyethylene-protected Walls 2, 4, 

only dry outward, the best fit c value was 1.33, whereas for the other kraft-faced walls, c = 0.37

effects of both c and τ, the function is plotted in Figure 47 with τ values that correspond to min

maximum values of time constant (days) for the polyethylene walls with c = 1.33, along with a 

for the kraft wall c value of 0.373. We applied a scale factor of 12 to the function here, which m

look similar to our OSB values of M5net.

Inspection of Figure 47 confirmed that small values of τ indicated more rapid drying. In the act

walls, the scale factor was allowed to vary to best match the overall M5net values for that wall b

within ±0.9% MC of the starting value. Starting values were determined by picking the highest 

for the wall after the injection and resetting time zero to correspond to that peak value for each 

the fit parameters was done in Excel (Microsoft, Redmond, Washington, USA) by minimizing 

squared errors (difference between function prediction and measured M5net value for each day 

walls, the scale factors varied between 8 and 18.5 with average near 12, whereas τ values range

with average of 17 for kraft walls and 27 for polyethylene walls. All kraft walls were required t

value. Similarly, all polyethylene walls shared a c value, but it was higher, reflecting the slowe

of these walls. For Figure 42, which compares τ values across all wall types, the fit was change

consistent c value (c = 0.76), allowing direct comparison of τ values. Two examples of these fit

Figure 48.

(A-2)

The time response of the MC sensors is somewhat delayed 

relative to the surface RH, because any RH fluctuations 

are damped by the storage capacity of the OSB sheathing. 

Furthermore, the moisture pins read the highest MC 

anywhere throughout the depth of the OSB. For these 

reasons, we refer to calculated values as “apparent” surface 

RH values.

0.15

0.10

0.05

0

0.25

0.20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

m(0-1) moisture content

h(0-1) relative humidity

Figure 46. Oriented strandboard sorption isotherm at 23 °C.
Research Paper FPL–RP–698

36

Appendix B—Moisture Content 

Decay Function

To help quantify the drying behavior of the OSB walls, the 

M5net values were fit to a near-exponential decay function. 

Background and details on the function can be found in 

Whitehead and others (2009). The equation is presented in 

Equation (B-1) and plotted in Figure 47 with a scale factor 

(K) of 12.

11

nse of the MC sensors is somewhat delayed relative to the surface RH, because any RH fluctuations 

he storage capacity of the OSB sheathing. Furthermore, the moisture pins read the highest MC 

ghout the depth of the OSB. For these reasons, we refer to calculated values as “apparent” surface 

dix B—Moisture Content Decay Function

y the drying behavior of the OSB walls, the M5net values were fit to a near-exponential decay 

round and details on the function can be found in Whitehead and others (2009). The equation is 

uation (B-1) and plotted in Figure 47 with a scale factor (K) of 12.

( )

1

1

1

c

ft K

c

t

τ

   

=  

      +      

(B-1)

(days), c is a dimensionless shape parameter, and τ is a decay time constant (days). The c value, 

greater than 0, determines how near this function is to pure exponential decay, which would have c =

e Eq. (B-1) would not apply). For the fits to polyethylene-protected Walls 2, 4, and 7, which could

d, the best fit c value was 1.33, whereas for the other kraft-faced walls, c = 0.373. To illustrate the 

and τ, the function is plotted in Figure 47 with τ values that correspond to minimum, average, and 

s of time constant (days) for the polyethylene walls with c = 1.33, along with a comparison at τ = 5 

l c value of 0.373. We applied a scale factor of 12 to the function here, which made the numbers 

our OSB values of M5net.

gure 47 confirmed that small values of τ indicated more rapid drying. In the actual fits to individual 

factor was allowed to vary to best match the overall M5net values for that wall but was required to be 

MC of the starting value. Starting values were determined by picking the highest daily M5net recorded 

r the injection and resetting time zero to correspond to that peak value for each wall. Optimization of 

rs was done in Excel (Microsoft, Redmond, Washington, USA) by minimizing the sum of the 

difference between function prediction and measured M5net value for each day after peak). Across all 

factors varied between 8 and 18.5 with average near 12, whereas τ values ranged from 3.8 to 103 

 17 for kraft walls and 27 for polyethylene walls. All kraft walls were required to share the same c

, all polyethylene walls shared a c value, but it was higher, reflecting the slower outward-only drying 

For Figure 42, which compares τ values across all wall types, the fit was changed to force a 

ue (c = 0.76), allowing direct comparison of τ values. Two examples of these fits are provided in 

this manner does not provide any estimates of the uncertainty in fit parameters. Much of the error in 

on method is a result of the bias of the model. An underlying assumption in the decay model is that 

(B-1)

where t is time (days), c is a dimensionless shape parameter, 

and τ is a decay time constant (days). The c value, which 

must be greater than 0, determines how near this function 

is to pure exponential decay, which would have c = 0 (in 

which case Eq. (B-1) would not apply). For the fits to 

polyethylene-protected Walls 2, 4, and 7, which could only 

dry outward, the best fit c value was 1.33, whereas for the 

other kraft-faced walls, c = 0.373. To illustrate the effects 

of both c and τ, the function is plotted in Figure 47 with τ 

values that correspond to minimum, average, and maximum 

values of time constant (days) for the polyethylene walls 

with c = 1.33, along with a comparison at τ = 5 for the kraft 

wall c value of 0.373. We applied a scale factor of 12 to the 

function here, which made the numbers look similar to our 

OSB values of M5net.

Inspection of Figure 47 confirmed that small values of τ 

indicated more rapid drying. In the actual fits to individual 

walls, the scale factor was allowed to vary to best match 

the overall M5net values for that wall but was required to 

be within ±0.9% MC of the starting value. Starting values 

were determined by picking the highest daily M5net recorded 

for the wall after the injection and resetting time zero to 

correspond to that peak value for each wall. Optimization of 

the fit parameters was done in Excel (Microsoft, Redmond, 

Washington, USA) by minimizing the sum of the squared 

errors (difference between function prediction and measured 

M5net value for each day after peak). Across all walls, the 

scale factors varied between 8 and 18.5 with average near 

12, whereas τ values ranged from 3.8 to 103 with average 

of 17 for kraft walls and 27 for polyethylene walls. All kraft 

walls were required to share the same c value. Similarly, 

all polyethylene walls shared a c value, but it was higher, 

reflecting the slower outward-only drying of these walls. 

For Figure 42, which compares τ values across all wall 

types, the fit was changed to force a consistent c value 

(c = 0.76), allowing direct comparison of τ values. Two 

examples of these fits are provided in Figure 48.

Using Excel in this manner does not provide any estimates 

of the uncertainty in fit parameters. Much of the error in 

this quantification method is a result of the bias of the 

model. An underlying assumption in the decay model 

is that all walls are driven to return to M5net zero after 

application of the water injection. Clearly, this assumption 

was not usually valid because the walls were subjected to 

Figure 47. Near-exponential decay function with c = 1.33, various τ, and one value at c = 0.373 

(similar to pure exponential decay).

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

0

2

4

6

8

10

12

Time (days)

Decay function (scaled to start at 12)

τ = 100, c = 1.33 τ = 27, c = 1.33

τ = 5, c = 1.33 τ = 5, c = 0.373

τ
Field Moisture Performance of Wood-Framed Walls with Exterior Insulation in a Cold Climate

37

Figure 48. M5net values after Injection 2 (Nov. 2015) in select north walls showing data and curve fit 

(XPS, extruded polystyrene).

a variety of different driving forces with time. However, 

the experimental observation that many walls returned 

to near zero before Injection 3 provides some support for 

this bias in the model. Another bias is the assumption of 

uniform decay which reflects a single physical mechanism 

decreasing the MC at MC5. Clearly, this assumption is also 

not usually valid since water is moving away from MC5 

because of leaving the system (escape to indoor or outdoor) 

and redistribution within the OSB. We have no data to show 

the magnitude of these two mechanisms, but we assume that 

most of the redistribution happens early; therefore, fitting 

the later data is able to capture much of the actual drying.

Further difficulty in quantifying the error of these decay 

constants, τ, resulted from the variance within the data. 

Some of this variance did not reflect actual changes in MC 

of the OSB but was the result of electrical connection faults 

between the wires and the moisture pins, which resulted 

in data that jumped between infinite resistance (MC near 

8%) and the actual value, as can be seen in some of the data 

0 20 40 60 80

XPS, kraft Fit

XPS, kraft Fit

14

12

10

8

6

4

2

0

M5net (%)

Time (days)

0 20 40 60 80

14

12

10

8

6

4

2

0

M5net (%)

Time (days)
Research Paper FPL–RP–698

38

Figure 49. Drying time constants for walls with polyethylene indoor barrier, including error bars from data 

variance but not model bias (CI, continuous insulation; MW, mineral wool; XPS, extruded polystyrene).

sets (see Figures 32 and 33, for example). Given all these 

difficulties, the τ values should be used only to compare 

walls experiencing similar conditions, hence the grouping 

of the polyethylene walls in Table 4 and the kraft walls in 

Table 5.

Finally, to give an indication of the variance within the 

existing data and its effect on the fit parameters given the 

mentioned constraints, error bars are provided (excluding all 

the model bias) by fitting the polyethylene data sets using 

MATLAB, which provides estimates of the error in τ for the 

95% confidence interval. These error bars are displayed in 

Figure 49.

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