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The use of nuclear moisture density meters to measure moisture in flat
roofing insulations
Hedlin, C. P.
https://publications-cnrc.canada.ca/fra/droits
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THE USE OF NUCLEAR MOISTURE DENSITY METERS TO MEASURE MOISTURE IN FLAT ROOFING INSULATIONS
Charles P. Hedlin
Division of Building Research, P r a i r i e Regional Station National Research Council of Canada, Saskatoon, Saskatchewan
Canada S7N OW9
.
ABSTRACTCommercial nuclear soil moisture-density m e t e r s were used t o m e a s u r e m o i s t u r e content of t h e r m a l insulation in flat roofs and t o m a p wet a r e a s . Variations i n bitumen and g r a v e l thickness and moisture distribution cause uncertainty in t h e s e measurements. In field t e s t s the standard e r r o r of estimate w a s 50 kg/m3 f o r moisture contents up to 250 kg/m3 and 100 kg/m3 for m o i s t u r e con- tents up to 400 kg/m3. Grid surveys often clearly defined wet a r e a s .
On a employ6 d e s appareils nucleaires commerciaux de m e s u r e de l a densit6 et d e 11humidit6 d e s sols afin de m e s u r e r l a teneur en humiditk d e llisolation thermique d e s toits plats e t d e d6finir l e s surfaces humides. Des variations d e 116paisseur du biturne . e t du g r a v i e r e t de l a rkpartition de llhumiditk ren- dent de t e l l e s m e s u r e s incertaines. L 1 e r r e u r - type dlkvaluation dans l e s e s s a i s s u r place e s t de 50 kg/m3 pour une teneur e n humidit6 allant jusqul'a 250 kg/m3 et de 100 kg/rn3 pour une teneur en humiditk allant jusqul'a 400 kg/m3. L e s
relev6s p a r quadrillage fournissent souvent une d6finition c l a i r e d e s s u r f a c e s humides.
2
-
Hedlin IntroductionWhen p r o b l e m s occur in f l a t roofs, t h e building owner m u s t de- cide whether complete r e p l a c e m e n t i s needed o r whether p a r t o r a l l of t h e roof c a n be salvaged. The d e c i s i o n m a y depend on the condition of t h e t h e r m a l insulation. If wet, i t will not provide the r e q u i r e d r e s i s t a n c e t o h e a t flow and t h e m o i s t u r e m a y con- t r i b u t e t o f u r t h e r d e g r a d a t i o n of the roofing s y s t e m o r l e a k into t h e building. E f f o r t s t o e s t i m a t e the m o i s t u r e content of i n s u l a - tions in conventional f l a t r o o f s a r e h a m p e r e d because i t i s hidden f r o m view by t h e m e m b r a n e on top and by t h e s t r u c t u r e on the u n d e r s i d e . Cutting into t h e m e m b r a n e g i v e s a useful but l o c a l - ized view of t h e insulation but naturally, t h i s o r any o t h e r method t h a t d a m a g e s t h e m e m b r a n e i s viewed with concern by i n t e r e s t e d p a r t i e s .
C o n s i d e r a t i o n h a s r e c e n t l y been given t o t h e use of n u c l e a r m o i s - t u r e - d e n s i t y m e t e r s , n o r m a l l y used on s o i l s , to identify w e t roof a r e a s and t o e s t i m a t e t h e amount of m o i s t u r e p r e s e n t . The i n s t r u m e n t r e s t s on t h e roof s u r f a c e and i s used in t h e b a c k - s c a t t e r m o d e , hence t h e m e t h o d i s n o n - d e s t r u c t i v e . A n u c l e a r s o u r c e d i s c h a r g e s n e u t r o n s which a r e slowed down when t h e y e n c o u n t e r hydrogen. I n t h i s s t a t e t h e y c a n be detected and counted. The n u m b e r of s u c h counts i n c r e a s e s with the hydrogen content in the f i e l d of view of the i n s t r u m e n t . Because of i t s hydrogen content, w a t e r c a n be detected by i t . The a c c u r a c y of t h e method i s affected by t h e f a c t t h a t i t i n d i s c r i m i n a t e l y s e n s e s hydrogen in a l l m a t e r i a l s , including t h a t i n t h e bitumens.
E x p e r i m e n t a l
A feasibility study of the method was undertaken at the P r a i r i e Regional Station of the Division of Building R e s e a r c h . National
3 - Hedlin
R e s e a r c h Council of Canada, a t Saskatoon. T h i s included l a b o r - a t o r y and field s t u d i e s of i n s t r u m e n t s e n s i t i v i t y t o m o i s t u r e i n s i m u l a t e d and r e a l f l a t roofing s y s t e m s .
T o i n t r o d u c e a d e g r e e of g e n e r a l i t y into the r e s u l t s , each r e a d - ing w a s converted f r o m an i n s t r u m e n t count into a fictitious w a t e r concentration using the m a n u f a c t u r e r ' s c a l i b r a t i o n data. The c a l i b r a t i o n v a l u e s w e r e b a s e d on usage with s o i l s and w e r e not intended f o r u s e i n studies of roofing m a t e r i a l s . As the i n s t r u m e n t r e s p o n d s t o a l l hydrogenous m a t e r i a l s i n the roof, t h i s value d o e s not r e p r e s e n t a b s o l u t e m o i s t u r e content. How- e v e r , i t d o e s give a u s e f u l b a s i s on which t o r e p r e s e n t m o i s t u r e v a r i a t i o n . It i s r e f e r r e d to h e r e a f t e r a s w a t e r equivalent (w. e . ) , kg/m3.
L a b o r a t o r y Studies
In the laboratory, a v a r i e t y of deck types and top coverings w e r e combined with i n s u l a t i o n s at s e v e r a l m o i s t u r e c o n t e n t s . Speci- m e n s of p e r l i t e - f i b r e insulation, 0. 6 m 2 x 0.05m t h i c k with m o i s - t u r e contents ranging f r o m d r y t o 650 kg/m3 ( s y m m e t r i c a l l y d i s - t r i b u t e d through the i n s u l a t i o n ) , w e r e placed on s t e e l and con- c r e t e d e c k s and c o v e r e d , in t u r n , w i t h a 4-ply a s p h a l t - o r g a n i c f e l t m e m b r a n e , an equivalent m e m b r a n e c o v e r e d w i t h a l a y e r of roofing g r a v e l O.013m thick and a 0. 05-m-thick l a y e r of c o n c r e t e paving stone. The w a t e r equivalent values d e r i v e d f r o m the i n s t r u m e n t r e a d i n g s w e r e plotted a g a i n s t the i n s u l a t i o n m o i s t u r e contents (Fig. 1). T h e slopes of t h e c u r v e s (which w e r e l e s s than unity in a l l c a s e s ) d e c r e a s e d a s t h e s e p d r a t i o n was i n c r e a s e d by roofing g r a v e l o r t h e paving stone. F o r the s a m e reason, r e a d i n g s v a r i e d depending on w h e t h e r the m o i s t u r e w a s uniformly d i s t r i b u t e d o r c o n c e n t r a t e d n e a r t h e t o p o r bottom of the i n s u l a - tion.
4 - Hedlin
INSULATION MOISTURE CONTENT,
K q / m 3
FIG. 1
E f f e c t of top c o v e r and insulation m o i s t u r e content on i n s t r u m e n t r e a d i n g I I I I I 1 ,+ M E M B R A N E , . 0 1 3 m /
-
GRAVEL COVER / /+
M E M B R A N E o NO MEMBRANE-
I
I I I 1 I 1To d e t e r m i n e the effect of a c o n c r e t e b e a m on the result,O. 05-m- thick paving s t o n e s w e r e p l a c e d on a c o n c r e t e m e m b e r , 0.25m wide and about O.5m h i g h . , T r a v e r s e s w e r e made with t h e m e t e r a c r o s s the a r e a of the b e a m ( a ) with no insulation and ( b ) with 0.05-m-thick d r y wood f i b r e insulation lying on the paving stone ( F i g . 2). T h e r e s p o n s e w a s significant when no i n s u l a t i o n was used but w a s s m a l l when insulation was p r e s e n t .
F i e l d Studies
F i e l d s t u d i e s w e r e c a r r i e d out in Saskatchewan and A l b e r t a . Through the c o - o p e r a t i o n of a number of building o w n e r s and other i n t e r e s t e d p a r t i e s , m e a s u r e m e n t s w e r e made on r o o f s in good condition and on wet r o o f s .
5 - Hedlin FIG. 2 m E c 7 5
2
*
'-I- S z 5 0 - W J I- a 2 5 Z>
g
5
3 Oa
L a
5
k'
2
Effect of c o n c r e t e bean1 below a 0 . 0 5 - m - t h i c k c o n c r e t e deck on i n s t r u m e n t r e a d i n g I I I I I 0
-
N O INSULATION, 0 0.6-0- "‘ '0. ,O .o' 0 0%-
-
?,'*
/ P a - m 0..0.0-6
o
,
,
0'690.0---4)
9-0-m-m-&l -m-m-
\ 0 . 0 5 r n T H I C K DRY WOOD FIBER
-
INSULATION ON THE DECKC O N C R E T E DECK C O N C R E T E
B E A M
1 I I I I I
I n surveying r o o f s , i t i s common t o e s t a b l i s h a "normal" r e a d - ing t h a t c o r r e s p o n d s t o a d r y condition. Affected a r e a s can be identified by significantly h i g h e r r e a d i n g s c a u s e d by m o i s t u r e . T o obtain a n o r m a l o r base r e a d i n g , a t l e a s t p a r t of the roof m u s t be d r y . I t m a y be t h a t n o d r y a r e a e x i s t s , but experience
s u g g e s t s t h a t t h i s i s not likely t o o c c u r . Usually a number of s i m i l a r , r e l a t i v e l y low r e a d i n g s a r e found. 0 . 3 0 2 01 0 0 . 1 0 . 2 0 3 D I S T A N C E O F I N S T R U M E N T FROM M I D D L E OF BEAM, m Uncertainty a s t o w h e t h e r m o i s t u r e i s p r e s e n t c a n a r i s e because t h e i n s t r u m e n t d o e s n o t d i s t i n g u i s h between t h e v a r i a b i l i t y d u e t o s t r u c t u r a l f e a t u r e s a n d non-uniformity of roofing application and t h a t c a u s e d by m o i s t u r e . M e a s u r e m e n t s w e r e m a d e t o e v a l u a t e t h e n a t u r e of c o n s t r u c t i o n v a r i a b i l i t y .
6 - Hedlin
w e r e s u r v e y e d . Readings w e r e m a d e at r e g u l a r l y spaced i n t e r - v a l s of about 3m. F o r e a c h roof, t h e m e a n value was found and the d e p a r t u r e of e a c h value f r o m the m e a n was calculated. These r e s u l t s w e r e u s e d t o plot a h i s t o g r a m and a frequency d i s t r i b u - tion c u r v e ( F i g . 3). 1 5 0
100
5 00
- 6 0 - 4 0 - 2 00
2 0 4 0 6 080
100
DEPARTURE FROM M E A N WATER E Q U I V A L E N T FOR R O O F ,
~ 9 / r n ~
I FIG. 3
Variation in i n s t r u m e n t reading about m e a n values f o r r o o f s due to c o n s t r u c t i o n variability. A h i s t o g r a m and c u r v e based on o b s e r v a t i o n s on eighteen d r y r o o f s .
C u r v e A
-
p e r cent probability that roof i s not d r yF o r t h i s g r o u p of r o o f s , c o n s t r u c t i o n v a r i a t i o n exceeded 30 kg/m3 w. e. about 5% of the t i m e . S t a t i s t i c a l l y , t h e r e f o r e , one would expect t h a t i n 95% of the c a s e s a deviation of that m a g n i t u d e would indicate that m o i s t u r e was p r e s e n t ( C u r v e A ) . When ap- plied t o individual c a s e s , t h i s i s only an a p p r o x i m a t e guide since construction v a r i a b i l i t y d i f f e r s c o n s i d e r a b l y f r o m roof t o roof.
7
-
HedlinNo distribution c u r v e w a s drawn f o r readings taken n e a r roof e d g e s but i t was a p p a r e n t that variability within 0.
bm
of the edges w a s much g r e a t e r than outside t h a t a r e a and would cause proportionately m o r e difficulty in detection of m o i s t u r e .Boundaries of wet a r e a s can be found f r o m surveys. A s e r i e s of r e a d i n g s and cut t e s t s , taken a c r o s s one such boundary, i l l u s - t r a t e the s h a r p n e s s of definition that m a y exist (Fig. 4).
1- m . c . 13 K q / m 3
0 I 2 3 4
D I S T A N C E , m FIG. 4
Lnstrurnent readings and insulation m o i s t u r e contents a c r o s s the boundary of a wet a r e a
T o e s t i m a t e the a c c u r a c y , m o i s t u r e m e a s u r e m e n t t e s t s were m a d e on eight wet roofs where cut t e s t s could be m a d e . The d e c k s w e r e of c o n c r e t e o r steel. Wood fibre insulation predom- inated but cellular g l a s s and c o r k w e r e also r e p r e s e n t e d . Sam- p l e s of insulation w e r e oven d r i e d t o d e t e r m i n e t h e i r moisture contents which ranged f r o m n e a r - d r y t o near-saturation. The r e s u l t s , plotted against instrument readings, a r e shown in Fig. 5. B a s e d on l i n e a r l e a s t s q u a r e s f i t s , t h e standard e r r o r of e s t i -
8
-
Hedlin" 0 100 2 0 0 300
WATER EQUIVALENT, K g / m 3
FIG. 5
M o i s t u r e content v s . i n s t r u m e n t reading b a s e d on cut t e s t s on eight wet roofs. A d i f f e r e n t symbol w a s g s e d f o r e a c h roof
m a t e f o r the m e a s u r e d m o i s t u r e content w a s 50 kg/m3 f o r m o i s - t u r e contents below 250 kg/rn3. F o r m o i s t u r e contents between 250 and 400 k g / m 3 , i t w a s 100 kg/m3. T h i s suggested t h a t the u n c e r t a i n t y of m e a s u r e m e n t i n c r e a s e d with m o i s t u r e content. Based on t h g s e r e s u l t s , t h e r e should be a b o u t a t w o - t h i r d s chance - of e s t i m a t i n g t h e m o i s t u r e content within 50 o r 100 kg/m3 up to the r e s p e c t i v e m o i s t u r e l e v e l s .
Conclusions
L a b o r a t o r y a n d field t e s t s with c o m m e r c i a l n u c l e a r s o i l m o i s t u r e - density m e t e r s indicated t h a t they r e s p o n d e d to m o i s t u r e i n insu- lation i n g r a v e l - c o v e r e d bituminous f l a t r o o f s . P r e s e n c e of m o i s - t u r e w a s d e t e c t e d by a d e p a r t u r e f r o m the b a s e reading. Inde-
9
-
Hedlint e r m i n a t e variation, s u c h a s t h a t due t o bitumen and g r a v e l t h i c k n e s s and, t o a l e s s e r extent, t o s t r u c t u r a l f e a t u r e s caused u n c e r t a i n t y in the m e a s u r e m e n t . V e r t i c a l d i s t r i b u t i o n of m o i s
-
t u r e in t h e insulation a l s o affected t h e result.F i e l d t e s t s on eight bituminous r o o f s indicated that m e a s u r e m e n t of m o i s t u r e contents w i t h an a c c u r a c y of better t h a n 5 0 kg/m3 w a s unlikely. B o u n d a r i e s of wet a r e a s often showed up c l e a r l y , p a r t i c u l a r l y when l a r g e amounts of m o i s t u r e w e r e p r e s e n t .
Acknowledgements
T h e a u t h o r wishes t o e x p r e s s a p p r e c i a t i o n t o M r . K e n McArton, who m a d e m a n y of the m e a s u r e m e n t s and helped t o p r o c e s s the r e s u l t s , and t o t h e building o w n e r s o r t h e i r r e p r e s e n t a t i v e s and m e m b e r s of the building industry who co-operated by making r o o f s available f o r t h i s work and, i n s o m e i n s t a n c e s , a s s i s t e d actively i n obtaining t h e readings. T h i s paper i s a contribution f r o m the Division of Building R e s e a r c h , National R e s e a r c h Council of Canada, and i s published with the a p p r o v a l of the D i r e c t o r of the Division.
