Build 155 — August/September 2016 — 75

FEATURESECTION

Reflecting on colourChoosing the right colour for a building cladding is about more than just looks. Some paint colours absorb heat and can cause problems, but use the appropriate reflectance value and you can specify with confidence.

BY MIKE CLOWES, RESENE TECHNICAL MANAGER

VIRTUALLY ALL of the energy the Earth receives comes in the form of radiation from our sun. The radiation that reaches us is about:

● 5% ultraviolet light of wavelength between 290–400 nm ● 50% visible light between 400–700 nm ● 45% infrared (or heat) ranging from 700–2,500 nm.

Ultraviolet light damages paintAll of these waves interact with the substrates they fall on, being absorbed, transmitted or reflected. All absorbed energy eventually gets converted to longer-wavelength heat.

The 5% of ultraviolet light is very active and causes damage when it impacts on organic materials, such as paint and human skin.

LRV measures visible light reflectedThe light reflectance value (LRV) of a colour, as measured by a spectrophotometer for example, is a measure of how much of the visible spectrum of light is reflected by that colour.

Light reflectance values are based on the way the colour looks. Black has a light reflectance value of 0% and absorbs all light. The surfaces are consequently very dark and can get very hot. In contrast, white has a light reflectance value of 100% and keeps a building light and cool. All colours fit within these two extremes, and it is virtually impossible to have a dark colour with a high LRV or a pastel shade with a low LRV.

Surface can affect LRVThe LRV of the same shade can be affected by the surface of the substrate. A glossy paint, even a jet-black glossy paint, will have some specular reflectance from the surface, rather like a mirror. Flat and textured paints do not have this property and reflect only in a diffuse manner.

Semi-transparent finishes more complexLRVs are particularly difficult to determine from semi-transparent finishes such as wood stains. Light travelling through these finishes gets absorbed and is reflected by the underlying timber.

Hence, a stain applied over pine will have a substantially higher LRV than the same stain applied over kwila. This is why there is not a standard LRV provided for wood stain colours.

Paint colour affects temperatureThe role paint can play in temperature control in buildings comes down to the colour. The ability of white to reflect visible light extends through the infrared, and because of this, white surfaces remain relatively cool to touch, even in direct sunlight.

The opposite is true of black and dark colours, which absorb light in this infrared area, resulting in significant heat build-up in the surface. As the emissivity of paints is not particularly good, the surface heat is conducted into the substrate and then radiated into the building.

Exteriors

76 — August/September 2016 — Build 155

ExteriorsFEATURESECTION

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Only some colours suit heat-prone surfacesTraditionally, LRVs have been used to define which colours are suitable for heat-prone substrates. However, visible LRVs ignore the effects of ultraviolet and infrared light.

Conflicting LRV requirements hard to manageThe LRV demands of various bodies can cause problems that are difficult to resolve.

Some councils will demand low LRVs so that roofs do not cause glare to neighbours or apply covenants to paint colours used on buildings erected in bush-clad suburbs. They may demand an LRV of less than 50% to manage the aesthetics of a development and ensure buildings are not too conspicuous.

Suppliers of building systems and elements, however, are cautious about the effect of the excess heat that may be absorbed and specify an LRV of not less than 40%, hence the dilemma.

Higher LRVs are often specified for building systems that may be sensitive to heat. This recognises that absorbed light will convert to heat but does not necessarily take into account the 45% of the sun’s energy emitted in the infrared range.

This can lead to the situation where two colours can have identical LRVs but dramatically different heat build-up depending on whether the pigmentation reflects in the infrared range or not.

TSR better measure of thermal stressTotal solar reflectance (TSR) takes into account all three forms of light to give a better measure of the reflectance of colour. TSR is the measurement used for heat reflectance of a colour. There is no direct relationship between LRV and TSR, but TSR can be used to determine if a colour with an LRV of less than 40 can be used for heat-sensitive substrates.

Some new paint products are formulated with special pigments that reflect much of the infrared heat so the colour stays much cooler than its standard colour counterpart, even though both look the same. This is because the total solar reflectance of the paint is much higher. In these situations, it is the heat reflectance of the colour that is important rather than its light reflectance.

While an LRV value is a useful indicator for the visual aesthetic properties of a colour, total solar reflectance is more useful to achieve a picture of the thermal stress a colour may inflict.