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Heat absorption and transfer - page 2

It should be, therefore, also no surprise that metal window frames heat up much quicker than wooden frames when exposed to direct sunlight. Add to this the choice of a low albedo dark or black colour and you created a glowing hot window frame – outside and inside. “Like a cat on a hot tin roof,” that’s an expression I was painfully reminded of when installing a TV antenna on the ‘almost’ tin roof. It was of zinc-alum really, but that wouldn’t make the point as well.

Let’s stay with the roof example for the moment. So, a metal roof heats up quicker than a concrete tile roof. Does this mean we shouldn’t use metal roofs? Not at all. As you can see from the thermal properties table (specific heat column), when you compare aluminium or steel with concrete, there is not much difference. Metal roofs have some advantages, though. Depending on the choice of colour, they are usually more reflective than tiles and, which brings us to the subject of thermal conductivity, they loose the stored heat much quicker at night.

Heat moves through a substance at a specific rate and thermal conductivity measures the efficiency of heat transfer within this substance. More specific, it determines the power of heat energy (Watts) which flows through an object of a particular length (metres) per degree Kelvin. The formula is, therefore, as follows: W / m °K. High values for substances indicate they are good heat conductors, whereas small numbers tell us that the material are good insulators.

Other terms often used are thermal conductance, the rate at which energy passes through the substance; and thermal resistivity, the inverse of conductivity, i.e. the ability of a substance to block heat transfer.

A window with an aluminium frame is perhaps a good example for the differences in thermal conductivity, but with approximately the same value in specific heat. Compare the values for aluminium and glass, specific heat and thermal conductivity, and you can see that both require about the same amount of calories to increase their temperature by one degree Celsius. However, the value for the heat conductivity of aluminium is around 300 times higher than that of glass. You could also say that heat travels much faster through aluminium than it does through glass.

This explains why an aluminium frame feels glowing hot when exposed to the sun, while the glass pane is relatively cool – not considering the direct radiation that reaches your hand through the glass, but not through the frame.

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Global Bioweather

Heat absorption and transfer - page 2

Heat Island

Urban