Surface emissivity – why this matters
Matt surfaces, such as concrete, have a high
emissivity level and are much better at absorbing an emitting
radiant heat than shiny surfaces, like galvanised steel.
For heat to be stored or released from an exposed soffit it must
of course pass between the surface and occupied space below, and
although heat moves relatively quickly through concrete, it is
generally more limited at its face, which acts to some extent as a
thermal bottle neck.
Where possible, encouraging turbulent air flow across the soffit
(as previously mentioned) will improve convective heat transfer
performance, however the radiant heat transfer rate is determined
by another factor called surface emissivity. This basically relates
to how shinny or matt a surface is, measured by a factor ranging
between 0 and 1. Emissivity matters because matt surfaces, such as
that of concrete, have a high emissivity level of between
0.85-0.95, making them very good at absorbing an emitting radiant
heat. In contrast to this, relatively shinny surfaces such as
galvanised steel have a much lower emissivity of around 0.22-0.28
that limits radiant heat flow. Since heat transferee to and from
the soffit is typically about 2/3 radiant and
1/3 convective, a low surface emissivity has
a significant impact on overall performance.
In practical terms, this very much limits the
FES performance of exposed composite floors (steel decking with
thin concrete topping), which maybe further reduced if permeable
ceiling tiles are used to visually screen the metallic soffit. This
can be seen in figure # which shows that the approximate cooling
output of an exposed composite floor is a modest 10-14
W/m2, falling to around 4-9 W/m2 when a
permeable/open ceiling is used. This contrasts with a value of
around 15-25 W/m2 for a plane, exposed concrete soffit
without any thermal enhancements (see below).
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