14 D+D SEPTEMBER 2014

Coatings Technology

Calculated ImpactCan WUFI calculate the impact of coatings permeance on wall systems?By Kevin Brown, Harold DeCelle and Kenneth Trimber, KTA-Tator Inc.

he permeance of coating systems

applied to the interior and exterior

walls of single-wythe CMU (con-

crete masonry unit) buildings, and

the number of maintenance re-

paints, can directly impact wall-sys-

tem performance.

The question is — by how much. Get-

ting that answer in advance on projects

could likely prevent many coating and

wall-system failures.

This article discusses the permeance of

coatings and presents the results of pre-

liminary thermodynamic modeling using

WUFI-ORNL/IBP (Oak Ridge National Lab-

In the “wet cup method,” Procedure B of ASTM E 96 Standard Test Methods for Water Vapor Transmission of Materials, dry paint films sealedon dishes of deionized water are measured for vapor permeability. Photo courtesy of KTA-Tator.

T

Coatings Technology15

oratory/Fraunhofer Institute for Building

Physics) to track the effects of a few inte-

rior/exterior coating-permeance scenarios

on the performance of wall systems.

As you may know, WUFI comes from the

German “Wärme und Feuchte instationär,”

which roughly translated, means “heat and

humidity transient.” WUFI is a software

family that helps design professionals cal-

culate real-world one- and two-dimensional

heat and moisture transport in multi-layer

building wall assemblies exposed to

weather.

The model examined in this article uses

two hypothetical but identical CMU build-

ings. Variables include building locations —

Providence, R.I. and Miami; the permeance

of interior wall coatings — no coating and 5

English perms; and exterior coating system

permeance — 40, 25, 15 and 5 English

perms applied to 8-inch (20-cm) concrete

block.

Vapor DriveFor background, let’s review the signifi-

cance of vapor drive. The following discus-

sion of vapor drive is from the

November/December 2012 Durability + De-

sign article, “Coating System Concerns for

Single-Wythe CMU Construction1.”

A “vapor drive” is caused by the difference in rel-ative humidity (RH) between the building interior(typically higher RH) and the exterior environment(typically lower RH), causing water molecules inthe interior air to pass through molecules of thewall components. If the wall is not insulated, andgiven certain exterior ambient climatic conditions,temperature will cycle within the block and fallbelow the dew point, causing the vapor to con-dense, wetting the interior of the block and po-tentially leading to coatings problems. The degreeof vapor drive is controlled by the porosity of the

denses to liquid. The most obvious signs thatthis is occurring is the presence of surface con-densation or efflorescence, but the most dam-aging effect of the moisture is when it becomestrapped behind the paint or within the fabric ofthe wall. Cures include good ventilation, a con-sistent heating level, a contiguous air/vaporbarrier and good wall insulation.

In the example of a building in a northern cli-mate, if moisture vapor cannot pass through theexterior coating at a sufficiently rapid rate, itwill build up and condense to liquid, potentiallyincreasing the vapor pressure behind the coat-ing at the wall interface, resulting in coatingblistering. The cyclic temperature conditionswill result in accumulation of more moisture inthe form of liquid, and expansion of blisters withincreasing periods of vapor pressure. Eventu-ally the coating system adhesion will fail.

While permeance is a key factor in se-

lecting coating systems for walls, other

factors affect wall performance including

air infiltration/exfiltration, stack effect,

wind load, insulation and mechanical pres-

surization, as well as defects in the cop-

ing, roof membrane, gutters, flashing,

mortar and sealants.

Determining Water-Vapor Permeance of CoatingsSelection of interior and exterior coating

systems should be based on the expected

vapor drive, and the vapor permeance of

the coatings. Laboratory water-vapor per-

meance testing measures the rate at

which a solid material lets moisture pass

from one side of a membrane to the other.

ASTM E 96 Standard Test Methods for

Water Vapor Transmission of Materials is

commonly used to test the properties of

coating films — both individual coats and

entire systems. Test conditions should be

chosen based on the material design pa-

wall, together with environmental factors, espe-cially:

Moisture gradients. Moisture vapor will natu-rally move from a higher concentration to alower concentration until in balance. With highvapor pressure to the interior of the wall, and lowvapor pressure to the exterior of the wall, vapordrive will be directed outward (and vice versawhen the relative vapor pressures are reversed).The greater the difference of this vapor pressureor “concentration gradient,” the greater is thevapor drive.

Temperature gradients. Moisture vapor willnaturally move from the warm side of a wall tothe cooler side. With higher temperatures to theinterior of the wall and lower temperatures to theexterior of the wall, vapor drive will be directedoutward (and vice-versa when the differences intemperature are reversed). The greater the“temperature gradient” (difference), the greaterthe vapor drive.In other words, the movement of moisture via

diffusion is a result of differences in vapor pres-sure that are related to the temperature andmoisture content of the air on both sides of thewall. The mixed climates found across the coun-try, both in the interior space and external to thebuilding, must be taken into consideration whendetermining the direction of vapor diffusion andwhen designing the painting system.

The practical problems that can occur if

vapor drive is ignored during building de-

sign and construction were described by

Cindy O’Malley, Chuck Duffin and Steve

Revnew in their paper “Use of Atlas Test

Cells to Assess the Performance of Coat-

ings with Varied Permeance over Concrete

Masonry Units,” which they presented at

The Society for Protective Coating’s Green-

COAT conference in 2012.

If the exterior coating material blocks or re-stricts this natural flow of warmer air to coolerair it will lead to a buildup of vapor. When cool-ing of the air (vapor) occurs, the vapor con-

16 D+D SEPTEMBER 2014

rameters for use, but Procedure B, wet cup

method, is frequently used for coatings.

Procedure B involves the application of a

coat, or the entire system of coatings, to

polyester sheets so that after drying, films

of consistent thickness can be removed

from the sheets to produce free-standing

films. The researchers cut the films into

discs and seal them to open-faced glass

dishes of a specific diameter — 4 inches (10

cm), for example — filled with deionized

water to three-quarters of their volumes.

They weigh the dishes and expose them in

a controlled temperature/humidity cham-

ber at approximately 23 C (73 F) and 50

percent RH for 17 days.

The researchers weigh the dishes at pre-

scribed intervals during the exposure pe-

riod and plot the results on a graph. When

the testing is done, they calculate perme-

ance, commonly reported in English perm

units. It is critical to know which method

generated the data, in order to compare re-

sults. Different test methods, even under

the same ASTM standard, can yield differ-

ent results.

The sophisticated computer program

WUFI-ORNL/IBP assesses the combined

heat and moisture transfer in building com-

ponents based on building type and local

interior and exterior environments. Build-

ing envelope components such as wall ma-

terials, windows and insulation are entered

into the program, and it calculates the im-

pact of the components on heating loss

and the direction of vapor diffusion and liq-

uid transport through the walls. WUFI-

ORNL/IBP has been used to determine the

effect that the interior/exterior placement

of air and weather barrier coatings can

have on the performance of building walls.

WUFI doesn’t house specific coating sys-

tem data by manufacturer or product type

but the advanced versions allow the user

to enter a permeance value. The perme-

ance value includes the entire coating sys-

Fig. 1: Providence – This graph of the exterior block face shows high moisture in winter. The graph isbased on 5-perm coatings on the exterior and interior surfaces. The extension of the graph above the redline indicates moisture build-up.

Efflorescence exuding from a pinhole in an exterior coating indicates the presence of moisture withinthe wall. WUFI may hold answers about the effects of coating permeance on wall-system ability tohandle moisture vapor. Photo courtesy of KTA-Tator.

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2.35

2.21

2.08

1.94

1.8

3.6

3.4

3.2

3

2.8

2.6

Concrete Brick

Water Con

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Water Con

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18 D+D SEPTEMBER 2014

tem (primer and topcoat) including re-

paints, if applicable. A few manufacturers

have permeance values for coating sys-

tems but many haven’t published this in-

formation.

If the permeance values aren’t known,

laboratory testing must determine them,

or else users must determine the values

based on experience.

The authors ran a few building scenar-

ios by inserting values for the permeance

of exterior coatings based on data deter-

mined in the laboratory2, together with

the anticipated reduction in permeance

after repainting. The authors used values

40, 25, 15 and 5 English perms for the ex-

terior coatings applied to 8-inch concrete

block.

Interior treatments included bare CMU

and low-permeance (5 English perms)

coatings. The authors entered the values

into WUFI to determine the predicted ef-

fect that changes in interior and exterior

coating permeance might have on the

performance of walls in identical, 8-inch

single-wythe CMU buildings in Provi-

dence and Miami. The interior building

environment stayed consistent with air

conditioning, heating set point 69.98 F

(21.1 C) and cooling set point 75.02 F

(23.9 C), no dehumidification, and low

density sprayed-polyurethane foam insu-

lation in the wall cavity.

WUFI Modeling ResultsThe authors drew the following conclu-

sions from the specific variables ana-

lyzed, but readers should not rely on the

results that follow for making building-

specific coating decisions since variables

such as the wall-system assembly, dehu-

midification, interior conditioning, insula-

tion and other factors will affect the

results.

The authors judged performance in

each scenario by the amount of moisture

Fig. 2: Providence – This biological graph shows the interior surface. The graph is based on 5-perm coatingson exterior and interior surfaces. The graph indicates potential for mold growth within the first season.

Moisture trapped in wall systems causes all sorts of damage, from biological growth to coatingdelamination. Here, moisture trying to escape from the wall has created blisters in exterior paint. Photo courtesy of KTA-Tator.

45 60 75

100

80

60

40

20

0

7.63 in (Interior Surface)– – LIM B I ––– LIM B II

Relativ

e Hu

midity

[%]

Temperature [°F]

Coatings Technology19

WUFI predicted would develop in the

walls, and the potential for mold growth.

The model included cycling for three years,

Oct. 1, 2014 through Oct. 1, 2017 using

“cold year” cycles.

MiamiThe wall system performed best with the

interior bare and a low-permeance (5 Eng-

lish perms) coating on the exterior.

When the interior side was bare, the

wall system performance, while still good,

decreased as the permeance of the exterior

coating system increased (from 15 to 40

English perms).

With a low-permeance system (5 perms)

on the interior, the performance of the wall

system was best with a low-permeance ex-

terior coating system (5 perms).

When the interior was coated with a

low-permeance system (5 perms), in-

creaing the permeance of the exterior coat-

ing system (from 15-40 perms) decreased

wall-system performance, creating poten-

tial for biological growth on the interior.

See Figure 3 on page 20.

The permeance of the interior coating

system influenced performance more (bare

CMU was better), than the permeance of

the exterior coating system (although

lower permeance was better).

ProvidenceThe wall system performed best with the

interior side bare and a high-permeance

coating (40 perms) on the exterior, the op-

posite of Miami.

Performance worsened when the interior

and exterior sides were both coated with

low permeance systems (5 perms). The

low permeance on the exterior caused poor

performance, the opposite of Miami. See

Figure 1 on page 16.

The preceding scenario (5 perms interior

and 5 perms exterior) created biological

growth potential. See Figure 2 on page 18.

Exterior coating permeance influenced

wall-system performance (the higher the

permeance the better) more than the inte-

rior. A 5-perm interior coating system

showed a slight performance decrease

compared to bare block.

OverallIn these scenarios, the Miami wall system

outperformed the Providence wall system.

The authors have seen this in existing

buildings. Southern buildings generally

sustain less peeling, blistering and sub-

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20 D+D SEPTEMBER 2014

strate damage such as spalling and mortar

deterioration than buildings in the North.

Much additional modeling needs to be

undertaken before relying on WUFI to se-

lect coating systems, but the preliminary

analysis conducted for this article indi-

cates that WUFI has a great deal of prom-

ise for use in the coatings industry.

Questions and answers regarding the

potential use of WUFI based on the initial

analysis include:

Q: Will interior/exterior painting strate-

gies that are successful in one geographi-

cal location perform the same in another

location?

A: Based on preliminary modeling of a few

scenarios using identical insulated build-

ing designs in Florida and Rhode Island,

the answer is no. The same coating sys-

tem can result in differences in the per-

formance of walls depending on the local

environment. In the models run by the au-

thors, an exterior coating with low perme-

ance provided the best performance in

Miami, but the same system provided the

worst performance in Providence.

Q: Does WUFI appear to be of value in

making judgments on the number of re-

paints that are feasible before problems

occur with the wall’s performance?

A: Based on laboratory-generated perme-

ance data of a few coating systems after re-

painting, it appears that WUFI could have

great utility in determining the number of

building-specific repaints that are feasible

before problems occur. However, more

analysis is required, and brand-specific per-

meance data is needed to better determine

the utility of the program for this purpose.

Potential for CoatingsThe modeling using the WUFI program

demonstrates WUFI’s value as a tool to as-

sist in selecting coatings for the interior and

exterior walls of single-wythe CMU build-

ings based on the permeance of the coat-

ings and the building-specific

interior/exterior climate.

The modeling also suggests that once

the permeance of given systems, by brand,

is established analytically for both the new

installation and each repaint, WUFI will be

able to predict the number of mainte-

nance-painting operations that are feasible

before problems with the building walls

can be expected.

The modeling also shows that the selec-

tion of coating systems depends on the

permeance of the systems and the unique

service environment for both interiors and

exteriors. That is, using the same interior

and exterior coating systems and mainte-

nance painting schedule on the walls of a

building located in one climate, such as

Miami, may not be the best approach for a

building in another climate, such as Provi-

dence.

Based on the success of the preliminary

modeling, the authors plan additional

modeling to include more building-specific

variables and environments to better ex-

amine the capability of WUFI for use in

coating system selection.

Assuming the more in-depth analysis is

promising, the WUFI database will need to

be populated with permeance data for a

variety of interior and exterior building-

wall paint systems, including the reduc-

tion of permeance after multiple repaints.

The authors are willing to coordinate the

collection of coating permeance data, and

independently review the data for submis-

sion if coating manufacturers are inter-

ested in participating.

References(1) “Coating System Concerns for Single

Wythe Concrete Masonry Unit (CMU)

Construction,” K. Trimber, Durability +

Design: the Journal of Architectural

Coatings, November/December 2012,

Vol. 2, No. 6. pp. 32-39.

(2) “Use of Atlas Test Cells to Assess the

Performance of Coatings with Varied

Fig. 3: Miami – This biological graph shows the interior surface. The graph is based on a 5-perm coatingsystem on the interior and a 40-perm coating system on the exterior. The graph indicates potential formold growth within the first season.

45 60 75

100

90

80

70

60

50

7.63 in (Interior Surface)– – LIM B I ––– LIM B II

Relativ

e Hu

midity

[%]

Temperature [°F]

Coatings Technology21

Permeance over Concrete Masonry Units

(CMU)”, Cindy O’Malley – KTA-Tator,

Inc. Chuck Duffin – Sto Corp, and Steve

Revnew – Sherwin-Williams, SSPC

GreenCoat 2012, Las Vegas NV, February

2012.

About the AuthorsKenneth A. Trimber, president of KTA-

Tator Inc., is a NACE-certified coatings in-

spector level 3, an SSPC-certified

protective coatings specialist and is certi-

fied at a level III coating

inspection capability in

accordance with ANSI

N45.2.6. Trimber has 40

years of experience in

coatings inspection,

testing and analysis, is

a past president of the Society for Protec-

tive Coatings (SSPC), and is chairman of

the SSPC committees on Surface Prepara-

tion, Visual Standards and Containment,

along with the SSPC Commercial Coatings

Committee (Architectural, Commercial, In-

stitutional).

Kevin J. Brown is manager of the Commer-

cial Services Group for KTA-Tator Inc.,

where he develops and implements main-

tenance programs for commercial clients

with architectural/commercial problems

related to paint failures. He’s a certified XL

tribometrist and regis-

tered roof observer.

Brown has a Bachelor of

Science degree and an

MBA from Gardner-

Webb University in Boil-

ing Springs, N.C., and

has more than 13 years of experience in

the field of retail facility management.

Harold DeCelle is a building inspection

technician for KTA’s Commercial Services

Group. He has more than 35 years of expe-

rience in construction and building design,

and a Bachelor of Science in Civil Engi-

neering (Structural) from the University of

North Carolina at Charlotte. DeCelle evalu-

ates commercial building envelopes for

KTA clients nationwide. He performs de-

structive and non-destructive testing in

forensic evaluations of existing buildings

and performs modeling using WUFI to aid

in restoration design. D+D

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