The effect of increased insulation on exposed bituminous roofing membranes

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The effect of increased insulation on exposed bituminous roofing membranes

Turenne, R. G.

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THI

THE EFFECT OF INCREASED INSULATION

ON EXPOSED BITUMINOUS ROOFING W R A N E S

Division of Building Research National Research Council of Canada

O t t a w a

January 1977

THE

_EFFECT

ON EXPOSED BIWINOUS ROOFING MEMBRANES

R.G. Turenne

Concern is expressed from time to t h e that increasing the amount

of insulation or specifying insufations having a loner k factor t o

improve the thermal resistance of conventional roofing systems might

adversely affect the performance of t h e membrane. Lower membrane

temperatures caused by increased thermal resistance could induce

higher stresses in the membrane, thus increasing the incidence of splitting failures. It is also feared that additional insulation could result in larger differential stresses

in

partially covered

with

between

snow

To evaluate these various factors, a specific roof construction

was studied. Roof insulation thichesses ranged from 0 to 4 in.

and

_outside

OaP with a IS mph wind and -45'F w i t h no wind. These conditions corresponded closely to the mean daily January temperature and the extreme minkmum temperature for Winnipeg, Manitoba during the past

32 years.

The membrane temperatures were calculated assuming two conditions:

a bare roof and one covered w i t h 12 in. of snow, The snow was assumed

to _{have a t h e m resistance of 10 units per foot. For simplicity,}

t h e underside of the roof deck was assumed to be unfinished and the interior temperature to be

70°F.

me design of t h e roof selected for this study was as follows: 4-ply built-up membrane

insulation (assumed 4 units of resistance per inch) 2 -ply vapour barrier

4- in. concrete deck (standard aggregate)

.

Assuming winter conditions of O'F, 15-mph wind and

no

the thermal resistance of t h i s construction w a s calculated as

Construction (heat flow up)

Top surface Membrane

Insulation of thickness "tmt

2 -ply vapcrur barrier

4-in. concrete deck

Bottom surface. [still air]

no insulation is used

If 1 in. of insulation is used

If

Resistance

R

Adding 12

in.

Construct ion

(heat flow up)

Top surface snow

Membrane

Insulation of thickness "ttl

2-ply vapow barrier

4-in. concrete deck

[standard aggxegate) -

Bottom surface [still air]

If no insulation is used

If 1 in. of insulation is used

I f 2 _{in. of insulation a r e used}

If 4 in. of insulation are used

Resistance

The same technique was used to calculate thermal resistance of- the roof system at - 4 5 ' ~ . With a no-wind condition, however, the

resistance of the top surface increases by 0.44 to 0.61. This amount

(0.44), therefore, was added t o all previously determined values,

The temperature

of

exceed 32'~ as long as it _is covered _{w i t h snow. In the ensuing}

calculations, therefore, the membrane surface temperature was

maintained at 3 2 O ~ whenever the resistance above the membrane was high enough to produce a temperature greater than 32'~ at the membrane

surface. The heat flow from the inside to the membrane surface and from the membrane surface to the outside air was then calculated, the difference between these values being the heat available for melting

(see Table I ) .

Discussion of Results

in

(a) The mean membrane temperature changes very little when the thickness of insulation is increased f r o m

X

outside temperature of

o°F,

the difference is 10°~.

(b] The temperature difference across a membrane is due to its

thermal resistance. It decreases as the _{thermal insulation} increases. Table I (Column 73 shows that at -45°F and no snow,

the temperature difference across a membrane placed over an uninsulated roof is 2 0 . 3 O ~ . I t is 6.4OF when placed over 1 in. of insulation and 2 _{. 2 a ~ with 4 in. of insulation.}

( c ) Adding insulation to a roof i n i t i a l l y increases the spatial

temperature variation of the membrane on a roof partially covered with snow. However, as more insulation is used, this temperature

difference decreases. For example, at -4S°F the difference in

temperature between arr area of membrane clear of snow and one

having a. 1-ft snow cover is 62.S°F when there is 1 in. of insulation

under

is increased to 4 i n , , the difference in temperature between the

two areas is reduced _{to 3g°F.} In _{column 4,} Table _{I additional}

values are listed for other thicknesses of insulation. These values are p l o t t e d against the insulation thiclmess in Fig, 1. The graph shows that insulation first increases the spatial temperature variation, but as the amount of insulation is

increased the temperature variation decreases.

[d) Adding insulation reduces or eliminates snow melting, especially

at lower temperatures, by decreasing the heat loss through the

roof. As ice on roofs may be a contributing factor to premature membrane failure, a measure minimizing snow melting could be desirable.

[e) The mean temperature of

the

increased thermal resistance; however, as the mean temperature

drops, so does the temperature difference across the membrane. The effect of increased insulation

on

the membrane temperature due t o solar radiation has nut yet been considered. Steady state _{calculatims were made to determine the}

possible range of temperatures that the membrane might experience

under severe exposure. A case

involving

was assumed:

Inside air t q e r a t u r e Solar radiation

Outside ai.r temperature -10°F day, - 3 0 ° ~ night

Sky temperature -30'F day, -SOOF night

Crmvective

Roof emissivity and absorption 1.0

The c a l m l a t i o n s gave the _{fo1,lowing results:}

Ruof Temperature day, F'

Roof Temperature night, OF

The heat capacity of the roof would likely result

in

roof. The calculated temperatures do, however, give a reasonable

coutparism for insulated roofs. They show very l i t t l e difference between _{a roof having a minimal thermal resistance and one having}

16 units.

Ix

insulation on raofs does not appreciably increase the severity of

therrnal cycling of the membrane due t o solar radiation.

' h e added thickness of insulation, however, means that the membrane and the stmctu.lra1 deck are spaced further a p a r t ,

The

properties of the insulation and the _{adhesives take} on _added

significance therefore, since thermal stresses shauld be transferred from the membrane t o t h e deck in well designed and well built roofs, These stresses must he considered by the designer and roofing con- tractor in selecting the insulation, the adhesive, t h e felts, and

t h e installation technique. Proper adhesion of all components

between the- membrane and the deck s h w l d be ensured. The insulation should have adequate shear and tensile strength. A l l layers should be firmly bonded together when more than one layer is used.

If

incidences of thermal shrinkage of membranes installed on roofs having

increased thermal resistance should be no more numerous than at

present.

Acknow ledgernmts

The author wishes to acknowledge the contribution of information

by Mr. K.R. Solvasan on the effect of s o l a r radiation and night time

cooling on the _{membrane. The suggestions} and comments _{made by} Mr.

T H I C K N E S S O F I N S U L A T I O N , I N - F I G U R E 1 CURVES S H O W I N G R E L A T l O N S H I F B E T W E E N T H I C K N E S S O F I N S U L A T I O N

A N D

TABLE

ckness

in.

From

Mean Membrane Difference

h e

Snow

Snow

Temp.

Conditions for a o O E Outside Temperatu're

Temp. Difference Across Membrane

F

NO

1

No Snow

20.5 29.0

SO.

1

Snow

Conditions for a - 4 5 ' ~ Outside Temperature

16.5 4.2 2.5 1 . 5 34.5 62.5 53. Q 41.0 10.0 2.3 1.3 0.7 10.0 2 , 5 2 . 0 1.4

-