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Factors influencing mould growth and modeling of aesthetic service life:- coated and uncoated wooden cladding

Lone Ross Gobakken 1 and Olav A. Høibø 2

1Norwegian Forest and Landscape Institute2Norwegian University of Life Sciences

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> Blue stain fungi and moulds affect the aesthetic service life of wooden cladding façades

> Main requirements for fungal growth: moisture, temperature and nutrients (substrate)

> Several factors are influencing service life of wooden cladding façades

> Wood properties> Craftsmanship> Maintenance> Design> Wood moisture content and wood temperature> Climate> Type of wood protection

Background

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> study selected factors that can influence growth of blue stain and surface mould on coated and uncoated wooden claddings

> address the importance and the possibilities of developing models that describe growth of blue stain and surface mould over time

Objectives

> 5 (6) field studies

> Sweden and Norway

> Combinations of wood substrates and coating typologies

> Recorded climate data

> Mould growth assessment

> Modelling

> Comparisons

> Simulations

M & M

Ås

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> Field study 1> Field study 2> Field study 3> Field study 4

Ås

• 5 modified wood substrates and 8 reference wood substrates

• 3 coating systems (WB + SB + SB/WB)• Mould growth assessed 1 time a year for 4.5 years

> Field study 1> Field study 2> Field study 3> Field study 4

Ås

• Norway spruce panels• 6 model paints – glossy, matt, soft, hard,

hydrophopic and a film with pores (air inclusion) + commercial standard paint

• Exposed for 3 years – assessed 11 times

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> Field study 1> Field study 2> Field study 3> Field study 4

Ås

• 7 wood substrates (including 3 modified substrates)• 3 coating systems – WB without fungicide, SB

without fungicide and ICP + unpainted• Exposed for 3 years – assessed 11 times

> Field study 1> Field study 2> Field study 3> Field study 4

Ås

• Norway spruce - low and high productive forest• Manufacturing characteristic x 4• Wood properties x 5• 1 coating systems - WB • Exposed 3 years – assessed 2 times per year

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> Field study 1> Field study 2> Field study 3> Field study 4> Field study 5

Ås

• Acetylated SYP (three treatment levels) and untreated SYP

• Not coated• Exposed facing south and north, September 2010 to

March/May 2011

> Assessment according to EN 927-3, 2000 and 2006

> The rating is based on a step-wise increase in mould growth coverage, but the shape and the pattern of the mould growth are also of importance to the determination (density and size)

> Mould rating using the standards makes the response variable ordinal

Mould assessment

No mould growth Heavy mould growth

0 1 2 3 4 5

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> Field study 1

> proportional odds logistic regression model

> Mould rating – a function of time, coating and wood substrate (species / sapwood-heartwood-mixed / modification or treatment type)

> Field study 2, 3 and 4

> ordinal logistic regression model

> Mould rating - a function of time / coating typology / wood substrate / manufacturing characteristics / wood properties / relative humidity / temperature

Statistical analysis and modelling

> Several factors affects surface mould growth

> The factors have a synergetic effect, and to some extent it can be difficult to localise the most important contributing factor

Results and discussion

Coating Wood substrate

Exposure time

Surfacestructure

Manufacturing characteristics

Material climatet/RH/wmc

Blue stainand

surface mould

Wood properties

Climatemicro/meso

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> Exposure time had the greatest impact on fungal growth

> Surface mould growth adapt a sigmoid growth curve as a function of time

The slope coefficient for mould coverage over time will decide the time when the panel will reach end of service life

Every combination of wood substrate and type of surface finishes will have an unique growth curve.

Exposure time

> Type of coating typology was, together with exposure time, the most significant factor for surface mould growth

> Clear differences in time of colonisation and speed in development of mould coverage between coating typologies

Wood coating

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> Type of wood substrate was a less important factor than coating –but still significant

> Copper treated wood, pine heartwood and larch heartwood had less mould growth than other substrates

> Thermally modified substrates resisted mould growth better than furfurylated and acetylated substrates

Wood substrates and wood properties

> Coated spruce claddings: wood properties and manufacturing characteristics had no significant effect on surface mould growth

> Effect concealed due to the use of a high performance coating system (?)

0

1

2

3

4

5

Stan

dard w

ith fu

ngici

de Soft

Mat

t

Ai r inclu

sion

Hydrop

hob

Gloss

yHard

Stand

ard w

ithout

fung

icide

Mo

uld

gro

wth

(0

-5)

> Surface structure (type of model paint) had significant effect on mould growth

> Hard and glossy model paint was most susceptible to mould growth

Surface structure

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> RH, precipitation and temperature were tested in initial models to try to capture the effects of the local climate

> RH and temperature were found significant and kept in a final model

> Simulations made over a wide range in RH and temperature indicated that a general increase will give more mould growth

> Cardinal direction had a clear effect on mould growth – but the effect can vary due to type of test material, test set-up and location.

Relative humidity, temperature and cardinal directionMould rating

Norway spruce claddings• coated and uncoated• exposed 1,5 years

Acetylated southern yellow pine• three treatment levels• uncoated • exposed 9 months

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> Material climate

> wood moisture content

> wood temperature

> Ambient air

> RH and temperature 10 mm in front of the panels

> Colour of the surface

> Open / shadowed environment

> Studies of user preferences

> aesthetic service life

> need for maintenance

Current work….

> Developing models for mould growth allow investigation of specific effects contributing to the variation in surface mould growth development over time

> Exposure time and type of surface finishing affect surface mould growth the most

> The individual mould growth curve and slope coefficient decide when a specific combination of wood substrate and surface finish reaches maximum mould rating and end of aesthetic service life

> Statistical models can be used to study the effects of climate change on deterioration and service life of exterior wooden claddings

Conclusions

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Thank you for your attention!

Photo: Oskar Puschmann, Skog og landskapPhoto: Oskar Puschmann,

Skog og landskap

Acknowledgement:Mats Westin SP Technical Research Institute of SwedenPatricia Lebow USDA Forest Service - Forest Products

Laboratory, Madison, USAHalvor Solheim Norwegian Forest and Landscape Institute Geir I. Vestøl Norwegian University of Life SciencesStig Bardage SP Technical Research Institute of SwedenMari S. Sivertsen Mycoteam as