Architects memo no.
73: September 2003
the amazing technicoloured dream coat
You will get no argument from this scribe regarding the point
that paint is one of the more remarkable and versatile building elements;
however, even the most enthusiastic devotee must recognise its limits.
We particularly refer to claims made for paint's ability in the climate
control area, especially in the area of insulative abilities.
The major contribution paint makes in this general area is primarily
centred around colour issues. Visible light and infra-red heat radiation
lie adjacent to one another in the electromagnetic spectrum and, as
a general rule, colours that reflect strongly in the visible spectrum
will also reflect in the infra-red area. This explains why white paints
will feel cool as they reflect not only light, but heat.
The converse is also generally true with black paints absorbing both
heat and light and, on a sunny day, feeling quite hot to the touch.
If a black roof were to be painted white, a significant reduction in
roof space temperature would be achieved. This is sometimes claimed
to show the 'insulating' properties of the paint when, in fact, the
phenomenon is purely due to reflectivity.
It should be noted that, if the roof were made of relatively new,
shiny galvanised steel or Zincalume, there would not have been a heat
problem in the first case. This is because polished bright metals are
the best reflectors of light. Advantage has been taken of this property
in the use of aluminium-flake containing paints to assist temperature
control of industrial installations, tanks, gas bottles and so on.
Recently decorative paints have been introduced containing aluminium
flakes in an attempt to conserve by reflection. The use of an aluminium
primer is a more efficient way to achieve such savings.
The astute reader will have noticed the use of 'weasel' words in the
foregoing section such as 'as a general rule', 'generally' and so on.
This is because there is some technology in our development labs that
may break the paradigm - but this could be the subject of another memo.
So, do paints have a role to play in insulation? To answer that question
one must look at how heat is conducted through a material. Basically,
when heat energy falls onto a surface it sets the atoms or molecules
in that surface jiggling. In this way, the heat energy transfers through
the mass of material. The key properties are: how easily the molecules
get turned on (specific heat) and how thick the layer is.
Copper is a good conductor with its atoms being readily excited. A
vacuum, that is a space where theoretically nothing exists, is the best
insulant of all. The vacuum flask, a vacuum contained in highly polished
metal coated glass, remains the epitome.
Vacuums are difficult to work with in the building game and thankfully,
still air gives a close approximation. The word 'still' is important
here. Heated free-flowing air can transmit heat by convection but, if
one traps the air, it is unable to deliver its heat. This is why fibreglass
balls are so efficient.
There are materials that one can add to paint that contain encapsulated
air, from micron-scale opacifiers to millimeter scale texturing pigments
such as Perlite. Materials, such as Perlite, have excellent insulative
properties. However their insulative ability is governed by the second
part of Kelvin's Law, that is, the actual thickness of the insulating
layer. The relationship of insulation to thickness is not just a simple
one, that is double the thickness and double the insulation, but by
the third power, that is double the thickness and one actually increases
the insulative capacity by eight times.
Adding an insulative additive (such as Perlite) to a standard paint
and applying it at typical paint spreading rates would offer nothing
in insulatory properties but would detract significantly from aesthetic
appeal. A coating incorporating such an insulating material would need
to be applied at about 1 centimetre thick to achieve insulating properties
that would make a difference!
We, naturally, would love the world to start specifying paints at
these sorts of spreading rates and we would willingly supply technical
services from our yacht in the Bahamas. We reluctantly concede that,
in this area of technology, there are more appropriate methods of insulation
that should be employed.