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Seasonal moisture variation in wood siding and framing: a case study
Ser TRI.
I392
ISSN 0701-5232
SEASONAL
MOISTURE VARIATION IN 11100D S I D I N G ArVD FRAPIING: A CASE STUDYby
D.L. Scott*
ABSTRACT
The seasonal variation in moisture content f o r several wood frame wall
components is recorded f o r a building located near Halifax, Nova S c o t i a ,
Conclusions are drawn regarding t h e p o t e n t i a l Ear deterioration, the
beneficial effecrs of protecting wood from exposure to weathering elements
and t h e importance of b u i l d i n g d e t a i l i n g .
*
Atlantic Regional Station, D i v i s i o n o f B u i l d i n g R e s e a r c h , E a t i o n a l R2so2rc> C o u n c i l Canada, H a l t f a x , Nova S c o t i a .INTRODUCTION
Thts
report documents a s i t e investigation o f m o i s t u r e levels in theexterior
wall
components o f a r e c e n t l y constructed two-storey h o u s e l o c a t e dnear
HaLifax, Nova
Scotia. The owner's initial i n q u i r y r e l a t e d t o problemswith maintaining a clear c o a t i n g on cedar s i d i n g , which in turn led to a
concern with moisture in the exterior w a l l elements. Maritime weather
conditions have long been known to contribute to t h e m o i s t u r e balance In
t ~ a l l s . This case study gives one example of the relationship between moisture sources, building details and drying potential of wood frame assemblies.
CONSTRUCTION SYSTEM
The b u i l d T n g was completed in 1981 u s i n g a p r o p r i e t a r y i n s u l a t e d wall system ( F i g . 1) and a conventional roof w i t h o u t overhangs. The stud w a l l
c o n s i s t s o f a 127 mm
(5
in.) t h i c k c o r e of p o l y s t y r e n e placed between 38 x1 4 0
mm
(2
x 6 in.) t o p and bottom plates, The core is notched on 400 mm( 1 6 in.) c e n t r e s to accept 38 x 8 9 mm ( 2 x 4 in.) studs on t h e i n t e r i o r face
and 1 9 x 6 4 mm (1 x 3 in.) s t r a p p i n g on t h e exterior. Since t h e core is 127 nm ( 5 in.) thick,a 12.5 m (% in.) air gap is created behind the 0.10 mm
( 4
mil) vapour barrier and i n t e r i o r d r y w a l l , On t h e exterior, No. I 5 a s p h a l t b u i l d i n g p a p e r was a p p l i e d t o g e t h e r w T t h 11 x 184 mm (1 x 8 in.) cedarbevelled siding exposed
165
mm ( 6 %in.)
to t h e weather. The exterior c o a t i n g consists of a p e n e t r a t i n g wood preservative followed by two coats of a w e a t h e rr e s i s t a n t n a t u r a l wood
f i n i s h .
The b u i l d i n g w a s t e s t e d f o r airtightness i n t h e spring af 1983 and found
t o have 2 . 4 air changes p e r hour at a p r e s s u r e difference of
50
Pa,i n d i c a t i n g better than average airtightness f o r new construction. M o s t of t h i s Leakage w a s r e p o r t e d to be at t h e sill plate and artic h a t c h d e t a i l s , Over a two-week p e r i o d in March, insfde temperatures were m a i n t a i n e d a r o u n d 2 1 ' ~ ( 7 0 ' ~ ) w i t h relative h u m i d i t y o f 4 5 5 5 % . These would be c o n s i d e r e d normal spring conditions in a residential b u i l d i n g l o c a t e d in a maritime
climate.
OBSERVATIONS FROM I N I T I A L S I T E VISITS
During an initial site visit in August 1982, siding moisture content
r e a d i n g s ranged between 12 and 18%. A short p i e c e of s i d i n g removed on the south wall revealed free water on t h e surface o f t h e a s p h a l t paper; t h e
moisture content of t h e framing on t h e southeast corner exceeded 22X. Beads o f water hung f r o m t h e butt edge of the siding on the north w a l l and water
a p p e a s e d to b e moving up between the l a p j o i n t s in t h e sid-ing by c a p i l l a r y a c t i o n .
There
w a s evidence of simflar moisture beads on the other exposuresb u t these had d r i e d , l e a v i n g a r e s i n - l i k e powder t h a t could b e removed easily
with a light wiping. These observations followed a two-month period d u r i n g
which precipitation was
40-602
above normal.To determine if the siding system w o u l d dry over t h e winter and s p r i n g ,
the D i v i s i o n of B u i l d i n g Research recommended a moisture content monitoring
MOISTURE
MONITORINGThe
moisture content of sections of corner f r a m i n g , s t r a p p i n g and s i d i n gwas m o n i t o r e d following i n s e r t i o n of m o i s t u r e p i n s . Pairs of p i n s 50 mm
( 2
in.)
l o n g and epoxy coated except f a r t h e i r t i p s , allowed m o i s t u r e contentr e a d i n g s to be taken a c c o r d i n g te the d e p t h they were d r i v e n i n t o the
assembly. The p i n s were sealed at; the surface of the s i d i n g w i t h silicone r o
avoid water entry along the p i n . Moisture r e a d i n g s w e r e taken by the owner
with a Delmhorst resistance t y p e meter an a daily b a s i s , with a few
e x c e p t i o n s , for the p e r i o d November 1982 to June 1983, except f o r a p e r i o d
when t h e meter was n o t functioning p r o p e r l y . R e a d i n g s taken i n July indicated
a continuing drying trend.
SUMHART
AND
ASSESSMENT O F MOISTURE CONTENT DATAEight o f t h e f a u r t e e n p o i n t s monitored were chosen as representative of
the m o i s t u r e c o n t e n t at the selected l o c a t i o n s . The m o i s t u r e c o n t e n t readings
at these stations were plotted for t h e p e r i o d November 1982 to J u l y 1983
(Figs. 2 to 9 ) . A general trend line h a s been sketched that approximates the
averages of r e a d i n g s taken. The arithmetic average of moisture r e a d i n g s for
each month w a s t h e n a d j u s t e d on the b a s i s o f t h e mean ambient temperature f o r
the month and the t i m e of day. I n general this adjustment moved t h e moisture
c o n t e n t upwagd, s i n c e
t h e
ambient temperature for most readings waslower
t h a nthe 21°c ( 7 0 F) calibration temperature for t h e meter. C o n s i d e r a t i o n should
b e given, however, to t h e dynamic environment in which a wall component
operates. The effect of sun on the wall, or heat b r i d g e s from t h e i n t e r i o r
d u r i n g c o l d weather, could increase the temperature o f t h e component and modify t h i s a d j u s t m e n t .
Conditions necessary for the deterioration o f wood t h r o u g h rotting have b e e n
dealt
w i z h in detail elsewhere1. O f s p e c i a l interest are the conditionso f moisture and temperature of the material. Wood r o t t t n g fungi do not
germinate on wood t h a t h a s a moisture c o n t e n t below t h e f i b r e saturation
p o i n t , commonly reached at around
25
to 30%. Wood cannot be c o n s i d e r e d immunefrom rot: until t h e m o i s t u r e c o n t e n t is below abgut 2 0 % . O p t i m u m temperature
conditions for f u n g i growth range f r o m 18 t o 35 C ( 6 5 ta 9 5 # ~ ) , w i t h l i t t l e or
n o growth at temperatures slightly above freezing.
The
m o i s t u r e data was d i v i d e d i n t o three s e t s : corner framing, strapping and t h e s i d i n g .CORNER FRAMING
The v a r i a t i o n in moisture content f o u n d i n e x t e r i o r corner framing is shown i n Figs. 2 to 4 . The highest ~ o i s t u r e content was found i n the n o r t k
e a s t c o r n e r in early w i n t e r , when adjusted values up to 2 4 % were r e c o r d e d . While t h i s v a l u e i s c l o s e to f i b r e saturation, the mean temperatures during
0
the winter period o f November to February ( 6 , 1 to -3,9
C
or 4 3 to 2 5 O ~ ) arewell b e l o w the optimum f o r d e t e r i o r a t i o n . The corner d e t a i l would b e more responsive to o u t s i d e cycles of moisture as a r e s u l t of b e i n g exposed on t w o a i d e s . T h e butt ends of t h e cedar s i d i n g also appear to have wicked in
moisture despite a good caulking job, and t h e b u i l d i n g paper, l a p p e d d o u b l e
t h i c k n e s s around the c o r n e r s , may store this water for r e - d i s t r i b u t i o n i n t o
the corner framing o r
back
out into the siding at a later time.Since cons5derabfe d r y i n g does take p l a c e through t h e spring p e r i o d (down t o 10 to 13%), it would appear that these framing members are not l i k e l y to deteriorate.
STRAPPING
The e x t e r i o r strapping was monirored on t h e west and s o u t h sides of the house ( F i g s . 5 and 6 ) . The m o i s t u r e c o n t e n r of the material increased f r o m
Rovember t o January and then d e c l i n e d steadily u n t i l last measured in July
1983. Peak adjusted moisture l e v e l s on the w e s t s i d e exceeded 26%
in
January; however the s t r a p p i n g w o u l d not be expected to deteriorate at prevailingw i n t e r temperatures. S t r a p p i n g in t h e s o u t h wall remained well below the
fibre saturation point over the e n t i r e period.
The cedar siding was monitored for moisture content at: a number af
locations. Readings are p r e s e n t e d f o r an area p r o t e c t e d f r o m the weather n e x t t o t h e main entry ( F i g . 7) and f o r t w o locations exposed to t h e south
( F i g s . 8 and 9 ) .
The m o i s t u r e content of the p r o t e c t e d s l d i n g varies between 12.5 and 13.5%, representing an equilibrium moisrure content w i t h a i r of
65-70%
relative humidity. Since outside air at the building location averages a b o u t
80X relative humidity, an equilibrium moisture c o n t e n t o f
16%
m i g h t beexpected.
The
difference in moisture c o n t e n t r e c o r d e d may represent areduction in absorption that could
be
attributed ta the unweathered exteriorc o a t i n g .
S i d i n g exposed to the rain ( F i g s . 8 and 9 ) showed peak a d j u s t e d r e a d i n g s a f j u s t under 19%. S i d i n g that was wedged o f f at t h e b u t t s to p r e v e n t
c a p i l l a r i t y (Pig. 9 ) had a h i g h e r moisture content t h a n adjacent s i d i n g left with the j o i n t s t i g h t ; t h i s may result from higher i n g r e s s of w i n d - d r i v e n
rain.
CONCLUSIONS
I.
Most exterior elements s t u d i e d showed relatively h i g h moisture c o n t e n t t h r o u g h t h e w i n t e r but effective d r y i n g d u r i n g the s p r i n g . Deterioration w o u l d not be expected to r e s u l t since the period o f high moisture c o n t e n tc o i n c i d e s w i t h low w i n t e r temperatures.
2.
M o i s t u r e c o n t e n t r e a d i n g s d i d n o t a p p e a r t o b e influenced b y o r i e n t a t i o n o f t h e w a l l .3 . The b e n e f i c i a l effect of protecting s i d i n g from direct exposure to
exposed elements s u p p o r t s t h e c o n c l u s i o n t h a t rain, s u n and w i n d have h a d a substantial i n f l u e n c e on t h e m o i s t u r e conrent of t h e e x t e r i o r components studied.
4 . Attempts to limit t h e ingress o f water b y c a u l k i n g j o i n t s are not always
successful.
In
some cases it may b e m a r e beneficial t o d e s i g n a j o i n t t h a t w i l l d r a i n and allow a i r circulation at butt j o f n t s to a s s i s t in d r y i n g , r a t h e r than to rely on surface caulking.Baker, M.C. Decay of Wood. Canadian B u i l d i n g D i g e s t No. 111, D i v i s i o n
of Building Research, National Research C o u n c i l Canada, Ottawa,
12.7 mm (112 In.) DRYWALL
-0.10
rnm
(4
m i l l VAPOUR BAERBER
12.5
mrn
(112 in.)
A I R
SPACE
38
x
89
mrn
(2
x
4
in.) STUDS
19
x 64 mrn
I1
x
3
in.)
STRAPPING
127
man
(5
in.)
P3LYSTYRENE
NO.
14
ASPHALT
PAPER
IE
x 184 mm
El
x 8
in.)
S I D I N G
F I G U R E
1
8
N O V M C JAN FEB M A R APR MAY JUNE JULY
M.C. READING m SPOT READING NOVEMBER 1962 - J U N E 1783 F I G U R E 2 N O R T H E A S T CORNER P O S T
8
NOV OEC JAN FEB MAR APR M Y LINE JULY
NOVEMBER 1782 - JUNE 15'83
--- M.C. READING ADJUSTED FOR TEMPERATURE
- AVERAGE M.C. READING
SPOT READING
FIGURE 3
8
NOV DEC JAN FEB M A R Am M Y JUNE JULY
I T ' ' I ' I I I I I I I
--- M.C. READING ADJUSTED FOR TEMHRATURE -
AVERAGE M.C. READING - - SPOT READING
4
3 1 I I I I 1 L I F 1 ... N O V E M B E R 1 9 0 2 - JUNE 1983 F I G U R E 4 SOUTHEAST C O R N E R P O S T 8NOV DEC J A N F E B M A R A m M Y JUNE JULY
JEMAWTURE - AVERAGE M.C. READING 5 SPOT RMPlNG NOVEMBER 1 9 8 2 - J U N E 1983 F I G U R E 5 W E S T S T R A P P I N G
8
NOV DEC JAN FEB MAR APH MAY JUNE JULY
N O V E M B E R 1 9 8 2 - J U N E 1983 AMRAGE h.C. READING m SPOT READING FIGURE. 6 SOUTH S T R A P P I N G 8
NOV DEC JAN FEB MAR APR M A Y JUNE AULY
I I ! I I 1 I t I I
- --- M.C. READING ADJUSTED FOE TEMPERATURE
AVERAGE M.C. READING - - - - - -
-
_ _ - -
-
---___
- - t SPOT REAOtRlG I L 1 F I I I L I I NOVEMBER 1982 - J U N E 1 9 8 3 P R O T E C T E D S I D I N G-- - - -- M . C . REAOlNG ADJUSTED FCR TEMPE=TURE
-
AVERAGE M . C . READING w E 3 C 2 16 0 3 2 *-
! L SPOT ZEADING 8 I I I 1 . J 1 INOV DEC J A N FES M A R APF! M A Y AUNE JULY
N Q V E M I E R 1 9 8 2 - J U N E 1783
FIGURE C
S O U T H S I D I N G (NOT V : E D G E D )
S
NOV GEC JAN FEa k A " AP? M Y JUNE JULY
t I
--- FE..C. READiNG ADJUjTEO FCL TEMPEMTURF
