Moisture content of exposed painted wood panels

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Moisture content of exposed painted wood panels

CANADA

I

I

- -Ser TKL B92 no.

79

c . 2

!

I

I

' I

MOISTURE CONTENT

OF

EXPOSED PAINTED

WOOD PANEIS

D . C . Tibbetts and D.

R.

Robson

3 3 + / 6 3 -

D I V I S I O N O F BUILD4NO R E S E A R C H N A T I O N A L R E S E A R C H C O Y N C I L 4 O T T A W A 1 C A N A D A

MOISTURE CONTENT O F EXPOSED P A I N T E D

WOOD P A N E L S

D. C. Tibbetts and D.

R,

Robson

T h e extensive u s e of f r a m e construction in the Atlantic Provinces

of Canada makes the sabject o f paint performance on wood s i d i n g s of particular interest in that r e g i o n . The problem of p e e l i n g is r e l a t e d

t o the p r e s e n c e of water but the source of the w a t e r is not particularly w e l l under stood. h April 1 9 6 1 one of the authors conducted a detailed

examination af the exteriors of 278 wood c l a d h o u s e s in the region.

T h e s e and subsequent studies on other houses of similar age and con- struction type suggested that paint failure by peeling is more probably due to exterior weathering factors than t o the condensation of m o i s t u r e

o r i g i n a t i n g behind the paint film. The most serious peeling was observed to occur most often on w a l l s e x p o s e d t o wind-driven rain. T h e r e was

some evidence that roof overhangs p r o t e c t e d t o p portions of walls but building orientatton remained the m o s t important f a c t o r .

Generally, north walls weT e free f r o m the t y p e of f a i l u r e common

to other exposures. The most s e v e r e paint: failures o c c u r r e d on s o u t h and east walls. ( M o s t wind-driven r a i n in the region comes f r o m southerly d i r e c t i o n s . ) Even on unheated unoccupied buildings having the same exposures f a i l u r e by peeling has been observed. It w a s

d e c i d e d to investigate how r a i n and outdoor humidity affected the r n o i s t u r e

content of wood behind painted surfaces. Any effect humidity in w a l l spaces and heat loss from typical occupancies might have on d r y i n g of w e t t e d w o o d siding from the back w a s ignored.

Accordingly, twelve panels, m a d e of the s a m e wood s i d i n g b u t applied in three different methods, w e r e painted and i n s t r u m e n t e d s o

that the wood m o i s t u r e contents could b e recorded r e g u l a r l y . Panels of each sf the three types w e r e exposed outdoors in f o u r directions a t

the DBR J N R c _{Regional Station in Halifax, Nova Scotia, in April 1965.}

DETAILS

OF

PANELS AND SENSING ELEMENTS

Bevelled c e d a r siding was applied 5S inches to the weather over

3

z-inch fir plywood on 2 - by 3 _{-inch s t u d s for a panel width} of 1 6 inches. A l l structural elements of the panels, including the sheathing, w e r e painted before the siding was applied, Ther e _{w e r e t h r e e methods of}

s i d i n g application but only two s t r u c t u r a l types of panels. Of t h e

and s i d i n g ,

No.

2 h a d the siding applied directly to the sheathing ( over # 1 5 asphalt sheathing paper ), a n d No. 3 w a s t h e s a m e as No. 2

except the s i d i n g was backprimed. Panels were assembled in s e t s of

three before exposure by bolting t o a common d r i p cap and base s t r a p ( F i g " l ) -

Following its application the siding w a s painted indoors w i t h a good quality oil type p r i m e r and t w o topcoats for a 4 to 5 millimetre film thickness. Caulking was done with a one part sealant after the

p r i m e r h a d d r i e d ( F i g . 2

1.

Sensing pins were a s s e m b l e d for each panel, c a r e being taken t o ensure that the w i r e leads and lucite holders f o r t h e pins w e r e

m o i s t u r e proof { F i g . 3

1.

T h e pins w e r e f i x e d with epoxy r e s i n in the h o l d e r s a n d centered behind the t h i r d piece of siding from the

bottom of each panel, penetrating 118 inch where the s i d i n g was

4

inch

thick.

PANEL ASSEMBLY AND INSTRUMENTATION

The c l a d panels w e r e assembled in s e t s of three in an existing f r a m e on the laboratory roof with s i d e s facing north, east, south, and

west { F i g . 4

1.

Leads from the sensing pins w e r e taken through a

w i r i n g harness t o connections in a waterproofed junction box ( Fig.

5

) ; additional l e a d s w e r e taken from the box t o the selector switch of a resistance t y p e of moisture meter i n s i d e the building. Because the pins w e r e llpermanently'r installed, meter voltages w e r e only applied

long enough to obtain- individual readings. A check w a s made f o r p o s s i b l e t h e r m a l effects on a typical installation subjected to the

anticipated temperature r a n g e , P e r i o d i c checks with a portable m e t e r

w e r e made during the exposure period. AT1 m e t e r e d readings w e r e

corrected f o r temperatures obtained simultaneously w i t h thermocouples

installed in the panels using the chart in ASTM D2Ok 6 - 6 5 ( 1 ).

During the twelve-month exposure period that began on I A p r i l

1955, the wood moisture contents were r e c o l - d e d o n l y once a week. The possibility of recording automatically was investigated, and in consulta- tion with a company specializing in this t y p e of equipment, instrumenta- tion involving a meter, timer, and chart recorder was developed t o

record the rnoistur e content o f each panel at preselected intervals of one hour to one day { Fig. 6 _).

In

July 1966 the automatic equipment was put into operation ; i t took readings every six hours, coinciding wiih the t i m e s that t e m p e r a t u r e s

and humidities w e r e observed by the local Meteorological Station.

Initially, r e a d i n g s were taken m o r e frequently than this, d u r i n g p e r i o d s

of heavy rains, t o observe t h e r a t e s of change in moisture content.

The study consisted of t w o phases. During Phase

I

moisture contents w e r e obtained

by

manual switching an a weekly b a s i s f a r twelve

months beginning April 1965. During P h a s e

IF,

m o i s t u r e contents w e r e automatically recorded every six hours far twelve months beginning July

1966.

During this Latter p e r i o d , weekly readings w e r e a l s o taken

manually u n t i l 19 December 1966.

Rain cups on the frame provided the rainfall amounts noted in

this r e p o r t . Information on rainfall, outdoor temperature and

humidity w a s p r o ~ i d e d by the Shearwater Naval A i r Station, 4% miles e a s t and I: miles south of the panel installation.

OBSERVATIONS FROM PHASE

I

F o x the twelve-month period beginning 1 April 3

965,

m e t e o r o -

logical data a n d rain cup measurements were compiled ( F i g . 7

3 .

Shown in

this graph a r e the equilibrium moisture contents (EMC) based on relative

humidity and in accordance with established tables { 21, the plotted

humidities and a bar graph to illustrate the monthly rainfall totals, N o

attempt was made ta correlate raincup cofiections with meteorological totals but in general the variations over the period are similar.

Monthly averages ef wood moisture contents, based on weekly readings recorded from the three panel t y p e s for four exposures, w e r e

plotted in association with the relative humidities for the same times

F i g 8 . It is apparent that the vented panel

(No.

1 ) attained a higher m o i s t u r e content f o r all exposures than did the non-backprimed ( N o . 2 )

a n d the baclcprimed ( No. 3 ) types in that order.

Table I relates panel t y p e s to exposure in decreasing moisture

contents. For this location the n o r t h exposure is, for all panel types, the one with the lowest wood moisture content as the result of wind-driven rain, W i t h the exception of a drying period during F e b r u a r y 1 9 6 6 t h e r e appeared to be a progressive accumulation of wood rnoistur e content over

the exposure p e r i o d .

The graphs in Figures 9 and 10 show the raincup measurements for the four exposures and the related directional effects. They also show how the d i r e c t readings of wood moisture content differ f r o m the

EMC

calculated from relative humidity records ; although the patterns

a r e _{similar, the table-derived EMC are higher than the measured values}

DISCUSSION

OF

PHASE I

Raincup totals confirm f i e l d observations in demonstrating that

most of the wind-driven rain in this region comes from southerly directions. T h e r e w e r e s o m e obvious departures f r o m this over the

exposure p e r i o d : in June 1965 most rain came frorn the north and e a s t in medium amounts, and in July 1965, when maximums w e r e low for a l l exposures, rain w a s f r o m the w e s t .

Although the open r a i n screen effect of a vented panel may, in theory, prevent r a i n in association with wind pressure frorn penetra-

ting the cladding, it is evident that the vented panels contained m o r e m o i s t u r e for all exposures over the entire period than did t h e other t y p e s . This may be due to the combined effects of surface wetting

and simultaneous gains from relative humidity increases at the back surface. T h e

higher

moistulr e contents of the non-backprimed type suggests that backpriming the siding is beneficial.

It

is a s s u m e d that h c r e a s e s in wood moisture during a r a i n

are due to w a t e r penetrating the paint film. A s an added precaution

a n d to ensure that rain from one direction did no^ penetrate the back of

panels opposite ( although this is improbable due to the t h i c h e s s of the plywood sheathing which was _{painted both s i d e s ) , the backs}

_of

_{all panel}

a s sembkies were covered with plastic.

W e e k l y readings do little to explain rates of wetting or drying during b d immediately following

a

rain. They serve a useful purpose, however, in indicating cumulative effects during prolonged p e r i o d s of

r ainf all.

The backprimed panels ( No. 3 ) had the l o w e s t moistux e _content

for all exposures. Although the vented p a n e l s ( No. 1 ) had the highest

content for south a n d east exposures, the other t w o types w e r e not substantially influenced by exposure, The lowest moisture gains in

a l l panels w e r e in those exposed to the north

-

substantiating previous f i e l d studies of houses.

OBSERVATIONS

FROM PHASE I 1

T o

complete the rather limited picture produced by the r e a d i n g s

taken rn anually each week, automatic r e c o r d i n g equipment was connected to the panel sensing elements in J u l y 1 9 6 4 , and was checked by the o r i g i n a l

method until

I 9

December

1966.

The automatic equipment was kept

in service for an additional. twelve months ( u n t i l July

1968).

Although this paper refers only t o r e c o r d s c o l l e c t e d d u r i n g one year by the

automatic method, those taken during a second year r e v e a l e d a similar

pattern when weather variations a r e taken into account and gave no

evidence that paint weathering had rnater ially influenced rnoistur e r e a d i n g s . This s t u d y is not concerned with paint performance but

r a t h e r with wood m o i s t u r e contents behind t h e film, but it m a y be

noted that there w e r e no obvious d e f e c t s in the film after three y e a r s .

From records taken every six hours d u r i n g May 1 9 6 7 , a graph

i l l i u s t r a t h g moisture variations in the south-exposed vented panel w a s prepared ( Fig, 1. I

1.

In

addition to the graph of relative humidity for that p e r i o d ,

the

amount of rainfall ( in inches ) is noted a n d appears to relate directly t o maximum wood rnoistur e _{contents. Moisture contents}

increased very rapidly during a r a i n and decreased at about the s a m e

rate when the rain stopped. It is not k n o w n whether the r e c o r d e d decreases are due to moisture leaving the panel o r , in p a r t , to moisture redistri- bution within the panel causing lower r e a d i n g s at the sensing elements.

Weekly averages b a s e d on r e a d i n g s recorded every six hours

were plocted for all panels f o r July 1956 to J u l y

1967,

along with the

r e l a t i v e humidities and rnonthly rainfall totals for the s a m e p e r i o d .

The graphs in Figures 12, 13 and 14 s h o w the relative humidities and

the wood m o i s t u r e content for each panel over this period.

DISCUSSION O F PHASE

I1

Examination of moisture contents recorded at; six-hour intervals indicated that many variations occur which could not b e anticipated from

w e e k l y observations. Significant changes were observed over short

p e r i o d s of time ; f o r example, the relative humidity i n c r e a s e d from 6 5

to 100 p e r c e n t , and with less t h a n

2

inch of rain, the moisture increased from 7 t o 2 0 p e r c e n t within a twelve-hour period. Generally a relative humidity of 100 percent d i d not cause t h e _{moisture content to exceed}

20 percent unless there w a s also rain. Increases and decreases in

m o i s t u r e content of 10 percent in twelve hours w e r e common. Although

weekly readings i n d i c a t e d cumulative effects over a twelve -month p e r i o d , they provided no indication of the m a n y cycles of wetting a n d drying within that period,

W h e n the weekly averages based on d a i l y r e a d i n g s for the vented panel are compared with the daily averages for the same period, the

variations a r e levelled out to such an extent as to eliminate any indication

of m o i s t u r e contents exceeding 1 5 percent. As might be anticipated,

there i s very l i t t l e indicated lag between humidity changes and changes in wood m o i s t u r e content,

Although the weekly averages based on daily r e a d i n g s a r e m o r e d e t a i l e d than the weekly r e a d i n g s observed in _{Phase I, an appreciation} of the extremes of change in reflecting what actually happens over short t i m e p e r i o d s depends on an examination of r e c o r d s obtained every six hours or at even shorter intervals.

As in Phase I, the vented panels had the highest m o i s t u r e

content followed by the non-backprimed and backprimed t y p e s respectively, but in the second Phase, only small differences could be observed t o

relate to orientation. Maximum values for l i m i t e d periods o c c u r r e d far south and w e s t exposures,

G

_ONC

_LUSION

In general, the panels exposed in a southerly direction had the

highest moisture content, and those facing north had the lowest m o i s t u r e levels. The vented panels had t h e highest moisture contents for all exposures followed by the n on-backprimed and the backprimed panels in descending order.

Measured wood m o i s t u r e contents rarely exceeded 2 0 p e r c e n t ,

except for short periods when high relative h u m i d ~ t i e s w e r e accompanied b y rain. It was observed from readings acquired every six hours that substantial increases and decreases could occur in less than twelve hours.

Average wood moisture contents f o r one-year p e r i o d s departed

very little from f i e l d measurements that had been made in April 1 9 6 1 when over 500 m o i s t u r e r e a d i n g s were taken from sidings of 278 houses in the

region. T h e majority of field readings w e r e less than 1 5 percent buc moisture contents near butt joints, window details, and corner boards

w e r e found t o be

XO

to 1 2 percent higher than at other locations on the walls.

This study of panels exposed outdoors suggests that weekly readings provide insufficient detail. when attempting to relate frequency and r a t e

of change of wood moisture contents to relative humidity and rainfall.

It

is as surned that wetting

by

r a i n is due to penetration of the paint f i l m , and although rapid lncreas e s c a n be noted during a rainfall, it is possible that similar d e c r e a s e s that occur when the rain stops might be due in part t o moisture redistribution.

The p r o b l e m s of m o i s t u r e redistribution, he depth of p e n e t r a t i o n

while there is a continuous film of water over the face of the panels, and

the possibility of indoor m o i s t u r e and temperature having an influence on the moisture content of the siding a r e under review f o r detailed investiga- tion at the Halifax Station,

REFERENCES

I . American Society for Testing and Materials, 1965. Standard Methods

of

Test for Moisture Content af Wood. ASTM

D2016-65.

2. Canadian Woods; Their Properties and U s e s . Second Edition, May

3951. Forestry Branch, F o r e s t Products Laboratories Division. Queen" Printer, Ottawa, Canada.

Figure 1. Front view of panel assembly be

-

f a r e siding application

-

furred panel at

left.

Figure 2. Caulking at ends of siding

-

aver prirne coat.

Figure 3. Moisture sensing assembly.

Figure 4. Completed assembly shewing N o r t h and W e s t exposure

-

r a i n cups above panels.

Figure

5.

Wiring harnes s for assembled panels

-

waterproof junction box at

top.

Figure

6 .

_{Automatic metering} and record-

100 % A H

,

OUTDOOR

F I G U R E

7

E Q U I L I B R I U M MOl S T U R E C O N T E N T S , R E L A T I V E H U M I D I T I E S A N D I N C H E S OF R A I N F A L L FROM D O T W E A T H E R R E C O R D S , A N D R A I N F A L L F O R F O U R E X P O S U R F S

F R O M

R A I N C U P S I N S T A L L E D A T P A N E L L O C A T I O N .

(1) Vented - N o t B a c k p r i m e d (3) Not V e n t e d A M J 1 A S O N D J F M (2) N a t V e n t e d - N o t B a c k p r i m e d - B a c k p r i m e d l l l l l I 1 1 1 1 t 1 W E S T -

-

- 7

-

- - -

-

-

-

A M J A S O N D J F M F I G U R E 8

M O N T H L Y A V E R A G E S O F WEEKLY MOI S T U R E R E A D I N G S FOR T H R E E PANEL T Y P E S W I T H FOUR

E X P O S U R E S , A M I) R E L A T I V E HUM1 D l T Y R E A D I N G S .

2 Z F Z

=

+ v w - * * o w r r 4 w l m r/l?cc m m a = m r n e MC,

Q,

C O R R E C T E D F O R TEMP. )Ir CI v rn 0 w w R A l N

C U P ,

ML

MC,

S, C l O R R E C T E D FOR TEMP. R A l N C U P , ML

(1) V e n t e d - N o t B a c k p r i m e d (2). N o t V e n t e d

-

N o t B a c k p r i m e d (3)

Not

V e n t e d - B a c k p r i m e d (4) EMC

-

D e r i v e d f r o m Weather R e c o r d s (5) R a i n C u p T o t a l s

-

M o n t h l y T h i s E x p o s u r e M O I S T U R E R E A D I N G S F O R W E S T

A N D

S O U T H E X P O S U R E S

F O R

T H R E E P A N E L T Y P E S W I T H R E L A T I V E MUMIDTTY R E A D I N G S A N D R A I N C U P T O T A L S .

D A Y S

1 P A N E L

-

S O U T H

MAY

1967

F I G U R E

11

HUM1

D I T Y

A N D

R A I N F A L L D A T A FROM D O T

WE.ATHER

R E C O R D S ,

H A L 1

F A X

F O R

M A Y

1967,

A N D

THE

M O I S T U R E

C O N T E N T O F

THE

V E N T E D

S O U T H

F A C I N G

P A N E L

B A S E D O N

R E A D I N G S

T A K E N E V E R Y

S I X H O U R S .

u M e r t h E x p o s u r e - 15 A

*

L

F I G U R E 1 2 HUM1 D l T Y A N D RAINFdLL D A T A W I T H W E E K L Y A V E R A G E M O I S T U R E CONTENT B A S E D ON D A I L Y R E A D I N G S F O R F O U R E X P O S U R E S O F V E N T E D P A N E L S (NO. P I .

I I l l I I I 1 3 I l l 1 N o r t h Exposure I - I I E I 1 1 I I I I I I I I I I I I I 1 1 1 I 1 1 1 - E a s t E x p o s u r e

-

'L I 1 1 1 I I I I l l I I I I - 25 _{1 1 1 1 I 1 1 1 1 1 7 1} - u S o u t h E x p o s u r e

-

15 Be - 5 I 1 1 1 I I I I l l I t I I F I G U R E 13 HUM1 D I T Y AND R A I N F A L L D A T A W I T H W E E K L Y A V E R A G E M O l S T U R E CONTENTS B A S E D O N D A I L Y R E A D I N G S FOR FOUR E X P O S U R E S O F N O N - B A C K P R I M E D PANELS ( N O . 2). BR 4385-7

25

-

,

_{i l l I I I I l l 1 I t 1 -} - u N o r t h E x p o s u r e + 15

*

West Exposure -

2

15 A z!9 51 I I 1 1 I I I l l I I l i J A S O N D J F M A M S J A H U M I D I T Y A N D R A I N F A L L D A T A W I T H WEEKLY A V E R A G E M O I S T U R E C O N T E N T S A B S E D ON D A I L Y R E A D I N G S FOR F O U R E X P O S U R E S O F B A C K P A I MED P A N E L S (NO. 31. BR I J 8 5 - B

Table 1 .

-

Panels related to direction of exposure in order bf decreasing moisture content.

Panel Type Vented Won-backprimed Backprimed No. I 2 3 '0 Expos ed Dir ection

South-East-West-North

West-East-Sauth-North West-South-East-North