The effects of thermal bowing on an insulated envelope can be serious potentially causing overstressing of insulating sandwich panels and associated joints.
An important factor to consider in the application of panels is the thermal bowing. The average results are shown on the adjacent graph based on steel faced panels and a control of bowing equivalent to span/240. Care should be exercised! Thermal bowing and structural bowing can be additional.
Cause of Thermal Bowing
- The form of construction of the insulating sandwich panel
- A change in differential temperature from inside to outside of the insulating sandwich panel compared to the differential in temperature from inside to outside the insulant panel at the time of manufacture
The change in differential temperature can be due to any cause but for insulated envelopes enclosing a controlled environment the change in differential temperature from inside to outside of the insulating sandwich panel compared to the time of manufacture is due to the following:
- Change of the temperature of the external face of the insulating sandwich panel to match the temperature of external environment
- Change of the temperature of the internal face of the insulating sandwich panel to match the temperature of the enclosed environment.
The temperature of the external enviroment varies throughout the day and year and thus the temperature of the external face of the insulating sandwich panel changes, with a slight modulating effect due to thermal inertia, to follow the external temperatures.
For Type 1, where insulating sandwich panels are shaded from the direct sun the temperature of the external face of insulating sandwich panel will adjust itself to match the varying external air temperature.
Type 1 Installation
For Type 2, where the faces of the insulating sandwich panels are not shaded, the faces of the insulating sandwich panels will be subject to the effects of solar radiation gain during the day, resulting in external face temperature of up to 70 degree celcius in the Middle East and radiation loss to chear skies at night resulting in a fall to a temperature of 0 degree celcius or lower at night.
Type 2 Installation
The peak and lowest temperatures of the face of insulating sandwich panels due to radiation effects is a function of the color of the face of the insulating sandwich panel and is particularly acute for panels faced with dark colored cladding. However, the temperature range due to radiation effects on light colored cladding can also be extreme.
Influence of Panel Cores
It can be shown that for insulant foams EPS, XPS, PUR, PIR etc., which have a very low modulus of elasticity in comparison to facing materials, that the insulant core has almost negligible influence on the magnitude of the thermal bowing movement of the insulant panel. It can be further shown that for unrestrained thermal bowing the effects of thermal bowing within normally experienced temperature ranges results in negligible stressing of the facing material or the important bond line between the insulant core and the facing materials.