How Many Times Can You Repaint the Exterior of Your Building? Kevin Brown KTA-Tator, Inc.

Determine the relationship between the permeance of the coating and water content within the wall assembly. Recognize the value of thermal-dynamic modelling (using WUFI -ORNL/IBP software in this case) when specifying coatings for use on building walls. Recognize that the geographical location of a building will influence coating selection and the number of times the building can be repainted. Explain the long-term cost savings that can be realized by selecting the correct coating system for the geographical location.

Moisture moves from warm to cold (thermal gradient) Moisture moves from more to less (concentration gradient)

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

Vapor Impermeable Vapor Barriers 1 perm or less Semi-Vapor Permeable Vapor Retarders More than 1 and less than 10 perms Vapor Permeable Breathable 10 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

WUFI does not have pre-programmed values for coating permeance. Permeance values can be obtained from the manufacturer or through laboratory testing and entered into the model. The values used must be representative of the entire system, not individual coats alone.

Exterior Paint Permeance 0 5 10 15 20 25 30 35 40 Interior Paint Permeance 0 15 40

Concrete Brick Water Content Source: WUFI Pro 5.3 Water Content analyzed for each model Practical water Content Free Saturation water content Maximum water content

Water Content (lb/ft 3 ) Relative Humidity (%) 0 0% 2.28 5% 2.98 69.5% 3.21 (W 80 ) 80% W 80 Practical Moisture Content (80% RH) W f Free Saturation point (100% RH) 3.47 91.5% 4.05 (W f ) 100%

Water content at outer layer of block (beneath the coating) Water content at inner layer of block (beneath the coating) Water content in interior of block

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

Exterior Interior Outer Block, 0 Interior Perms, (lb/ft 3 ) 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