Hygric Redistribution in 

Insulated Assemblies: 

Retrofitting Residential 

Envelopes Without Creating 

Moisture Issues 

J. Smegal and J. Lstiburek 

Building Science Corporation

January 2013
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iii

Hygric Redistribution in Insulated Assemblies: 

Retrofitting Residential Envelopes Without Creating Moisture Issues

Prepared for: 

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy 

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

J. Smegal and J. Lstiburek 

Building Science Corporation 

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40337-00 

January 2013 
iv

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v 

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................ viii

Definitions................................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

1 Problem Statement............................................................................................................................... 1

1.1 Introduction ..........................................................................................................................1

1.2 Background ..........................................................................................................................1

1.3 Relevance to Building America’s Goals ..............................................................................2

1.4 Cost Effectiveness ................................................................................................................3

1.5 Tradeoffs and Other Benefits ...............................................................................................4

2 Experiment ............................................................................................................................................ 5

2.1 Research Questions ..............................................................................................................5

2.2 Technical Approach .............................................................................................................5

3 Testing and Analysis............................................................................................................................ 6

3.1 Drainage and Drying Testing ...............................................................................................6

3.1.1 Research Goal/Questions To Be Answered .............................................................6

3.1.2 Testing System .........................................................................................................7

3.1.3 Matrix for Testing ..................................................................................................12

3.1.4 Testing Protocol .....................................................................................................12

3.1.5 Results/Analysis .....................................................................................................13

3.1.6 Summary ................................................................................................................17

3.2 Hygric Redistribution.........................................................................................................18

3.2.1 Research Goal/Question To Be Answered ............................................................18

3.2.2 Testing System .......................................................................................................18

3.2.3 Matrix for Testing ..................................................................................................20

3.2.4 Results/Analysis .....................................................................................................22

3.2.5 Summary ................................................................................................................26

3.3 Field Testing ......................................................................................................................27

3.3.1 Enclosure Wetting Systems ...................................................................................27

3.3.2 Instrumentation ......................................................................................................28

3.3.3 Observations and Data Analysis ............................................................................31

3.3.4 Comparison of Walls A and B ...............................................................................32

3.3.5 Test Wall Deconstruction ......................................................................................42

3.3.6 Summary ................................................................................................................45

3.4 Conclusions ........................................................................................................................46

3.5 Future Work .......................................................................................................................47

References................................................................................................................................................. 48

Appendix: BEopt Output Graphs ............................................................................................................ 49
vi 

List of Figures

Figure 1. Moisture movement and condensation on interior surface of exterior insulation .............. 2

Figure 2. Moisture balance of wetting and drying ................................................................................... 7

Figure 3. Wall balance schematic.............................................................................................................. 8

Figure 4. Drainage and drying balance..................................................................................................... 9

Figure 5. Location of wood MC sensors................................................................................................... 9

Figure 6. Installed wood MC sensor and thermistor ............................................................................. 10

Figure 7. Typical test wall schematic...................................................................................................... 11

Figure 8. Interior surface of typical test wall.......................................................................................... 11

Figure 9. Exterior surface of typical test wall......................................................................................... 11

Figure 10. Two-inch-long plastic spacers .............................................................................................. 12

Figure 11. Spacers installed in top of drainage gap............................................................................. 12

Figure 12. Smooth plastic housewrap drainage test............................................................................. 14

Figure 13. Repeated drainage gap wetting analysis, smooth housewrap .......................................... 15

Figure 14. Repeated drainage gap wetting analysis, corrugated housewrap .................................... 17

Figure 15. RH sensor in the drainage space .......................................................................................... 19

Figure 16. Wetting system and drainage space RH sensors................................................................ 19

Figure 17. Layout for sheathing MC sensors and stud space RH sensors as shown from the 

“interior”.............................................................................................................................................. 20

Figure 18. Example of drainage mesh between two sheet materials .................................................. 21

Figure 19. Example of a bumpy housewrap........................................................................................... 21

Figure 20. Measured RH at nine locations in the drainage space ....................................................... 22

Figure 21. Comparison of surface areas for diffusion from wetting system...................................... 23

Figure 22. Measured RH in the five stud spaces ................................................................................... 24

Figure 23. Measured MC sensors in all five stud spaces at the mid height of the wall..................... 25

Figure 24. Measured MC sensors in all five stud spaces 16 in. from the bottom of the wall............ 26

Figure 25. Exterior insulated wall on all four orientations (Wall A) ..................................................... 27

Figure 26. Direct applied stucco wall on all four orientations (Wall B)............................................... 27

Figure 27. Exterior wetting apparatus..................................................................................................... 28

Figure 28. MC pins installed in plywood sheathing through the wetting system .............................. 29

Figure 29. RH and temperature sensor installed in a stud bay (with MC pins installed below) ....... 30

Figure 30. Typical wall construction and sensor configuration........................................................... 30

Figure 31. Lower OSB sheathing measured MC comparison between north Wall B (direct applied 

stucco) and north Wall A (exterior insulation) ................................................................................ 33

Figure 32. Middle and upper OSB sheathing measured MC comparison between north Wall B 

(direct applied stucco), and north Wall A (exterior insulation)...................................................... 34

Figure 33. East Wall B (direct applied stucco) and east Wall A (insulating sheathing) lower OSB 

sheathing measured MC.................................................................................................................... 34

Figure 34. East Wall B (direct applied stucco) and east Wall A (insulating sheathing) middle and 

upper OSB sheathing measured MC ................................................................................................ 35

Figure 35. South Wall B (direct applied stucco) and south Wall A (exterior insulation) lower OSB 

sheathing measured MC.................................................................................................................... 35

Figure 36. South Wall B (direct applied stucco) and south Wall A (exterior insulation) middle and 

upper OSB sheathing measured MC ................................................................................................ 36

Figure 37. West Wall B and south Wall A lower OSB sheathing measured MC comparing exterior 

insulation............................................................................................................................................. 36

Figure 38. West Wall B and south Wall A middle and upper OSB sheathing measured MC 

comparing exterior insulation........................................................................................................... 37

Figure 39. Example of air leakage condensation durability issues ..................................................... 38

Figure 40. Measured interior and ambient temperatures...................................................................... 39

Figure 41. Measured interior and ambient RH ....................................................................................... 39

Figure 42. Comparison of the hours of potential condensation between Wall B and Wall A from 

December 18, 2009, to May 30, 2010, using measured interior dew point and sheathing 

temperature......................................................................................................................................... 40
vii 

Figure 43. Comparison of the hours of potential condensation between Wall B and Wall A from 

September 19, 2010, to March 23, 2011, using measured interior dew point and sheathing 

temperature......................................................................................................................................... 41

Figure 44. Comparison of condensation potential between north walls A and B, January to May 

2010...................................................................................................................................................... 42

Figure 45. Comparison of condensation potential between north walls A and B, September to 

March 2010 .......................................................................................................................................... 42

Figure 46. Exterior OSB sheathing on the north orientation immediately following deconstruction43

Figure 47. Exterior OSB sheathing on the north orientation immediately following deconstruction 

(photo taken from the opposite direction of Figure 46).................................................................. 44

Figure 48. Exterior OSB sheathing on the east orientation immediately following deconstruction 44

Figure 49. Exterior OSB sheathing on the south orientation immediately following deconstruction45

Unless otherwise noted, all figures were created by BSC.
viii

List of Tables

Table 1. Total Energy Savings for Each Retrofit Strategy at Various Locations (MBtu/yr)................. 3

Table 2. Predicted Annual Savings ........................................................................................................... 4

Table 3. Testing Matrix for Drainage and Drying Testing ..................................................................... 12

Table 4. Values of Maximum Storage Amounts on Nonabsorptive Surfaces for Context to Testing 

Storage Amounts................................................................................................................................ 14

Table 5. Comparison of Drainage Tests on Smooth Plastic Housewrap ............................................ 16

Table 6. Comparison of Drainage Tests on a Corrugated Plastic Housewrap ................................... 16

Table 7. Hygric Redistribution Wetting Schedule.................................................................................. 20

Table 8. Intentional Wetting Event Schedule and Location.................................................................. 28

Table 9. Comparison of Drying Rates for Wall A and Wall B Following Interior Wetting Event in 

January 2011....................................................................................................................................... 33

Table 10. Number of Hours of Potential Air Leakage Condensation Based on the Interior Dew 

Point and Sheathinga

.......................................................................................................................... 40

Unless otherwise noted, all tables were created by BSC.
ix

Definitions

BEopt Building Energy Optimization

Btu British thermal unit

DOE U.S. Department of Energy

ffpic Foil-faced polyisocyanurate

fl oz Fluid ounce

ft2 Square foot

g Gram

L Liter

lb Pound

m

2 Square meter

MBtu Million Btu

MC Moisture content

mL Milliliter

Ng Nanogram

Pa Pascal

RH Relative humidity

o.c. On center

OSB Oriented strand board

XPS Extruded polystyrene

Yr Year
x 

Executive Summary

This research report was prepared by the Building Science Corporation Building America 

Research Team as part of the Energy Efficient Housing Research Partnerships project. The 

project title is Hygric Redistribution in Insulated Assemblies. 

The exterior retrofit of frame assemblies can be risky when impermeable insulating sheathings of 

relatively low thermal resistance are used. For example, basic hygrothermal analysis 

demonstrates that removing the existing cladding and installing a board foam insulation with low 

vapor permeance (such as 1⁄2-in. foil faced rigid insulation) directly over the sheathing membrane

under a replacement cladding (such as vinyl siding) is likely to cause moisture problems in cold 

climates. One such problem is known as a “cold sided vapor barrier.” Tens of thousands of such 

retrofit and renovation projects have been completed, however, without apparent problems. 

There is also a perception that when water is intentionally drained in a drainage cavity between 

exterior insulation and the existing structure following an energy retrofit, moisture-related 

durability concerns exist for the sheathing and wood structure. 

This experimental program includes a literature review of previous research, laboratory testing 

programs, and an analysis of full-scale test wall performance in the U.S. Department of Energy’s 

climate zone 4C. 

The following four questions were proposed to determine the level of risk associated with adding

drained exterior insulation to the exterior of an existing home during an energy retrofit: 

1. How much water is stored in the drainage cavity, and does it pose a moisture-related 

durability threat?

2. If water becomes trapped in the drainage cavity, where does the water go, and what 

impact does it have on the durability of the wall system? 

3. Why is there a discrepancy between the basic hygrothermal analysis of vapor 

impermeable, relatively low R-value exterior insulations, and field performance?

4. Are there alternative solutions to energy retrofits that decrease the risk of moisturerelated durability problems? 

The team found that there is virtually no moisture durability risk to the sheathing when water is 

drained repeatedly in the drainage gap between the exterior insulation and the sheathing 

membrane, provided there are no other moisture-related issues unrelated to the drainage cavity 

such as air leakage condensation, or bulk rain water leakage. 
1 

1 Problem Statement 

1.1 Introduction

Many thousands of homes in the United States, particularly in cold climates, need an energy 

retrofit to reduce the energy consumed for space conditioning and increase occupant comfort. 

Straightforward insulation strategies that do not increase the risk of moisture-related durability 

issues to the enclosure are needed. 

The most straightforward solution is adding insulation to the exterior during an energy retrofit, 

especially where it is preferred not to disrupt the interior. Potential complications with enclosure 

and cladding details resulting from the increased thickness of the enclosure wall must be 

considered. Typically, insulation thicknesses of 1 in. and less do not require modifications to 

other details, but insulation thicknesses of 2 in. and greater will require modifications to window 

detailing and cladding attachment. When exterior insulation between 1 and 2 in. is installed, 

modifications to the enclosure depend on the specified construction details and material and 

cladding selection, and must be considered on a case-by-case basis. 

1.2 Background

The exterior retrofit of frame assemblies appears to be risky when impermeable insulating 

sheathings of relatively low thermal resistance are used. The most typical of these is foil-faced 

polyisocyanurate (ffpic). Basic hygrothermal analysis, using hygrothermal simulation software 

or mathematically derived procedures, demonstrates that wrapping the exterior of a wood frame 

house with a sheathing membrane, followed by installing 1⁄2-in. foil-faced rigid insulation under a 

replacement cladding (such as vinyl siding), is likely to lead to moisture problems in cold 

climates. This problem is referred to as a “cold sided vapor barrier.” 

The Building Science Corporation (BSC) Building America Research Team examined this 

potential problem as part of the Energy Efficient Housing Research Partnerships project. In its 

experience prior to this research project, though, the BSC team had seen many such retrofit and 

renovation projects completed in cold climates without apparent problems. 

The research team hypothesized that such retrofit applications actually work because the 

relatively cold exterior vapor barrier acts as a dehumidifier for the wall. The BSC team believes

that the condensate on the back of the condensing surface—the interior facing side of the 

impermeable insulating sheathing—does not cause damage because this condensate drains 

harmlessly to the outside. This condensing surface is exterior to the water management layer (the 

previous cladding or sheathing membrane) (Figure 1). The drainage that will occur is not 

reflected in hygrothermal simulations. This research also assumes that the wall did not suffer 

durability issues before the retrofit, which means there was not enough moisture movement 

(through vapor diffusion or air leakage) from the interior to result in condensation on the interior 

surface of the exterior structural sheathing. This condensation is likely to have caused moisturerelated durability issues without exterior insulation. Adding any amount of insulation to the 

exterior will increase the durability of the wood sheathing by increasing the surface temperature 

and decreasing the potential for condensation, as long as condensation that can occur to the 

exterior of the sheathing is adequately drained. 
2 

Figure 1. Moisture movement and condensation on interior surface of exterior insulation

This has huge positive implications for the retrofit of frame walls. It makes available a less costly 

approach to exterior insulation approaches where risk from moisture damage is low. The

effectiveness of the drainage needs to be determined, however, as well as the reduction (penalty) 

in thermal resistance resulting from an air gap that facilitates drainage. This research examined 

only the effectiveness of the drainage gap and the moisture-related durability of the enclosure. 

The BSC team did not investigate the potential loss of thermal resistance resulting from a 

drainage cavity. 

1.3 Relevance to Building America’s Goals

Overall, the goal of the U.S. Department of Energy (DOE) Building America Program is to 

reduce home energy use by 30%–50% (compared to 2009 energy codes for new homes and preretrofit energy use for existing homes). To this end, BSC conducts research to “develop marketready energy solutions that improve efficiency of new and existing homes in each U.S. climate 

zone, while increasing comfort, safety, and durability.” 

This research project explores retrofit insulation strategies to existing buildings, particularly in 

cold climates. The main goals of adding the exterior insulation are to reduce the energy 

consumption and increase the occupant comfort. There is also evidence of significantly 

decreasing the condensation potential on the interior of the sheathing by using exterior insulation

(Smegal and Straube 2009). Once the information from this research project is disseminated, 

along with appropriate cladding attachment details and finishing details, the results from this 

research could be quickly scalable to all retrofit projects.

The Building Energy Optimization (BEopt) tool was used to optimize the amount of exterior 

insulation installed as a retrofit on the exterior of the house strictly from a cost and source energy 

perspective without analyzing increased occupant comfort and increased moisture durability 

benefits. The simulated house is approximately 2,300 ft2

, with a window area of 410 ft2, which is 

approximately 18% of the exterior wall area. For the benchmark, it was assumed that the 2 × 4 

walls are uninsulated because of the age of the house for retrofit. The first analysis point for all 

Foil-faced polyisocyanurate 

insulation

Sheathing membrane

Plywood sheathing

2x4 framing

Fiberglass batt

Gypsum Board

Moisture movement

(Air leakage or diffusion)
3 

simulations was installing R-13 cavity insulation. It is assumed for prediction purposes that the

interior living space could be disrupted to install the cavity insulation. In reality, in some cases, it 

might not be possible to not affect the interior spaces during relatively simple retrofits.

After adding the cavity insulation, five different thicknesses of exterior polyisocyanurate board 

insulation were simulated. Polyisocyanurate was chosen because it has the highest R-value per 

inch of all foam board insulations, and is less expensive than extruded polystyrene (XPS) (Grin

and Lstiburek 2011). 

Table 1 summarizes the results of this analysis, and the appendix includes a copy of the analysis 

bar charts and optimization curves. For each city (Boston, Massachusetts; Duluth, Minnesota; 

Kansas City, Missouri; and Dallas, Texas), the optimized solution (based strictly on energy and 

cost) is indicated in bold. In climate zone 4 and above, BEopt showed that the optimized solution 

for retrofit is using 1.5 in. of exterior insulation. An attempt was made to estimate other 

construction related costs such as wood strapping, fasteners, and window boxes,among others, 

for thicker exterior insulation installation.

Table 1. Total Energy Savings for Each Retrofit Strategy at Various Locations (MBtu/yr) 

Boston, 

Massachusetts

Duluth, 

Minnesota

Kansas City, 

Missouri

Dallas, 

Texas

DOE Climate Zone 5 7 4 3

Benchmark Uninsulated 0 0 0 0

R-13 Cavity 69 100 65 46

1-in. Exterior Insulation 88 128 82 57

1.5-in. Exterior Insulation 92 135 87 60

2-in. Exterior Insulation 96 141 90 62

3-in. Exterior Insulation 101 148 94 97

4-in. Exterior Insulation 104 153 97 66

1.4 Cost Effectiveness

Adding exterior insulation not only increases the installed insulation R-value, but also increases 

the whole-wall R-value and improves the insulative performance of the stud space insulation 

because it minimizes the thermal bridging through the framing (Smegal and Straube 2009). The 

cost of adding exterior insulation depends on the type of insulation added, the thickness, and any 

other changes to the enclosure resulting from increased thickness, such as modifying the window 

and door installations. At an exterior insulation thickness of 1 in. or less, there are no significant 

changes to any of the other enclosure detailing. 

BEopt predicted that for DOE climate zones 4, 5, and 6, the maximum total energy savings 

occurred with 1.5 in. of exterior insulation. From RSMeans CostWorks2011 (Reed Construction 

Data, May 2011), the incremental cost of adding 1.5 in. of exterior ffpic to the wall system is 

$1.56/ft2

. Monetary savings based on decreased energy for space conditioning will depend on the 

type and thickness of exterior insulation, the climate, and the type of fuel used for space 

conditioning. Table 2 gives the BEopt predictions of the amount of money saved. These savings 

are predicted when a house with an enclosure with R-13 batt has 1.5 in. of ffpic installed to the 

exterior. The house in the simulation is a Building America benchmark house and might not 
4 

accurately reflect the condition of older homes, so the savings might be underestimated. Also, in 

some areas, old homes are sometimes uninsulated in the walls. Adding 1.5 in. of ffpic to an 

uninsulated wall assembly was not calculated, but the savings can be expected to be about 

$1,000/yr for the simulated building in a cold climate. This analysis also did not account for 

airtightness changes, but applying a drainage membrane and continuous exterior insulation could 

improve the airtightness of the building as well. 

Table 2. Predicted Annual Savings

Representative City DOE Climate 

Zone R-13 Cavity 1.5-in. (R-9.75) Exterior Insulation Savings 

Boston, Massachusetts 5 6,031 5,879 $152

Duluth, Minnesota 7 5,741 5,603 $138

Kansas City, Missouri 4 4,965 4,874 $91

It cannot be ignored that an initially slightly more expensive system might have to be 

implemented to save a significant amount of energy over the entire life of the structure, which 

will be much longer than a standard mortgage. Research has shown that walls exceeding an Rvalue of 35 can financially pay back during the life of the initial mortgage through energy 

savings while reducing greenhouse gases (Grin 2008). Because the building enclosure is 

designed to use less energy, the energy and greenhouse gas emissions savings extend for the life 

of the building, not just the initial mortgage.

1.5 Tradeoffs and Other Benefits

Adding exterior insulation will result in a more comfortable living space, but this significant 

benefit is difficult to quantify in terms of dollars. Also, by adding exterior insulation, the 

temperature of the condensation plane in cold climates (the interior surface of the exterior 

sheathing) will become warmer, which will decrease the risk of condensation and moisturerelated durability risks such as mold. By increasing the durability of the enclosure, the 

sustainability of the building is also increased. By doubling the life of a building using the same 

amount of resources to construct it, the building is twice as resource efficient. 

Also, note that it is possible to improve the airtightness of the enclosure during a retrofit like this, 

which will result in further energy savings. A ventilation strategy, however, might be required to 

bring adequate fresh air to the interior space. 
5 

2 Experiment 

2.1 Research Questions

The key research question is this: How risky is adding drained exterior insulation in an energy 

retrofit of an existing home? Drained exterior insulation in this project refers to exterior board 

foam insulation that is installed over a sheathing membrane, with the intentional drainage cavity 

of the wall between the insulation and sheathing membrane. The durability in question is the 

durability of the wood sheathing. Some perceive that adding exterior insulation to the exterior of 

the wood sheathing will cause moisture-related durability problems with the wood sheathing. 

To answer the key question, the mechanisms of wetting and drying will be examined: 

• How much water can be stored in the drainage cavity, and does it pose a moisture-related 

durability threat?

• If water becomes trapped in the drainage cavity, where does the water go, and what 

impact does it have on the durability of the wall system and the moisture content (MC) of 

the sheathing? 

• Are there alternative solutions to energy retrofits that decrease the risk of moisturerelated durability problems?

2.2 Technical Approach

This research project was divided into several areas of work. 

A literature review was conducted before testing, to determine what testing has been conducted 

and what research has been published on moisture-related issues with exterior insulation 

retrofits. There is no specific literature review section in this report, but the relevant information 

is discussed in each section. 

Laboratory testing was conducted to determine the amount of water drained and stored in the 

drainage cavity of various wall systems by measuring the amount of water stored after water is 

poured into the drainage cavity. The sheathing MC was monitored during and following water 

application to the drainage cavity to see what effect, if any, the stored water had on the MC of 

the sheathing. 

In a similar test, water was intentionally stored between the exterior insulation and drainage 

plane to determine what impact that had, if any, on the MC of the sheathing and wall cavity. 

Moisture stored in the drainage cavity is widely perceived to cause moisture-related durability 

issues for the sheathing. 

Full-scale comparative field testing of exterior insulated walls in DOE climate zone 4C with 

other more traditional wall construction strategies has been completed. This testing focused on 

determining moisture-related durability risks caused by exterior wetting between the wood 

sheathing and exterior low permeance insulation. The conclusions from this testing are relevant 

to all climate zones, particularly cold climates.
6 

3 Testing and Analysis

The experimental testing program was designed to assess the moisture-related risks of draining 

the enclosure wall system between the insulation and the sheathing on various drainage plane 

materials. Testing was designed to compare various drainage plane materials, and to determine if 

certain materials altered, either positively or detrimentally, the inherent moisture-related risk. 

Field tests were conducted on full-scale test walls that compare side-by-side, exterior insulated 

walls with more standard construction techniques. In the full-scale field testing, a known volume 

of water was injected using a wetting apparatus directly against the exterior surface of the 

sheathing, simulating a deficiency in the drainage plane. 

3.1 Drainage and Drying Testing

Drainage and drying testing is important in determining the amount of water that can be drained 

from the enclosure system, and to quantify the amount of stored water following initial drainage. 

This moisture could be drained intentionally from flashed openings in the enclosure or as a result 

of diffusion condensation through the enclosure. Typically there are two options. The water can

evaporate and diffuse away in six possible directions, or be dried by ventilation (air movement). 

Determining the drying rates of various wall systems is important because the drying rate is 

related to the moisture durability and moisture storage capacity of a wall system. In this case, 

drying rates were not analyzed because the temperature and relative humidity (RH) conditions 

around the test wall, which affect the drying rate, could not be tightly controlled. Tight control is 

important because fluctuations in the ambient RH will affect hygroscopic materials in the wall 

system and the weight of the wall will change with changes in RH (Smegal 2006). In future 

testing, an enclosure could be built around the testing apparatus to tightly control the temperature 

and RH so walls can be compared with the same boundary conditions. Drying rates of different 

walls will depend on their construction and material properties. Drying rates of test walls during 

field testing for exterior insulated walls and more typical construction are illustrated in 

Section 3.3. 

Previous research conducted at Pennsylvania State (Schumacher et al. 2003) and the University 

of Waterloo (Smegal 2006) used a similar testing technology of a balance that gravimetrically 

measures the mass of water in a 4 ft × 8 ft wall panel during drainage testing. This research was 

limited, however, to total moisture storage in the drainage cavity and did not address the manner 

or rates of hygric redistribution. Further, the effects of repeated wetting were not investigated. 

No literature was found that indicates that the MC of the sheathing was measured during 

drainage and drying testing. This research project extends previous research by measuring the 

impact of drainage and storage on the MC of the wood-based sheathing. 

3.1.1 Research Goal/Questions To Be Answered

There is a perceived concern in the industry that designing the enclosure wall system with the 

sheathing membrane and drainage plane between the exterior insulation and the sheathing might 

result in moisture-related issues for the wood-based sheathing. Moisture-related issues are caused 

by wetting that exceeds the drying capacity of the system and overcomes the safe storage ability 

(Figure 2). Measuring the amount of water that drains, and how much is stored, after one wetting 

or several wettings will help determine which enclosure assemblies decrease the exposure to 

moisture and reduce the risk even further. 
7 

Figure 2. Moisture balance of wetting and drying

This testing was done with no temperature gradient across the wall. It is well documented that 

solar heating of the cladding will drive moisture into the enclosure (solar inward vapor drives)

(Straube 2009; Smegal 2006; Straube and Burnett 2005). Future testing could incorporate 

simulated inward solar drives that would determine the effects on drying and hygric 

redistribution in the drainage cavity and enclosure wall. 

3.1.2 Testing System

Figure 3 shows the schematic of the wall balance used for testing. The test wall was nearly 

balanced with counterweights so the mass on the 25-lb load cell was approximately 5 to 10 lb. 

By using a smaller load cell, the precision of the load cell can be increased with limited cost. 

Figure 4 depicts the actual balance used in the laboratory. The test balance is a steel structure that 

is self contained so that it can be moved around the laboratory. It is not anchored to the floor. 

Test walls were outfitted with nine wood MC sensors to continuously monitor the wood MC of 

the sheathing. Figure 5 shows the layout of all nine sensors, and Figure 6 illustrates an individual 

wood MC sensor and thermistor. The sensors were continuously monitored using a Campbell 

Scientific CR1000 data logger and multiplexer. The MC sensors measure a resistance across the 

wood that correlates to an MC. As the MC increases, the electrical resistances of the wood 

decrease (Straube et al. 2002).

wetting

drying

safe storage 

capacity
8 

Figure 3. Wall balance schematic
9 

Figure 4. Drainage and drying balance

Figure 5. Location of wood MC sensors 
10 

Figure 6. Installed wood MC sensor and thermistor 

Wood MCs of plywood are a close approximation, and should not be judged with a pass/fail 

criterion. When MC measurements are analyzed in the laboratory or in the field, the reading 

should be kept in context and good building science judgment is required to determine the 

moisture risk to the plywood. For example, elevated wood MCs in the cold winter months are 

much safer from a mold growth perspective than similar MCs in the summer, when mold will 

grow more quickly. Also, a high MC for a short period followed by drying is not necessarily 

risky, as wood-based materials are able to manage high MCs for short periods without exceeding 

the safe storage capacity of the assembly.

In general, decay will not occur unless the MC is greater than fiber saturation for a prolonged 

period of time (Steffen 2000). Fiber saturation is commonly reached at an MC of approximately 

25% to 30% (Baker 1969). Sustained MCs above 28% for more than 4 weeks would generally be 

cause for concern. 

All of the test walls (Figure 7) were 4 ft in width and 8 ft in height, consisting of a 2 × 4 wood 

framed cavity at 16-in. o.c., plywood sheathing with a 1⁄8-in. horizontal seam, a 6-mil 

polyethylene air and vapor barrier on the interior (Figure 8), a sheathing membrane, and 3⁄4-in. 

ffpic insulation on the exterior (Figure 9). Installing the ffpic on the exterior and polyethylene on 

the interior means that drying of the entire system can occur only through ventilation and 

diffusion of the drainage cavity to the top and bottom of the drainage space. Moisture can still 

redistribute into the sheathing and stud space, but that will not result in any change of mass of 

the system. 
11 

Figure 7. Typical test wall schematic

Figure 8. Interior surface of typical test wall Figure 9. Exterior surface of typical test wall

This testing is in support of retrofit applications, and in many cases, retrofits may not have a 

polyethylene vapor barrier on the interior to limit the flow of water vapor. This testing, then, 

C

Foil-faced polyisocyanurate 

insulation

Sheathing membrane

Plywood sheathing

2x4 framing

Fiberglass batt

Polyethylene air/vapor 

barrier

Moisture content 

sensor
12 

might be conservative. Historically, however, oil-based paints that acted as a vapor control layer

were used. On most walls in retrofit situations, there are likely several layers of paint depending 

on the age of the building and occupant behavior. Having a vapor control layer on the interior 

decreases the ability of inward drying in laboratory testing and will result in the highest 

sheathing MC possible under the test conditions. This means that it is the worst case scenario for 

sheathing wetting under the specific test conditions. 

3.1.3 Matrix for Testing

Tests have been conducted on a smooth plastic housewrap and a corrugated plastic housewrap. A 

wall with liquid applied membrane will also be tested. While designing the testing program, 

multiple tests were conducted on the smooth and corrugated housewraps, and on different wall 

balances in an attempt to show repeatability of the testing procedure. Repeatability of testing 

results was shown to be quite good previously when similar tests were conducted numerous 

times or in different laboratories (Smegal 2006). 

Table 3. Testing Matrix for Drainage and Drying Testing

Test Equipment Sheathing Membrane

1 BSC Wall Balance Smooth Plastic Housewrap

2 BSC Wall Balance Corrugated Plastic Housewrap

3 BSC Wall Balance Smooth Plastic Housewrap

4 Alternative Wall Balance Corrugated Plastic Housewrap

3.1.4 Testing Protocol

The testing program was designed based on the testing and research conducted at the University

of Waterloo on dozens of different wall configurations. A drainage trough at the top of the wall 

was constructed using plastic housewrap. Five narrow 2-in.-long spacers (Figure 10) were 

inserted into the top of the drainage space to ensure that water entered the drainage space (Figure 

11). 

Figure 10. Two-inch-long plastic spacers Figure 11. Spacers installed in top of 

drainage gap
13 

In the case of the smooth plastic housewrap, the top edge of the ffpic was pressed tight against 

the housewrap, and water could not easily enter the drainage cavity. Once water penetrated the 

top edge, water drained to the bottom quickly and uniformly. 

Before the test began, a wall calibration procedure was undertaken. Calibration weights were 

added and removed to ensure that results could be verified during the data analysis that followed 

the testing. Calibration weights were also used at the end of the test before the wall was detached 

from the balance. Calibration is required to zero the balance and determine the linear relation to 

the system as built. It is completed at the start of every test to verify the system parameters once 

the wall has been hung. The readings on the load cell were taken continuously but the output was 

a 15-min average during drying. 

Based on previous research and testing, the BSC team decided to follow the same testing 

protocol as Smegal (2006). The first application of water was 1.5 L (50 fl oz) into the drainage 

space. This volume was chosen because it is a manageable amount that ensures that the drainage 

space is wetted as much as possible. Approximately 15 min after the first application was 

complete, the drainage space received a second injection of 1.5 L (50.7 fl oz). Fifteen minutes 

was used based on previous tests of dozens of wall systems. The drainage appeared to stop for all 

walls at the 15-min mark, but minimal drying of stored moisture had occurred. The second 

application was used to determine if water is being absorbed into the wall system. With 

nonhygroscopic materials on both surfaces of the drainage cavity, the team expected that the first 

and second applications of water would result in approximately the same amount of storage. 

Fifteen minutes after the second application of water, the free drainage had generally stopped. 

This is referred to as the storage amount.

3.1.5 Results/Analysis 

Two tests were conducted on smooth plastic housewrap, stapled to the sheathing, with ffpic 

applied directly to the surface. It should be noted that the housewrap manufacturer recommends 

using cap staples to attach the housewrap. Otherwise, any warranty is voided. Field observations 

have demonstrated, though, that the housewrap is often installed with standard staples without 

caps. 

The installation of housewrap without cap staples will give a worst case scenario for drainage 

and drying. The gap provided by the caps, especially against a relatively hard surface such as the 

surface of board foam insulation has been shown to be large enough to improve drainage and 

ventilation drying (Smegal 2006). A future drainage test may be conducted using smooth plastic 

housewrap installed with cap staples to quantify differences in both drainage and drying. 

Figure 12 shows the initial drainage test. It starts with two calibration weights that were added, 

100 g and 200 g (3.5 oz and 7 oz). At the 100-g (3.5-oz) weight, the measurement is within 2 g 

(0.07 oz), or 0.67 g/m2

 (0.002 oz/ft2). Fifteen minutes after the first pour, the storage was 

approximately 80 g (2.8 oz), and 15 min after the second pour (indicated by the vertical dashed 

line), the amount stored was 85 g (3 oz) or 28.8 g/m2

 (0.09 oz/ft2). Because the gap was so small, 

it does appear that there was still some drainage from the cavity at the 15-min mark. The two 

storage amounts were nearly identical. This was expected because neither drainage plane surface 

was absorptive.
14 

As a comparison to add perspective, when water was spray applied to a single nonabsorptive 

polyethylene sheet or a sheet of Plexiglas, the surface storage amounts were consistently 35 g/m2

(0.11 oz/ft2) and 65 g/m2 (0.21 oz/ft2), respectively. Drainage tests conducted on a 1-mm gap 

with both sides constructed of Plexiglas stored 24 g/m2 (0.08 oz/ft2) (see Table 4). More was 

stored on a single sheet than in the narrow drainage cavity because the cavity was too narrow to 

allow the moisture to form beads of water (Smegal 2006). 

Table 4. Values of Maximum Storage Amounts on 

Nonabsorptive Surfaces for Context to Testing Storage Amounts 

Maximum Storage g/m2

(oz/ft2)

Water Sprayed Against Smooth Polyethylene Sheet 35 (0.11)

Water Sprayed Against Plexiglas Sheet 65 (0.21)

Water Poured Between Two Layers of Plexiglas 

Approximately 1 mm Apart 24 (0.08) 

Figure 12. Smooth plastic housewrap drainage test

After the initial drainage and drying test, the drainage cavity was subjected to repeated wettings 

over a few days, to help determine if repeated wettings in the drainage gap to the interior of 

exterior insulation would result in any moisture-related durability issues. It was found that there 

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14:15 14:45 15:15 15:45

Change in Mass (g)

 
15 

was a correlation between the sheathing MC and repeated wettings, as shown in Figure 13, but 

the scale of the MC is between 9% and 12% as an average for all nine MC locations. This scale 

is used to show the correlation, but it should be noted that moisture durability is generally not an 

issue until the MC is greater than 20%. The average ±1 standard deviation of all the MC readings 

is also shown on the series of MC sensors. 

The total increase in the weight of the wall following the six wettings was approximately 200 g 

(7 oz). This is equivalent to approximately 67 g/m2 (0.22 oz/ft2). The surfaces of the drainage 

cavity are nonhygroscopic, which means that the increase in the weight of the wall was a result 

of redistributed moisture into the sheathing and stud cavity. 

Calculations show that an increase of 200 g (7 oz) of water, assuming that the added water is 

entirely in the plywood sheathing (which is likely not the case because some is stored in the 

drainage cavity), would correspond to an MC increase in the plywood of approximately 1.3%. 

This also assumes that the moisture is evenly distributed in the sheathing and is not stored in the 

stud space cavity as vapor. No RH sensors were installed in the cavity for this test, although 

there is evidence of moisture vapor increases during cavity wetting in the hygric redistribution 

test discussed later.

Figure 13. Repeated drainage gap wetting analysis, smooth housewrap

A second drainage test was conducted on smooth plastic housewrap to test the repeatability of 

the testing program. The results of the storage following the first and second cavity wettings are 

shown in Table 5, along with the results from the previous test. Using a sample size of two, the 

difference in results was nearly half. It is expected that with a greater number of tests, the 

9

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11-Aug 12-Aug 13-Aug 14-Aug 15-Aug 16-Aug 17-Aug

Measured moisture content (%)

Change in Mass (g)

p g 

Mass of test wall MC Sheathing Average Sheathing MC +1 SD Sheathing MC -1 SD
16 

repeatability would show a small standard deviation in the storage amounts. The repeatability is 

also expected to improve when there is some absorptivity and a more significant amount of water 

is stored. The amount of water stored during these drainage tests is quite small, even smaller than 

the amount of water that can be stored when sprayed against a polyethylene sheet.

Table 5. Comparison of Drainage Tests on Smooth Plastic Housewrap

Initial Storage 

g/m2 (oz/ft2)

Final Storage 

g/m2 (oz/ft2)

Test 1 – Initial Drainage Test on 

Smooth Plastic Housewrap 26 (0.09) 28 (0.09) 

Test 3 – Second Test on Smooth 

Plastic Housewrap for Repeatability 15 (0.05) 19 (0.06) 

Two tests were also conducted on a wall with a corrugated plastic housewrap. Intuitively, this 

material will store more water because there is more surface area for drainage and available 

surface for water storage. The gap is also wider, and previous testing (Table 4) demonstrated that 

a very small gap will store less water when the water is not able to form beads of water on the 

surface. One test was conducted at the BSC laboratory in Waterloo, and the second test was 

conducted at the laboratory of an industry partner on a different balance apparatus. 

Table 6 shows the results from the two tests. The final storage results were the same for each test 

and very similar to the amount of water stored when water is sprayed against a polyethylene 

sheet to add context to the storage amounts. This is a very small storage amount, and shows there 

is no absorptivity of the materials in the drainage cavity. It is unclear why the initial storage was 

higher than the final storage for the first test. The corrugated plastic housewrap did store more 

water than the flat plastic housewrap, although the results are still quite similar with respect to 

the moisture loading on the wall and the effects of moisture on durability. 

As stated previously, by confirming with calibration weights, the total weight of the wall is 

accurate to approximately 2 g, which is equivalent to 0.67 g/m2

. In the case of the corrugated 

plastic housewrap tests, this represents approximately 2% of the final storage amount. 

Table 6. Comparison of Drainage Tests on a Corrugated Plastic Housewrap

Initial Storage 

g/m2 (oz/ft2)

Final Storage 

g/m2 (oz/ft2)

Test 2 – Initial Drainage Test on Corrugated 

Plastic Housewrap 42 (0.14) 36 (0.12) 

Test 4 – Second Test on Corrugated Plastic 

Housewrap on a Different Balance Apparatus 35 (0.11) 36 (0.12) 

The BSC team observed that the drainage did occur more quickly using the corrugated 

housewrap, but both wall types were able to drain quite quickly and effectively. In a smaller gap, 

a large amount of water will be pushed laterally to the edges, but in a larger gap, the water will 

drain straight down in the path that it was poured into the wall. This was made evident by a very 
17 

small amount of leakage out the sides of the flat plastic housewrap wall, through the tape, and 

onto the exterior face of the ffpic. 

A repeated wetting test was also conducted on the corrugated plastic housewrap wall, and the 

results are shown in Figure 14 (with the same scale as the previous test depicted in Figure 13). 

There were two extra wetting events compared to the flat plastic housewrap (Figure 13), but the 

entire wall system with corrugated plastic housewrap gained more than twice the mass as the 

smooth plastic housewrap. The drainage cavity materials are nonhygroscopic, which means that 

more than twice the water vapor diffused into the sheathing and stud cavity from the drainage 

space. This happens because more water is stored after each wetting event, and the vapor can 

very easily move into the plywood sheathing through the vapor permeable drainage plane. The 

increase in MC from repeated wettings is still less than 20%, the minimum threshold for 

moisture-related durability concerns. 

Figure 14. Repeated drainage gap wetting analysis, corrugated housewrap

3.1.6 Summary

The larger gap provided by the corrugated plastic housewrap allowed more rapid drainage than 

the flat plastic housewrap; however, it did store nearly twice as much water following drainage. 

To keep that in perspective, the corrugated plastic housewrap stored approximately the same 

amount of water as a sheet of polyethylene with water sprayed against it, and only approximately 

half as much as a sheet of Plexiglas with water sprayed against it.

9

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30-Aug 31-Aug 1-Sep 2-Sep 3-Sep 4-Sep 5-Sep 6-Sep 7-Sep 8-Sep 9-Sep

Measured moisture content (%)

Change in Mass (g)

 

Mass of test wall Average Sheathing MC (9 sensors) MC -1 SD MC +1 SD
18 

Following repeated wettings of the entire drainage cavity, there were no moisture-related 

durability issues of the sheathing under laboratory conditions. 

The repeated wetting events on the corrugated plastic housewrap resulted in a higher adsorption 

to the plywood sheathing because more water is stored following each wetting event, likely 

because of the increased surface area for water deposition. 

It is likely that using cap staples as recommended by the product manufacturer to install the 

smooth housewrap will result in quicker, more vertical drainage similar to the corrugated 

housewrap. Because there is not significantly more surface area resulting from corrugations of 

the housewrap, though, storage would likely have been similar to that of the smooth plastic 

housewrap. 

This test can be used to compare the performance of different wall systems in the laboratory, but 

it is difficult to correlate the results to leaks that occur in real buildings. The volume and the 

frequency of water that might leak in the field are based on many variables and entirely 

unpredictable. 

3.2 Hygric Redistribution

This testing program was designed to more accurately determine what happens to water that 

becomes trapped in the drainage cavity behind vapor impermeable exterior insulation, or 

sustained long-term wetting in the drainage cavity. The potential improvements resulting from a 

corrugated housewrap instead of a smooth housewrap will be investigated. The hypothesis was 

that a corrugated housewrap could allow more hygric redistribution (diffusion) of water vapor in 

all directions in the drainage cavity. 

3.2.1 Research Goal/Question To Be Answered

Here, the main question is what happens to water and the hygroscopic wall materials during 

sustained long-term wetting in the drainage cavity behind vapor impermeable exterior insulation. 

Does the water evaporate and move in all directions through the cavity? Is there a risk to the 

moisture durability of the wall system? Do the results differ if a different corrugated housewrap 

is used?

3.2.2 Testing System

The same balance used for drainage and drying testing, described earlier in the report, was used 

to hold the 8-ft-tall by 6-ft, 8-in.-wide test wall, so that it could be monitored gravimetrically. 

Both MC and RH sensors were used to measure the amount of moisture in the stud spaces, 

sheathing, and drainage cavity. The wall section is the same as the previously described test 

(refer to Figure 7) using a corrugated plastic housewrap as the sheathing membrane.

To accurately measure the RH in the drainage space without affecting the drainage gap, the RH 

sensors were installed from the exterior of the ffpic in a pocket that was formed on the interior so 

that it did not affect the drainage cavity (Figure 15). Figure 16 shows the distribution of the RH 

sensors, and Figure 17 depicts the layout of both the RH sensors and the sheathing MC sensors. 

A wetting apparatus was installed on the interior surface of the exterior insulation to allow a 

known amount of water to be injected at a controlled time and location. The wetting apparatus 
19 

consisted of a storage medium 11 in. wide by 20.5 in. tall installed directly against the surface of 

the ffpic as shown in Figure 16 and indicated by the blue square in Figure 17. A perforated tube 

connects the storage medium to the exterior of the wall for water injections. This enables wetting 

to a known area in the drainage cavity without disturbing the wall system. 

The test wall was built of 2 × 4 framing on standard 16-in. centers and insulated with fiberglass 

batt insulation. To mimic standard cold-climate construction and to limit immediate drying of the 

injected moisture, a Class I vapor retarder was installed on the interior side of the framing. A 1⁄2-

in. plywood sheathing was installed because this would most likely be used in a retrofit situation 

instead of oriented strand board (OSB). 

Figure 15. RH sensor in the drainage space

Figure 16. Wetting system and drainage space RH sensors 
20 

Intentional wetting was conducted on the schedule shown in Table 7. Initially 1 oz was injected 

twice daily, but this was increased to 1.5 oz twice daily. The volume was increased for two 

reasons: (1) it was unclear if maximum storage was being reached because no water was draining

out during injection; and (2)1.5 oz, injected twice daily, is a standard intentional wetting volume 

during the full-scale field testing discussed in Section 3.3. The total amount of water injected 

over 7 days was 13 oz (390 mL). A small amount of water did drain within a few minutes of 

injecting1.5 oz of water, but water was injected slowly to maximize the water stored in the 

wetting system prior to any drainage. 

Table 7. Hygric Redistribution Wetting Schedule

Date (2011) Amount Time

Wednesday, September 14 1 oz (30 mL) Morning and afternoon

Thursday, September 15 1 oz (30 mL) Morning and afternoon

Friday, September 16 1.5 oz (45 mL) Morning and afternoon

Sunday, September 18 1.5 oz (45 mL) Morning

Monday, September 19 1.5 oz (45 mL) Morning and afternoon

Tuesday, September 20 1.5 oz (45 mL) Morning and afternoon

Wednesday, September 21 1.5 oz (45 mL) Morning

Figure 17. Layout for sheathing MC sensors and stud space RH sensors 

as shown from the “interior” 

3.2.3 Matrix for Testing

The initial test was conducted with a corrugated housewrap, and subsequent testing parameters 

were determined based on the analysis results. Initially the BSC team thought that there would be 

MC 1 MC 2 MC 3 MC 4 MC 5

MC 6 MC 7 MC 8 MC 9 MC 10

MC 11 MC 12 MC 13 MC 14 MC 15

RH 1 RH 2 RH 3 RH 4 RH 5
21 

significant measurable redistribution in the drainage cavity of water vapor, so a second test 

would be conducted with a smooth plastic housewrap of similar vapor permeance to demonstrate 

the advantages of a corrugated housewrap. The test results, however, show very little measured 

lateral redistribution of water vapor in the drainage cavity. 

The team proposed using a drainage mesh (Figure 18) to increase the depth of the drainage 

cavity and to minimize the barriers to vapor diffusion or potentially a “bumpy” housewrap 

(Figure 19). This also minimizes barriers to lateral diffusion but keeps the drainage space 

smaller. 

The time frame of this research project, however, did not allow further tests to be conducted on 

these proposed materials. 

Figure 18. Example of drainage mesh between two sheet materials

Figure 19. Example of a bumpy housewrap
22 

3.2.4 Results/Analysis

Testing began on September 13, 2011 with 1 day of data before wetting to establish the 

background levels of moisture in the test wall. Wetting began on September 14. There were no 

variations in the RH sensors in the drainage cavity (Figure 20), which was contrary to 

expectations, although the drainage space RH sensors did appear to trend with the laboratory RH. 

Because this was the first test of this kind, it is unclear if there was a sensor-related issue 

resulting from the method of RH installation, or if there was no hygric redistribution in the 

drainage cavity. In future tests simulated solar energy could be added to the test wall to see if the 

increased energy load will help distribute moisture. 

Figure 20. Measured RH at nine locations in the drainage space

Because there was no apparent lateral diffusion, moisture physics principles were more closely 

examined in an attempt to understand the behavior. 

First, it became apparent that the amount of surface area for diffusion through the face of the 

housewrap was significantly higher (225 in2) compared to the effective area of diffusion from the 

edges of the wetting paper (12.6 in2) (Figure 21). 

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Measured Relative Humidity (%)

Drainspace RH1 Drainspace RH2 Drainspace RH3 Drainspace RH4 Drainspace RH5 Drainspace RH6

Drainspace RH7 Drainspace RH8 Drainspace RH9 Laboratory RH First Wetting last Wetting
23 

Figure 21. Comparison of surface areas for diffusion from wetting system

When considering vapor permeability properties, the following two values are of significant 

importance: 

• Permeability of air = 185 ng/Pa·s·m

• Permeance of the housewrap = approximately 1,800 ng/Pa·s·m

2

Therefore, the thickness of air required to have the same vapor resistance as the housewrap is

• Permeability of air/permeance of material = 0.103 m or 4.0 in. 

This means that for every water vapor molecule that moves through the housewrap, to dry to the 

air gap, a water vapor molecule has to move 4 in. through the air in the gap around the wetting 

paper. This is further complicated by the fact that the gap is not full of air, but rather repetitive 

layers of corrugated housewrap, decreasing the permeance even further. 

The two factors of increased surface area for diffusion, along with the comparatively low vapor 

permeance of the housewrap, mean that water vapor will move through the housewrap into the 

plywood very easily relative to diffusing through the drainage gap. Plywood is a very adsorptive 

material for water vapor, and becomes significantly more vapor permeable as the RH increases 

(ASTM E96 wet cup test).1

Several conclusions can be drawn from this analysis: 

 

1 ASTM Standard E96 is available for purchase at http://www.astm.org/Standards/E96.htm. 
24 

• Measuring significant RH increases in the drainage gap is unlikely with this testing setup.

• Changing the gap material, the thickness of the gap, and the amount of water might allow 

measurement of more diffusion in the drainage gap. 

• Using a smaller scale test setup with RH sensors installed much closer to the wetting 

system might be required to show what hygric redistribution does occur. 

Analysis of the RH sensors in the each of the five stud spaces (Figure 22) shows that the stud 

space directly behind the wetting system (RH2) does experience increased RH starting 2 days 

after the first wetting. The maximum RH measured at RH2 is approximately 72%. On September 

22, there is a slight increase in the RH in both adjacent stud spaces (RH1 and RH3), but only of 

4% to 5%. This percentage range is comparatively insignificant.

Figure 22. Measured RH in the five stud spaces

There were no MC sensors in the sheathing directly adjacent to the wetting system. MC 7 was 

approximately 1 in. above the top edge of the wetting system, and MC 12 was approximately 7 

in. below the bottom edge of the wetting system. 

The MC at the measurement location MC7 started increasing 1 day after the first wetting and 

increased to a maximum of approximately 13% (Figure 23). At MC12 beneath the wetting 

system, there was also an increase, but it was much more modest, with a maximum of 

0

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Measured Relative Humidity (%)

 

Studspace RH1 Studspace RH2 Studspace RH3 Studspace RH4

Studspace RH5 Laboratory RH First Wetting Final Wetting
25 

approximately 11%. It started to increase almost 3 days after the first wetting (Figure 24). These 

MCs are very low and do not indicate any risk of moisture-related durability issues. 

Figure 23. Measured MC sensors in all five stud spaces at the mid height of the wall

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Wood Moisture Content (%)

 

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26 

Figure 24. Measured MC sensors in all five stud spaces 16 in. from the bottom of the wall

3.2.5 Summary

After adding a total of 13 oz (390 mL) of water to the wetting system, the maximum measured 

MCs just above and below the wetting system in the plywood sheathing were 13% and 11%, 

respectively. There was an increase in the stud space RH (RH2) corresponding to the placement 

of the wetting system, to a maximum of 72% and a slight increase of the stud space RH sensors 

in the adjacent stud spaces. This correlates with increased vapor permeability of the plywood 

sheathing with increased RH. 

There did not appear to be an increase in the drainage space RH sensors corresponding to the 

wetting events, which was initially unexpected. Further analysis showed that, in fact, it is much 

easier under these test conditions for water vapor to move into the plywood than move 

significant distances in the drainage cavity.

Several ideas might improve and further this testing for successive tests: 

• Smaller test walls with a higher density of RH and MC sensors can be used to further 

investigate hygric redistribution. 

• Conducting a test with a polyethylene sheet behind the housewrap will prevent moisture 

transport into the wall and control one of the boundary conditions for this testing. 

0

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Wood Moisture Content (%)

 

MC 11 MC 12 MC 13 MC 14 MC 15 First Wetting Last Wetting
27 

• Potentially, more water, longer wetting, a larger gap, and less material in the gap might 

increase the potential for diffusion of water laterally through the drainage gap. 

• Simulated solar energy could be used to add energy (drive) to the system, although this 

might only complicate matters because it will be difficult to achieve a constant 

temperature and energy input across the wall. 

3.3 Field Testing

Full-scale comparative field testing of exterior insulated walls in DOE climate zone 4C for 

comparison with other more traditional wall construction strategies is complete. This testing 

focused on determining the hygric redistribution of water applied between the sheathing and low 

permeance exterior insulation to assess the moisture-related durability risks to the enclosure. The 

performance of an exterior insulated wall (Wall A) shown in Figure 25 was compared to a more 

standard construction practice, direct applied stucco (Wall B), shown in Figure 26. 

The two comparison walls were constructed on all four orientations of the test hut and were 

exposed to natural environmental conditions. 

Figure 25. Exterior insulated wall on all four 

orientations (Wall A)

Figure 26. Direct applied stucco wall on all 

four orientations (Wall B)

3.3.1 Enclosure Wetting Systems

A wetting apparatus was installed on both the interior and exterior of each test wall to allow a 

known amount of water to be injected at a controlled time and location. The wetting apparatus 

consisted of storage media 11 in. wide by 20.5 in. tall installed directly against the interior and 

exterior surface of the sheathing (Figure 27). A perforated tube connected each of the storage 

media to the interior of the test hut to allow access for water injections. This enables wetting to 

either the interior or exterior independently without opening and disturbing the wall system. The 

wetting system was designed to simulate a window leak, and was used to help determine the 

drying potential of a wall system. On the analysis graphs the exterior wetting events are indicated 
28 

by red vertical dashed lines and the interior wetting events are indicated by blue vertical dashed 

lines. When an intentional wetting event occurred repeatedly over multiple days, only one line is 

shown, indicating the first wetting. There were five intentional wetting events as shown in Table 

8. For each wetting event, 1.5 oz (45 mL) was injected twice a day for 5 days. This is a total of 

15 oz (~450 mL) into the wetting apparatus directly against the sheathing. For the entire test 

period 60 oz (~1.8 L) was injected against the exterior of the sheathing, and 15 oz (~450 mL) 

was injected against the interior surface of the sheathing. 

Figure 27. Exterior wetting apparatus

Table 8. Intentional Wetting Event Schedule and Location 

Location Start Date Amount

Wetting Event 1 Exterior July 5, 2010 15 oz (450 mL)

Wetting Event 2 Exterior August 30, 2010 15 oz (450 mL)

Wetting Event 3 Interior January 19, 2011 15 oz (450 mL)

Wetting Event 4 Exterior May 9, 2011 15 oz (450 mL)

Wetting Event 5 Exterior August 9, 2011 15 oz (450 mL)

3.3.2 Instrumentation

Each of the test walls was outfitted with a series of temperature, RH, and wood MC sensors. 

These sensors were continuously monitored and recorded throughout the testing period using a 

data acquisition system. Photographs of the individual sensors are shown in Figure 28 and 29. 

Variations on a “typical” sensor package (illustrated in Figure 30) were used for each test panel. 

MC pins were installed in the framing lumber and the sheathing (from the interior) in all wall 

systems (Figure 28). Wood MCs can be determined from the electrical resistance of wood based 

on the Garrahan equation (Onysko et al. 2010; Garrahan 1988). These pins can be used to 

measure MC at any depth chosen because the pins are electrically insulated except for the tips. 
29 

Measurements are most commonly taken at 1⁄4-in. (6-mm) tip depth. In this study, MC

measurements were taken at two depths on the lower OSB near the wetting system so the 

moisture gradient could be determined. The wood MC pins were installed in combination with a 

temperature sensor in all locations. To correct the MC readings for temperature effects, a hole 

was drilled to the same depth as the MC pins and a temperature sensor was installed inside. 

Figure 28. MC pins installed in plywood sheathing through the wetting system

RH sensors were installed in the middle of each stud cavity, and in the drainage space of each 

wall. The RH sensor was always installed in combination with a temperature sensor, both of 

which are protected by a vapor permeable, water resistant cover (see Figure 29). RH and 

temperature sensors were installed at the midpoint of the stud space, between the drywall and the 

sheathing, as well as in some drainage cavities. 
30 

Figure 29. RH and temperature sensor installed in a stud bay (with MC pins installed below)

Figure 30. Typical wall construction and sensor configuration

One of the most significant advantages to using a test hut with removable/replaceable walls for 

analysis compared to instrumenting walls in existing buildings is that the experimenter can 

deliberately and easily stress the walls with high moisture loads, either in terms of vapor (e.g., 

≥50% interior RH) or liquid water (e.g., intentional wetting systems). In most cases, building 

owners are not interested in participating in research conducted on their enclosure walls by 

adding moisture. In addition, it is often difficult to determine the performance of a wall system 

 Relative Humidity and Temperature Sensor Package 

 Moisture Content Pins and Temperature Sensor Package

 Temperature Sensor

 Moisture Content Wafer and Temperature Sensor Package

 Moisture Content Pins B

C

DEGH A

80"

64"

48"

32"

16"

0"

Assembly

Layer Function Description

A Interior Finish gypsum wall board

B Vapor Control poly

C Studspace fiberglass batt

D Framing 2x6

E Sheathing 1⁄2” OSB

F Sheathing Membrane #15 felt

G Drainage Gap none

H Cladding stucco

Elevation Section

Sensor Package

Layer Code Location

A taem Interior finish, exterior face, 48"

C r/tcnm Studspace, interstitial, 48"

D m/tdeb Framing, exterior edge, bottom plate

D m/tdib Framing, interior edge, bottom plate

E m/teem Sheathing, exterior, 16"

E m/teim Sheathing, interior, 16"

E m/tenm Sheathing, interstitial, 48"

E m/tenm Sheathing, interstitial, 80"

G r/tgnw Drainage space, interstitial, 32"

G r/tgne Drainage space, interstitial, 64"

H them Cladding, exterior surface, 48"
31 

without stressing the moisture tolerances of a wall to determine the comparative risk of certain 

construction techniques. 

3.3.3 Observations and Data Analysis

This analysis contained is limited to observations and data collected during the monitoring 

period (December 17, 2009, to November 3, 2011) for the exterior insulated Wall A (refer to 

Figure 25) and a traditional stucco Wall B (directly applied to building paper over wood 

sheathing; see Figure 26). Four analysis criteria were used to compare the performance of the test 

walls: 

• Sheathing wood MC measurements under normal operating conditions 

• Sheathing wood MC measurements during wetting and drying from an interior wetting 

event 

• Sheathing wood MC measurements during wetting and drying from an exterior wetting 

event 

• Dew point analysis and potential air leakage condensation at the condensation plane 

(interior surface of the exterior sheathing)

• Qualitative visual observations during deconstruction. 

For this report, the focus was on the measured MC of the sheathing at the four measurement 

locations. The four sheathing moisture measurements are as follows: 

• 16 in. from the bottom of the bottom plate at the interior edge of the sheathing 

• 16 in. from the bottom of the bottom plate at the exterior edge of the sheathing 

• 48 in. from the bottom of the bottom plate at the center of the sheathing 

• 80 in. from the bottom of the bottom plate at the center of the sheathing 

The two MC sensors at 16 in. were used to evaluate the performance after wetting events 

because they were installed in the sheathing in direct contact with the wetting system (Figure 

28). The MC sensors at 48 in. and 80 in. from the bottom plate were not affected by either the 

interior or exterior wetting events, and are a good indication of how the sheathing MC is affected 

by the construction assembly under normal operating conditions. 

Sheathing moisture is used as the performance criterion because this is the first location where 

vapor diffusion condensation would occur in a cold climate during the heating season. In the case 

of these test walls, the most significant moisture risks were at the locations of the wetting 

systems, allowing comparison of the sheathing moisture performance under significant moisture 

stresses. MCs of the sheathing are used as a comparison instead of pass/fail criteria for the wall 

assembly. Generally, under normal conditions, the following criteria are used to assess the risk of 

various test wall assemblies: 
32 

• Peak sheathing with an MC < 20%, no mold growth, very little risk

• Peak sheathing with an MC of 20%–28%, potential for eventual mold growth, depending 

on frequency and length of wetting and temperatures during wetting. This design can be 

successful but conservative assessments usually require corrective action to be taken.

• Peak sheathing with an MC >28%, moisture-related problems are expected and this 

design is not recommended. 

Predicted wood MCs of wood-based sheathing are generally assessed with respect to relative risk 

instead of judged on pass/fail criteria. The predicted MC should be kept in context and good 

scientific judgment is required to determine the moisture risk to the sheathing. For example, 

elevated wood MCs in the cold winter months when the wood substrate is on the cold side of the 

assembly are much safer from a mold growth perspective than similar MCs in the summer, when 

the temperatures are in the correct range for optimal mold growth. Also, high MCs for a short 

period followed by drying are not necessarily risky, because wood-framed structures are able to 

manage high MCs for short periods without exceeding the safe storage capacity of the assembly. 

The safe storage capacity is the amount of moisture an assembly is able to manage without 

suffering any moisture-related issues. The baseline wood MC is a factor in the safe storage 

capacity because the lower the wood MC is during normal operation (without wetting events), 

the more moisture the wood can handle before reaching any durability risks. If the measured 

wood MC is consistently higher, even if there are no moisture durability risks, there is less 

moisture buffering capacity in the wood before reaching moisture-related durability risk levels.

3.3.4 Comparison of Walls A and B 

Comparing the overall MC measurements at the lower measurement location (both interior and 

exterior) indicates that the MC was higher on Wall B relative to Wall A for the same sensor 

(interior edge compared to interior edge, and exterior edge compared to exterior edge) for the 

entire monitoring period (all four orientations on Figure 31, Figure 33, Figure 35, and Figure 37). 

The only exception is on North Wall B, where the interior edge MC sensor did not appear to 

respond to the first two wetting events. The reason for this is unknown. 

Even though the research focused on exterior wetting events, one interior wetting event was 

conducted to compare the moisture-related durability performance of the two walls. Following 

the intentional interior wetting event (January 2011), Wall A dried more quickly in all cases than 

Wall B. This is because there is very little drying potential to the exterior in both Wall B and 

Wall A. Wall B was constructed with Class I polyethylene vapor control (<0.1 perms) on the 

interior, so drying to the interior is not possible. Wall A has a Class III interior vapor control 

layer ( <1 perms < 10, latex paint on the drywall) and is thus able to dry to the interior. Table 9

shows the amount of time, in days, following the interior wetting event until the measured MC

was less than 20%. The next wetting event was May 5, 2011, so if the measured sheathing MC

was still above 20% MC at that time, a value of >106 days was used.
33 

Table 9. Comparison of Drying Rates for Wall A and Wall B 

Following Interior Wetting Event in January 2011

Wall B

(Direct Applied Stucco)

Wall A

(XPS Exterior Insulation)

North >106 days 68 days

East >106 days 53 days

South 68 days 29 days

West 95 days 47 days

At the midheight and upper MC measurement locations ( all four orientations on Figure 32, 

Figure 34, Figure 36 and Figure 38), which are largely unaffected by the intentional wetting 

events, the MC measurements on Wall A on all orientations does not exceed 11%, and is always 

less than Wall B. Wall B measurements do not exceed 17%, which is also considered a safe level 

of moisture in the OSB sheathing. 

Figure 31. Lower OSB sheathing measured MC comparison between north Wall B 

(direct applied stucco) and north Wall A (exterior insulation)
34 

Figure 32. Middle and upper OSB sheathing measured MC comparison between north Wall B 

(direct applied stucco), and north Wall A (exterior insulation)

Figure 33. East Wall B (direct applied stucco) and east Wall A 

(insulating sheathing) lower OSB sheathing measured MC
35 

Figure 34. East Wall B (direct applied stucco) and east Wall A 

(insulating sheathing) middle and upper OSB sheathing measured MC

Figure 35. South Wall B (direct applied stucco) and south Wall A 

(exterior insulation) lower OSB sheathing measured MC
36 

Figure 36. South Wall B (direct applied stucco) and south Wall A 

(exterior insulation) middle and upper OSB sheathing measured MC

Figure 37. West Wall B and south Wall A lower OSB sheathing measured MC 

comparing exterior insulation
37 

Figure 38. West Wall B and south Wall A middle and upper OSB sheathing 

measured MC comparing exterior insulation

3.3.4.1 Comparison of Sheathing Temperatures and Interior Dew Point

The measured and calculated performance improvements to the building enclosure using exterior 

insulation are not new ideas. In 1964, Hutcheon demonstrated how the temperature gradients 

across a masonry wall changed when the insulation was moved from the interior of the structure 

to the exterior of the structure (Hutcheon 1964). He showed that condensation issues at 35% 

interior RH were solved by moving the insulation to the exterior. These same principles 

have been more recently illustrated by Straube using more current construction practices

(Straube 2011). 

The analysis in this report discusses the potential for interior air leakage condensation based on 

the measured interior air dew point and the measured sheathing temperature. This analysis is a 

measure of moisture durability risk, not actual moisture durability. This is because it is assumed 

that the vapor control of the enclosure is adequate and condensation will occur only if there is a 

path for air leakage from the interior to the sheathing and the sheathing is below the dew point of 

the air.

Field experience with airtightness and blower door testing has demonstrated that the enclosure 

will always have some air leaks, and their location and size depend on the type and quality of 

construction. It is not uncommon to find air leakage pathways through electrical outlets, 

switches, and other interior finish penetrations. This means that with air permeable cavity filled 

insulation, which is the most commonly used, evidence of moisture condensation on the interior 

surface of the exterior OSB behind penetrations can be found. Figure 39 shows an example of an 

air leakage condensation problem behind an electrical outlet in a 6-year-old house with an 

interior poly vapor barrier. The OSB and framing are stained and dark with surface mold and the 

nails are corroding. 
38 

Figure 39. Example of air leakage condensation durability issues

The condensation potential is directly related to the interior conditions and the sheathing 

temperature. Generally speaking, dry air has less moisture available to condense so there will 

be less concern for lower humidity interior conditions. For this analysis, two time periods were 

used over the colder winter months, and these two periods are shown on both the temperature 

(Figure 40) and RH (Figure 41) graphs. The interior temperature was approximately 20°C for the 

entirety of both comparison periods, with some small variations. The interior RH was set to 40% 

for the winter months and maintained with a humidifier controlled by the data acquisition 

system. 

The temperature and RH are used to calculate the hourly dew point of the interior air for 

comparison to the sheathing temperature. The temperature of the sheathing is taken at the 

midheight midthickness of the sheathing and not at the interior surface, but the difference in 

temperature is negligible over half the thickness of the OSB. 

The comparison results of this analysis are summarized numerically in Table 10 for both 

comparison time periods, and graphically in Figure 42 and Figure 43 for the individual analysis 

time periods. The results show a significant decrease in the number of measured potential hours 

of air leakage condensation when 38 mm (11⁄2 in.) of XPS is installed as exterior insulation.
39 

Figure 40. Measured interior and ambient temperatures

Figure 41. Measured interior and ambient RH

-10

-5

0

5

10

15

20

25

30

35

13-Dec-09 21-Feb-10 2-May-10 11-Jul-10 19-Sep-10 28-Nov-10 6-Feb-11 17-Apr-11 26-Jun-11 4-Sep-11 13-Nov-11

Temperature (C) 

Interior Temp - Location 1 Interior Temp - Location 2 Interior Temp - Location 3 Exterior Temperature

December 18, 2009 

to May 30, 2010

Sept 19, 2010 to March 23, 2011

0

10

20

30

40

50

60

70

80

90

100

13-Dec-09 21-Feb-10 2-May-10 11-Jul-10 19-Sep-10 28-Nov-10 6-Feb-11 17-Apr-11 26-Jun-11 4-Sep-11 13-Nov-11

Relative Humidity (%)

Exterior RH Interior RH - Location 1 Interior RH - Location 2 Interior RH - Location 3
40 

Table 10. Number of Hours of Potential Air Leakage Condensation 

Based on the Interior Dew Point and Sheathinga

Potential Hours of Condensation 

Between December 18, 2009, and 

May 30, 2010 

(3,936 Total hours)

Potential Hours of Condensation 

Between September 19, 2010, and 

March 23, 2011 

(4,441 Total hours)

North Wall B 1,252 2,417

North Wall A

(XPS) 91 551 

East Wall B 1,165 2,293

East Wall A

(XPS) 72 478 

South Wall B 1,050 1,980

South Wall A

(XPS) 51 320 

West Wall B 741 2,011

West Wall A

(XPS) 94 518 

a Temperature measure of moisture-related durability risk, not moisture-related durability

Figure 42. Comparison of the hours of potential condensation between Wall B and Wall A 

from December 18, 2009, to May 30, 2010, using measured interior dew point and 

sheathing temperature

0

500

1000

1500

2000

2500

North East South West

Hours of Potential Condensation 

1.5" Exterior XPS - Wall A OSB Sheathing - Wall B
41 

Figure 43. Comparison of the hours of potential condensation between Wall B and Wall A 

from September 19, 2010, to March 23, 2011, using measured interior dew point and 

sheathing temperature

The degree of potential risk in terms of vapor diffusion and air leakage moisture condensation is 

proportional to the length of time that the sheathing temperature is continuously below the dew 

point without any drying potential, and the magnitude of the difference between the dew point 

and the sheathing temperature. This means that a sheathing temperature 10° below the dew point 

will condense more water than a sheathing temperature 2° below the dew point, all other factors 

being equal. If the temperature of the sheathing is below freezing, condensation will occur as 

frost or ice, and then melt when the sheathing temperature increases.

Figure 44 and Figure 45 show the measured interior dew point and simultaneous measured 

sheathing temperature for walls A and B for both analysis time periods on the north orientation. 

The green line is the measured sheathing temperature of Wall A, and the blue line is the 

measured sheathing temperature of Wall B, depending on the comparison graph. 

The black line is the measured interior dew point. When the sheathing temperature falls below 

the dew point temperature, condensation will occur if the interior air contacts the interior surface 

of the sheathing. 

These two figures show that the sheathing temperature for Wall B was much lower than Wall A

for extended periods of time, which correlates with the numerical results of Table 10. This means 

that even with 1.5 in. of exterior XPS insulation, there is still a potential for a small amount of 

condensation, but significantly less than most building code minimum approved wall systems. If 

condensation does occur in Wall A, it will be able to dry much more quickly to the interior 

because latex paint is the only form of vapor control, compared to Wall B with a Class I 

polyethylene vapor barrier. 

 

0

500

1000

1500

2000

2500

North East South West

Hours of Potential Condensation 

1.5" Exteror XPS - Wall A OSB Sheathing - Wall B
42 

Figure 44. Comparison of condensation potential between north walls A and B, 

January to May 2010

Figure 45. Comparison of condensation potential between north walls A and B, 

September to March 2010

3.3.5 Test Wall Deconstruction

The test walls were deconstructed on November 2 and 3, 2011. The deconstruction was 

conducted carefully and systematically so that each component of each wall system could be 

photographed, examined, and compared. Disassembly was critical because inspecting all aspects 
43 

of all layers of the test walls was very informative. It was important to see if the visual 

observations correlated to and validated the measured results (Lstiburek and Parthenon 2011). 

With this qualitative comparison analysis it is important to keep in mind the boundary conditions 

that the walls were subjected to, both on the interior and exterior. Figure 40 and Figure 41 show 

the interior and exterior temperature and RH, and Table 8 shows the amount of water added to 

the surface of the sheathing.

The testing was conducted for approximately 2 years, so the conditions required extrapolation to 

determine what the OSB might look like after many years in service.

On all orientations, the exterior of the OSB on Wall A looked like new except for the immediate 

vicinity of the wetting apparatus where water was held between the XPS and sheathing directly 

against the OSB. The exterior of the sheathing on Wall B on all four orientations was 

considerably darker and stained over a much greater area. These observations show that 

measurements of lower MCs in Wall A correspond to a much cleaner and newer looking OSB 

sheathing on all four orientations. 

Photos of the north orientation are shown in Figure 46 and Figure 47 from two different 

directions. Photos of the east and south orientation are shown in Figure 48 and Figure 49, 

respectively. Unlike the other orientations, there was no photo taken of the entire west elevation 

immediately following deconstruction. 

Figure 46. Exterior OSB sheathing on the north orientation immediately following deconstruction
44 

Figure 47. Exterior OSB sheathing on the north orientation immediately following deconstruction 

(photo taken from the opposite direction of Figure 46) 

Figure 48. Exterior OSB sheathing on the east orientation immediately following deconstruction
45 

Figure 49. Exterior OSB sheathing on the south orientation immediately following deconstruction

3.3.6 Summary

The following list is a point form summary of the conclusions of full scale wall testing of an 

exterior insulated wall assembly compared side-by-side with a more traditional wall assembly 

without exterior insulation in a DOE 4C climate zone typical of the Pacific Northwest. These 

conclusions are based on measured data over the test period as well as qualititave observations 

during the deconstruction that occurred on November 2, and 3, 2011. 

• MC monitoring of the midheight and upper sheathing showed that the sheathing MC of 

Wall B was always higher than Wall A, but did not exceed criteria for moisture-related 

durability concerns. 

• Following the interior wetting event, Wall A dried more quickly to safe levels than Wall 

B, because the vapor control layer on Wall A was Class III and the vapor control layer on 

Wall B was Class I, which eliminated drying to the interior.

• Following the exterior wetting events, the drying rates were relatively similar between 

Walls A and B. Generally the MC was higher on Wall B, so even though the drying rates 

were similar, the MC of the sheathing on Wall B remained elevated compared to Wall A. 

• In dew point analysis, Wall B had significantly higher moisture durability risks with 

respect to interior air leakage condensation than Wall A. This is because the exterior 

insulation in Wall A keeps the sheathing at a higher temperature, reducing the number of 

hours that the sheathing is below the interior air dew point. This is not a durability 

problem unless interior air reaches the surface of the sheathing. 

• Observations from the wall deconstruction showed that the exterior surface of the OSB 

for Wall B was quite stained behind and around the wetting apparatus. There was also 
46 

staining on the upper portion of the OSB on Wall B, as well as a general overall darker 

appearance to the entire OSB sheet compared to Wall A. The OSB on Wall A looked like 

new except directly behind and under the wetting apparatus. 

Data analysis and wall deconstruction demonstrated that there were no moisture-related 

durability concerns of the wood structural sheathing in climate zone 4C when 1.5 in. of exterior 

XPS insulation was installed to the exterior of OSB structural sheathing and Tyvek DrainWrap. 

Water was intentionally placed in direct contact with the exterior surface of the sheathing behind 

the low vapor permeance XPS exterior insulation four times through the monitoring period, no 

moisture-related durability concerns were measured or observed. The overall performance of the 

exterior insulated wall was better under all comparison criteria than that of the direct applied 

stucco wall.

3.4 Conclusions

Four research questions were presented at the start of this report to help determine any moisturerelated durability risks of adding drained exterior insulation to the existing sheathing in an 

energy retrofit of an existing home. 

1. How much water can be stored in the drainage cavity, and does it pose a moisture-related 

durability threat?

Laboratory tests were conducted on a drainage balance of drained exterior insulation on a stud 

wall, using both smooth and corrugated plastic housewrap. The range of water amounts stored in 

these drainage tests was 19 g/m2 to 36 g/m2 when the entire surface area of the drainage cavity 

was wetted. This amount of water in the drainage cavity after a large wetting event proved 

insignificant to the moisture durability of the wall system. 

Repeated wettings of the drainage cavity (6–8 times) did increase the overall mass of the wall 

and the MC of the sheathing (less than 2%). This shows that it is unlikely for any amount of 

water that is stored in the drainage cavity to result in moisture durability risks of the sheathing, 

assuming there are no other simultaneous moisture sources such as air leakage condensation on 

the sheathing. 

Even when water was directly applied to the sheathing (simulating a deficiency in the sheathing 

membrane) during field testing, the exterior insulated wall performed better than the direct 

applied stucco wall in side-by-side comparisons on all four orientations in climate zone 4C. 

2. If water becomes trapped in the drainage cavity, where does the water go, and what impact 

does it have on the durability of the wall system? 

During hygric redistribution testing in the laboratory, water was injected into storage media in 

the drainage cavity to simulate water trapped in the drainage cavity (not in direct contact with the 

plywood sheathing). This water appeared to mostly diffuse across the sheathing membrane and 

adsorb into the plywood sheathing. The RH in the stud space increased during wetting. 

Mathematical analysis also confirmed that most of the moisture diffused into the wall relative to 

redistribution in the drainage cavity. After injecting 13 oz of water over a week, there was no 

risk of any moisture-related durability issues. 
47 

During field testing when water was trapped against the sheathing, the water was able to dry to 

the interior because there was no Class I vapor control layer on the interior, resulting in a more 

quickly drying plywood sheathing and overall lower sheathing wood MC than the comparison 

standard stucco construction wall. 

3. Why is there a discrepancy between the basic hygrothermal analysis of vapor impermeable, 

relatively low R-value exterior insulations and field performance? 

For the purposes of this report and analysis, the BSC team assumed that the discrepancy in field 

performance and hygrothermal analysis was based on the fact that hygrothermal analysis does 

not take into account the effects of drainage that occurs when any moisture condenses on the 

interior surface of the exterior insulation. Further study in this might be conducted in the future. 

4. Are there alternative solutions to energy retrofits that decrease the risk of moisture-related 

durability problems? 

Initial testing has shown that even with a flat plastic housewrap with no extra drainage gap, there 

is little risk of any moisture-related durability issues from water being drained through or trapped 

in the cavity. That being said, it is always beneficial to design the system with a factor of safety, 

assuming that there will be unanticipated design deficiencies. Adding more width to the gap will 

ensure that water drains easily and some ventilation will occur, but that could also lead to 

decreasing the energy gains of exterior insulation. Also, it is good practice to drain penetrations 

to the front of the insulation to minimize the water draining behind the insulation.

3.5 Future Work

Within the Building America Program, there are currently no plans or budget to expand on the 

knowledge and information included in this report. The BSC team, however, feels that a greater 

understanding of the potential moisture-related durabiltiy issues could be gained through further 

testing in the following areas:

• With all tests, a greater moisture load should be added to see at what point the wall will 

experience moisture-related durabilty issues. This will help constrain the upper bounds of 

the moisture durability for the wall system. It could be argued that the added water used 

in this testing program is not representative of moisture loads in the field in some cases. It 

is known, though, that the amount of water that enters a drainage cavity is related to 

many variables, including amount of driving rain, enclosure moisture management 

details, and the quality of workmanship. 

• Other sheathing membranes and drainage mats could be tested to improve the 

understanding of how various wall system compontents affect the moisture-related 

durability of wall systems. Other test variables such as temperature gradients across the 

wall or solar heating of the cladding could also be added to more closely simulate field 

conditions. 

• Thermal losses for drained exterior insulation should be measured and correlated to the 

size of the drainage gap to determine if there is any measurable effect on performance.
48 

References 

Baker, M. (1969). “Decay of Wood.” Canadian Building Digest 111. Ottawa, Ontario: National 

Research Council Canada.

Garrahan, P. (1988). Moisture Meter Correction Factors. Ottawa, Canada: Forintek Canada 

Corp. 

Grin, A. (2008). Evaluation of High Performance Residential Housing Technology. MASc 

dissertation. Waterloo, Ontario: University of Waterloo.. Chapter 5, pp. 77–89. 

Grin, A.; Lstiburek, J. (2011). Building America Special Research Project: High Performance 

Hybrid Assemblies. Building America Program. 

Hutcheon, N. (1964). CBD-50 Principles Applied to an Insulated Masonry Wall. Ottawa, 

Ontario: National Research Council Canada. 

Lstiburek, J,. Parthenon, Eh!, ASHRAE Journal (March 2012) 

Onysko, D., Schumacher, C., Garrahan, P., (2010). “Field Measurements of Moisture in Building 

Materials and Assemblies: Pitfalls and Error Assessment.” Presented at the Building Enclosure 

Science and Technology (BEST2) Conference. April 12-14, 2010, Portland, OR 

Reed Construction Data (2011). RSMeans CostWorks 2011. 15th Annual Edition. Norwell, MA: 

Reed Construction Data. Retrieved May 17, 2011. 

Schumacher, C., et al. (2003). “Ventilation Drying in Wall Systems.” Presented at the 

International Building Physics Conference, September 14-18, 2003, Leuven, Belgium. 

Smegal, J. (2006). Drainage and Drying of Small Gaps in Wall Systems. MASc dissertation. 

Waterloo, Ontario: University of Waterloo. 

Smegal, J.; Straube, J. (2009). RR-0903 Building America Special Research Project – High-R 

Walls Case Study Analysis. Accessed November 2012: www.buildingscience.com. 

Steffen, M. (2000). Moisture and Wood-Frame Buildings. Building Performance Series No. 1. 

Ottawa, Ontario: Canadian Wood Council. 

Straube, J.; Onysko, D.; Schumacher, C. (October 2002). “Methodology and Design of Field 

Experiments for Monitoring the Hygrothermal Performance of Wood Frame Enclosures.” 

Journal of Thermal Envelope and Building Science (26:2). 

Straube, J.; Burnett, E. (2005). Building Science for Building Enclosures. Westford, MA: 

Building Science Press. 

Straube, J. (2011). BSD-163: Controlling Cold-Weather Condensation Using Insulation. 

Building Science Corporation. Accessed November 2012: www.buildingscience.com.

Straube, J. (2009). RR-0904: Adhered Veneers and Inward Vapor Drives: Significance, 

Problems, and Solutions. Accessed November 2012: www.buildingscience.com. 
49 

Appendix: BEopt Output Graphs

Boston
50 
51 

Duluth
52 
53 

Kansas City
54 
55 

Dallas
56 
DOE/GO-102013-3720 ▪ January 2013

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.