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Thermal
Properties
Service Temperature
While ambient temperatures seldom exceed 25°C, surface temperatures
on roofs and within double skin rooflights can exceed 80°C in
Northern European summer conditions, and are much higher in many areas
of Africa, Asia, Australia, Middle East, USA. Temperatures below -20°C
on external surfaces in winter are not uncommon.
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Marlon CS Longlife
will retain its physical properties over the range of temperatures
found in these locations.
In comparison, PVC sheet is quite brittle at -20°C, and is unsuitable
for use in conditions where surface temperatures exceed 60°C.
GRP sheet will perform very well mechanically in this temperature
range but prolonged exposure to the higher temperature will cause
relatively rapid discolouration. |
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Thermal Expansion
Polycarbonate has a high coefficient of thermal expansion; the table
below and graph opposite illustrate the relative expansions of polycarbonate
and other common building materials. It is evident from these figures
that Marlon CS Longlife expands approximately 3 times as much as aluminium
under the same temperature rise. The high expansion coefficient means
clearance must be allowed in the holes drilled for fixings and sheet
lengths have to be limited so that there is not excessive differential
movement at end laps. |
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Solar Heat
Gain
The greenhouse effect - solar heat gain - described on the light transmission
section, is an important phenomenon in any building which incorporates
Marlon CS Longlife or any other glazing material. Measures must be
taken to avoid internal temperature increases due to solar heat gain
becoming excessive, to the detriment of building occupants or contents.
When designing any building incorporating rooflight or glazing materials,
consideration must be given to controlling the solar input by using
tinted material, and/or providing screening or ventilation systems |
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| Values for the
light transmissions and shading coefficients of the clear and tinted
forms of Marlon CS are given below. |
| Condensation
& Insulation
The moisture content of the air in a building and the temperature
of the inside surface of the rooflight determine the occurrence
of condensation. Occupancy and activities within a building determine
the air moisture content - relative humidity. When the inner surface
reaches the dewpoint temperature moisture starts to condense from
the air. The graph shows the relationship between dewpoint temperature,
air temperature and relative humidity. |
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Thin single skin
sheets of any roofing material, either metal or rooflight, have high
thermal transmittance and therefore provide little insulation and
produce a relatively high incidence of condensation. However, two
thin skins of rooflight material with a still air space of 20 to100mm
between them, forming a double skin rooflight, will achieve higher
thermal efficiency and reduce condensation risk.
The thermal transmittance of a double skin rooflight is 2.8W/m2 °C
while for a single skin rooflight it is 5.7W/m2 °C. A triple skin
rooflight has a thermal transmittance of 2.0W/m2 °C.
Double skin site assembled rooflights or factory assembled insulating
rooflights can be made from Marlon CS Longlife as necessary to suit
many types of roof system and construction. In addition to providing
natural daylighting, double skin and triple skin rooflights, in particular
factory assembled insulating rooflights, provide high levels of energy
conservation with consequent fuel economies. |
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