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.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.