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Water Vapor Permeance – Why Does it Matter?
(Part 2 of a 2 - Part Series)
Harold DeCelle & Kevin BrownKTA-Tator, Inc.
Part 1 – Define water vapor permeance, how to measure it in coatings, and how to interpret the results (presented February 24, 2016)
Part 2 – Determine the effect of coating permeance on the performance of wall assemblies and the number of times a building can be repainted.
Recognize the differences in “vapor drive” and “water-vapor permeance of coatings.”
Identify differences in the performance of a building wall assembly when an impermeable, semi-permeable, and permeable coating system is used.
Identify differences in the performance of identical building wall assemblies located in different climate zones.
Recognize the value in using WUFI in selection and arrangement of materials within a wall.
Moisture moves from warm to cold
(thermal gradient)
Moisture moves from more to less (concentration gradient)
Psychrometric chart represents
“Warm to cold”
“More to less”
Cold climate: vapor drive primarily from interior to exterior
Hot, humid climate: vapor drive primarily exterior to interior
Source: Whole Building Design Guide, a program of the National Institute of Building Sciences, “Moisture Management Concepts,” by Ted J. Kesik,
illustrations by Ashleigh Uisaka
Permeance – Permeance is measured in perms. One perm is one grain of water vapor per hour
flowing through one square foot of a layer, induced by a vapor pressure difference of one
inch of mercury across the two surfaces.
“Perm Rating” = Resistance to vapor drive
Vapor Drive
Permeability is a property of a material. Permeance depends on the thickness of the
material and therefore is a performance evaluation and not a property of a material.
Permeability = Water Vapor Permeance X Thickness
Permeability should only be calculated if the test specimen is homogeneous (not laminated) and at
least ½” thick.
Vapor Impermeable –Vapor Barriers1 perm or less
Semi-Vapor Permeable –Vapor RetardersMore than 1 and less than 10 perms
Vapor Permeable –Breathable10 perms or more
Source: ASHRAE Journal, February 2002, “Moisture Control for Buildings,” by Joseph Lstiburek., P.Eng.
Interior Paint
Exterior Paint
1.5” Concrete Brick layers
Cavity filled with Sprayed Polyurethane foam open cell insulation
Boundary Conditions: ASHRAE Year 3
Orientation – East
Interior ASHRAE 160P
Air Conditioning 69.98-75.02 °F
Models simulate 3 years
Exterior Paint Permeance
0
5
10
15
20
25
30
35
40
Interior Paint Permeance
0
15
40
Water Content analyzed for each model
Practical water Content
Free Saturation water content
Maximum water content
Concrete Brick Water ContentSource: WUFI Pro 5.3
Water Content (lb/ft3)
Relative Humidity
(%)
0 0%
2.28 5%
2.98 69.5%
3.21 (W80) 80%
3.47 91.5%
4.05 (Wf) 100%
W80 – Practical Moisture Content (80% RH)
Wf – Free Saturation point (100% RH)
Climate Zone 4&5 marine, Oceanic CFB cool pacific NW
Exterior Paint
Permeance
Interior Paint Permeance
0 0 15 40
5 0 15 40
10 0 15 40
15 0 15 40
20 0 15 40
25 0 15 40
30 0 15 40
35 0 15 40
40 0 15 40
Outer Block, 0 Interior Perms, (lb/ft3)Ext. Perms Start End Min. Max Delta
0 3.21 4.23 3.21 4.25 1.02
5 3.21 3.45 2.49 3.67 0.24
10 3.21 3.40 2.45 3.61 0.19
15 3.21 3.38 2.43 3.59 0.17
20 3.21 3.37 2.42 3.58 0.16
25 3.21 3.36 2.42 3.56 0.15
30 3.21 3.37 2.42 3.58 0.16
35 3.21 3.36 2.42 3.57 0.15
40 3.21 3.36 2.42 3.57 0.15
Exterior Interior
Outer Block Layer, Ending Water Content (lb/ft3)
Eastern Exposure
Insulation Layer, Ending Water Content (lb/ft3)
Eastern Exposure
Inner Block Layer, Ending Water Content (lb/ft3)
Eastern Exposure
Climate Zone 1, AW-tropical wet/dry
Outer Block Layer, Ending Water Content (lb/ft3)
Eastern Exposure
Climate Zone 4&5 Marine, DFA Humid cold
Outer Block Layer, Ending Water Content (lb/ft3)
Eastern Exposure
Climate Zone 4&5, dry semi-arid
Outer Block Layer, Ending Water Content (lb/ft3)
Eastern Exposure
Outer Block Layer, Ending Water Content (lb/ft3)
Interior coating permeance – 0
Eastern Exposure
Outer Block Layer, Ending Water Content (lb/ft3)
Interior coating permeance – 15
Eastern Exposure
Outer Block Layer, Ending Water Content (lb/ft3)
Interior coating permeance – 40
Eastern Exposure
182.5SEP
365MAR
547.5SEP
730MAR
912.5SEP
1095MAR
DaysMAR
W80
Wf
W80
Wf
182.5SEP
365MAR
547.5SEP
730MAR
912.5SEP
1095MAR
DaysMAR
Capabilities
Determine the appropriate selection and arrangement of materials
Takes into account vapor diffusion movement
Understands moisture storage functions of porous hygroscopic material
Input the interior conditions
Input exterior climate and orientation
Ability to review Seasonal changes in water content
Limitations
WUFI does not house specific coating system data by manufacturer or product type
The models assume the wall assembly is constructed perfectly
Coating permeance –is it considered for maintenance painting projects?
How many times can a building be painted before moisture related problems occur?
Consider Modeling?
US Perms
Generic Coating System System Code
69 Acrylic Primer/Acrylic Mid Coat/ Acrylic Topcoat 1
62 Acrylic Primer/ Acrylic Mid-Coat/ Styrenated Acrylic topcoat
3
54 Acrylic Primer/ Acrylic Mid-Coat/ Styrenated Acrylic topcoat
4
46 Acrylic Primer/ Styrenated Acrylic Mid Coat/ Styrenated Acrylic topcoat
2
29 Acrylic/Acrylic 5
24 Acrylic/Acrylic 8
15 Styrenated Acrylic /Styrenated Acrylic 9
11 Acrylic block filler/Acrylic mid coat/Acrylic topcoat 6
9 PVA block filler / Acrylic Mid Coat/ Acrylic topcoat 7
Source: Use of Atlas Test Cells to Assess the Performance of Coatings with varied permeance over CMU, Cindy O’Malley, KTA, Chuck Duffin, STO and Steve Revnew, Sherwin Williams
Can be estimated mathematically.
Laboratory testing:
ASTM E96 / E96M – 15, Standard Test Methods for Water Vapor Transmission of Materials
ASTM D1653 – 13, Standard Test Methods for Water Vapor Transmission of Organic Coating Films
System Code
Initial System (US Perms)
Cumulative after 1 Recoat Calculated by
Laboratory(US Perms)
Cumulative after 1 Recoat
Calculated Mathematically
(US Perms)
% Difference between
Laboratory and
Mathematical calculation
1 69 44 35 -22%
2 46 39 23 -41%
3 62 30 31 3%
4 54 26 27 4%
5 29 22 15 -34%
6 11 11 6 -50%
7 9 9 5 -50%
8 24 16 12 -25%
9 15 10 8 -25%
System Primer (mils)
Topcoat1 (Mils)
Topcoat2 (Mils)
Topcoat3 (Mils)
Topcoat4 (Mils)
Topcoat5 (Mils)
Topcoat6 (Mils)
Perms
1 18 6 24
2 18 6 6 16
3 18 6 6 6 8
4 18 6 6 6 6 4
5 18 6 6 6 6 6 2
6 18 6 6 6 6 6 6 1
City OEM (24 Perms)
R1 (16 Perms)
R2 (8 Perms)
R3 (4 Perms)
R4 (2 Perms)
R5 (1 Perms)
Miami, FL
Providence, RI
Colorado Springs, CO
Seattle, WA
Risk based on water content in Outer Block Layer with Interior coating permeance – 15
No Risk (80% RH or below)
Moderate Risk (81-90% RH)
High Risk (91% RH or above)
Thermodynamic Potential: “Warm to cold” and “More to less.”
Hydrophilic (hygroscopic) and hydrophobic materials. Understand the moisture storage function of a hydrophilic material (W80, Wf).
WUFI simulation modeling shows the importance of material selection and how properties such as climate, capillary conduction and permeance are used to predict moisture related problems.
WUFI modeling does not account for construction defects and does not house specific coating system data.
Seasonal changes impact water content.
Use caution to calculate coating permeance mathematically. Laboratory testing of multiple coats combined is preferred.
Combination of WUFI modeling and permeance testing of select coating systems can determine how many times a building can be repainted.
