Publisher’s version / Version de l'éditeur:
Technical Note (National Research Council of Canada. Division of Building Research), 1975-05-01
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Effect of Increased Thermal Resistance on Conventional Roofing
Systems
Turenne, R. G.
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NRC Publications Record / Notice d'Archives des publications de CNRC: https://nrc-publications.canada.ca/eng/view/object/?id=a97f5f6f-29f6-458f-8585-d98e0120e352 https://publications-cnrc.canada.ca/fra/voir/objet/?id=a97f5f6f-29f6-458f-8585-d98e0120e352
r-NATIONAL RESEARCH COUNCIL OF CANADA
No.
DIVISION OF BUILDING RESEARCH
592
DOM
201
PREPARED By
R.G. Turenne CHECKED BY M.C.B. APPROVED BY L.W.G.
PREPARED FOR
SUBJECT
General information
EFFECT OF INCREASED THERMAL RESISTANCE ON CONVENTIONAL ROOFING SYSTEMS
May 1975
Roofing contractors are concerned that increasing the amount of insulation or specifying insulations having a lower
k factor to improve the thermal resistance of conventional roofing systems might adversely affect the performance of the 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 roofing membranes partially covered with snow, due to greater temperature differences between snow covered and bare membrane areas.
To evaluate these various factors, a specific Toof
construction was studied. Roof insulation thicknesses ranged from
o
to 4 in. and two outside temperatures and wind conditions were assumed; O°F with a 15 mph wind and -45°F with no wind. Theseconditions corresponded closely to the mean daily January temperature and the extreme minimum temperature for Winnipeg, Manitoba during the past 32 years.
The membrane temperatures were calculated assuming two conditions: a bare roof and one covered with 12 in. of snow. The snow was assumed to have a thermal resistance of 10 units per foot. For simplicity, the underside of the roof deck was assumed to be unfinished and the interior temperature to be 70°F.
_.
-2-The design of the 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, IS-mph wind and no snow, the thermal resistance of this construction was calculated as
follows:
Construction (heat flow up) Top surface Membrane
Insulation of thickness
"t"
2-ply vapour barrier4-in. concrete deck
Bottom surface (still air)
Resistance, R 0.17 0.33 4.00 x t 0.12 0.32 0.61 1. 55 + 4.00 x t If no insulation is used R
=
1.55If 1 in. of insulation is used R
=
5.55If2 in. of insulation are used R
=
9.55If4 in. of insulation are used R
=
17.55Adding 12 in. of snow on the roof changes the thermal resistance as follows:
Construction (heat flow up) Top surface Snow
Membrane
Insulation of thickness "t" 2-ply vapour barrier
4-in. concrete deck (standard aggregate)
Bottom surface (still air)
Resistance, R 0.17 10.00 0.33 4.00 x t 0.12 0.32 0.61 11.55 + 4.00 x t
-3-If no insulation is used R = 11.55
If 1 in. of insulation is used R = 15.55
If2 in. of insulation are used R = 19.55
If4 in. of insulation are used R
=
27.55The same technique was used to calculate thermal resistance of the roof system at _45°F. With a ｮｯｾｷｩｮ､ condition, however, the resistance of the top surface increases by 0.44 to 0.61. This amount
(0.44), therefore, was added to all previously determined values. The temperature of the upper surface of the membrane cannot exceed 32°r as long as it is covered with snow. In the ensuing calculations, therefore, the membrane surface temperature was .
maintained at 32°r whenever the resistance above the membrane was high enough to produce
a
temperature greater than 32°F 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 1).
Discussion of Results in Table I
(a) The mean membrane temperature changes very little when the thickness of insulation is increased from 1 to 4 in. For an outside temperature of OOF, the difference is 2 1/2°F; for -45°F the difference is 10°F.
(b) The temperature difference across a membrane is due to.its thermal resistance. It decreases as the thermal insulation increases. Table I (Column 7) shows that at _45°F and no snow, the temperature difference across a membrane placed over an uninsulatcd roof is 20.3°F. It is 6.4°F when placed over I in. of insulation and 2.2°F with 4 in. of insulation.
(c) Adding insulation to a roof initially 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 -45°F the difference in temperature between an area of membrane clear of snow and one having a i-ft snow cover is 62.5°F when there is 1 in. of
insulation under the membrane; if the thickness of insulation is increased to 4 in., the difference in temperature between the two areas is reduced to 39°F. In column 4, Table I additional values are listed for other thicknesses of inSUlation. These values are plotted against the insulation thickness 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.
---
-4-(d) Adding insulation reduces or eliminates snow melting, especially at lower tcmperatures, 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 membrane may be lower due to 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 daily variation of the membrane temperature due to solar radiation has not yet been considered. Steady state calculations were made to determine the possible range of temperatures that the membrane might experience under severe exposure. A case involving the following winter condi-tions was assumed:
Inside air temperaturc Solar radiation
Outsidc air temperature Sky temperature
Convective coefficient h c Roof emissivity and absorption
70°F 154 Btu/hr ft2 -10°F day, _30o
r
night -30°F day, -50or
night 0.8 Btu/m ft 2 of 1.0 The calculations gave the following results:Roof Temperature Roof Temperature
R day, of night; of
1.38 75.3 -0.7
5.38 76.7 -24.4
9.38 77.0 -29.6
17.38 77 .2 -33.1
The heat capacity of the roof would likely result in a smaller variation between day and night, particularly for the uninsulated roof. The calculated temperatures do, however, give a reasonable comparison for insulated roofs. They show very little difference between a roof having a minimal thermal resistance and one having 16 units. It may be concluded, therefore, that increasing the insulation on roofs does not appreciably increase the severity of thermal cycling of the membrane due to solar radiation.
The added thickness of insulation, however, means that the membrane and the structural deck are spaced further apart. The
-5-significance thercfore, sincc thermal stresses should he transferred from the membrane to the deck in well designed and well built roofs. These stresses must be considered by the designer and roofing con-tractor in selecting the insulation, the adhesive, the felts, and the installation technique. Proper adhesion of all components
between the membrane and the deck should be ensured. The insulation should have adequate shear and tensile strength. All layers should be firmly bonded together when more than one layer is used.
If these recommendations are followed, splitting failures and incidences of thermal shrinkage of membranes installed on roofs having increased thermal resistance should be no more numerous than at present.
The author wishes to acknowledge the contribution of
information by Mr. K.R. Solvason on the effect of solar radiation and night time cooling on the membrane. The suggestions and comments made by Mr.
c..].
Shirtliffe are also appreciated.•
e
TABLE I
e
Temp. Difference
Insulation ｾｬ･｡ｮ Membrane Difference Surface Temp. Across Membrane Heat Loss Heat for
Thickness Tern . of Due to Snow of of Prom Interior Melting 2
"t" in. No Snow 1 ft Snow of No Snow 1 ft Snow No Snow 1 ft Snow Btu/hr-ft 2 Btu/hr-ft
I
I
Conditions for a OOp Outside Temperature
0 16.5 37.0 20.5 8.3 32 16.5 10.0 30.2 27
1 4.0 33.0 29.0 2.1 32 4.2 2.3 7.0 3.8
2 2.5 32.5 30.0 1.2 32 2.5 1.3 4.0 0.8
4 1.5 26.0 24.5 0.7 25.8 1.5 0.7 2.2 None
Conditions for a -45°F Outside Temperature
0 2.5 37.0 34.5 37.5 32.0 20.3 10.0 30.2 23
1 -30.0 32.5 62.5 11.7 21.3 6.4 2.6 7.0 None
2 -36.0 17.0 53.0 7.0 16.0 3.8 2.0 4.0 None
•
60 u.. 0 w u 50 Z w c::: w -45 0F ( OUTSIDE u.. u.. TEMPERATURE) c 40 w c::: :::> t -4: c::: w a.. 30 ｾ W t -- l 0 0 F ( OUTSIDE TEMPERATURE) 4: t -4: 20 a.. Vl 10o
o
2 3 4THICKNESS OF INSULATION, IN.
FIGURE
CURVES SHOWING RELATIONSHIP BETWEEN THICKNESS OF INSULATION AND SPATIAL TEMPERATURE DIFFERENCE OF MEMBRANES PARTIALLY COVERED WITH SNOW
