The truth about dark roofs and cool roofs

It seems like a no-brainer: cooling of buildings and neighbourhoods can be helped by having light coloured roofs. But the question has caused no end of controversy. Let’s settle it once and for all.

What does the science say?

The technical name for a roof that reflects heat is that it has a high albedo.

High-albedo roofing has two scientifically proven benefits: it helps to mitigate the urban heat island effect and reduce cooling energy demand and costs. It can also increase thermal comfort in non-conditioned buildings.

Benefits are both direct and indirect. 

Direct benefits to individual buildings occur by reducing absorbed shortwave radiation through the roof. Neighbourhood-scale indirect benefits result from reduced ambient air temperatures, particularly when high-albedo surfaces are deployed on a large scale. 

These conclusions come from several sources, including one, (published in the journal Energy and Buildings) which comprised whole-building energy simulations of a set of archetypical single family residential buildings in three locations with distinct characteristics in the Los Angeles area (one coastal and two inland). This location is about the same degrees latitude north of the equator as many Australian cities are south of it. 

The simulations show that benefits from the indirect effect can be the same order of magnitude as the direct effects. The systematic replacement of dark surfaces with white could lower heat wave maximum temperatures by 2 degrees Celsius or more, according to other research from Yale University. 

A recent study published in the Proceedings of the National Academy of Sciences found that in the absence of adaptive urban design, and separate from climate change, urban expansion increases average temperatures by 1-2°C. therefore if those roofs were painted a light colour this effect would be cancelled out.

But these benefits depend on the climate and building characteristics. For heating-dominated climates, white roofs may not be appropriate.

The highest energy and thermal comfort benefits were observed in the Los Angeles study in a low-performance building (defined by airtightness and ceiling insulation levels). There, simulations indicated an energy savings of 41 per cent and thermal comfort improvement of 23 per cent due to a combination of direct and indirect effects.

Incidentally, Los Angeles uses the albedo effect to cool itself also by painting several streets in a light grey paint to reduce road-top temperatures by as much as 5.5°C. 

What roofs for which parts of Australia?

Research commissioned by the RTAA, performed by the University of Newcastle, found that light coloured tiles yielded “energy savings between 25-36 per cent compared to dark coloured tiles”. 

Lower cooling energy demand was found even with insulation or sarking, although it is smaller. 

The sustainability guide Your Home 5th edition states that light coloured roofs are estimated to reflect up to 70 per cent of summer heat gain – around 50 per cent more than a dark roof. Dr Chris Reardon, principal author, said: “In cooling-dominated climates, you need more than one layer of sarking – two layers separated by 25 mm gap plus bulk insulation”. 

A dark roof could be beneficial in colder, heating-dominated climates like Canberra and possibly even Melbourne, but it shouldn’t be considered north of Brisbane.

Many thermal rating tools, such as those used to assess NatHERS ratings, work off historical climate data. Houses built now might not be prepared for the climate change affected temperatures of the future. The publication says that it would be prudent to look at projected temperatures over the 50-80 year lifetime of a house, with CSIRO research predicting temperatures up to 4°C warmer on average by 2100. 

Therefore even in southe000rn parts of Australia, homes should be properly insulated to protect both heat and cold extremes, and light roofs should be installed.

Other types of roofing

Of course other environmentally sound types of roof are possible. Roofs may be “green” – a layer of sedum vegetation –  or covered in photovoltaic panels. 

A study in Adelaide found that besides delivering cooling in summer, green roofs also act as an insulating layer to keep buildings warmer in winter. It found that covering 30 per cent of the roof area in vegetation would reduce the electricity consumption by 2.56 (W/m²/day).

Building control

As we reported before, some councils are intrinsically opposed to light-coloured roofs and turn down planning applications.

“White roofs would generally be incongruous with heritage items and are likely to be incongruous with the character of council’s heritage conservation areas because they would be in stark contrast to the overall character of those areas,” one council spokesman has said. 

“Development controls in such areas require council to have regard for the colour and finishes of materials, assessing each case on its merits. In some cases a white roof may be acceptable as part of a contemporary in-fill design. 

“White or light coloured roofs are also very reflective in nature, and given the undulating landscape in the Woollahra area it is necessary to consider the impact light reflected from a white roof would have on neighbouring properties overlooking the roof when considering any application.” 

But although a surface may reflect more heat it does not necessarily produce more glare or light reflection: pale colours with a matt finish, for example.

• See our story Cool roofs versus dark roofs: special report

Why do light roofs cool and dark roofs heat?

All materials have the capacity to absorb and emit the energy they receive from the sun. This can be used to maximise the proportion of incident solar energy absorbed and control how much is emitted. 

Absorption is a measure of how much incident radiation is received by a material. The emissivity of the surface of a material is a measure of its effectiveness in emitting energy.

The emitted energy is in the form of long wave or thermal radiation. In scientific terms:

Since energy cannot be lost or destroyed, any radiation not absorbed into a material will be reflected. 

The radiation that is absorbed will have the effect of heating the material, which then itself will emit heat (infrared radiation) in order to reach the same temperature as its surroundings. 

If a material absorbs a great deal of solar radiation it is said to have a high absorption coefficient. If it emits a lot of radiation it is said to have a high emissivity coefficient. 

The converse is true for low absorption or emissivity rates. 

• An absorption coefficient of 1 would mean that all of the solar spectrum radiation was absorbed, and of 0 would mean that none of it was absorbed and all of it was reflected. By definition a true “black body” surface has an absorption coefficient of 1
• An emissivity coefficient of 1 would mean that all of the incident radiation was re-emitted as infrared radiation, and of zero would mean that none of it was

(In practice these extremes are not reached.)

Selective surfaces

The need for spectral selectivity is dependent on the intensity of the incoming solar radiation and the temperature at which the surface is to operate. 

A totally black absorber and a totally white reflector will absorb and reflect respectively all solar energy arriving on them, regardless of wavelength. 

Selective absorbers will absorb only in the spectral region of the solar radiation and be transparent in the thermal infrared. Selective reflectors will reflect only in the thermal infrared and be transparent in the spectral range. This is used in the design of photovoltaic solar panels.

In general the solar energy absorbed by a wall or roof can be approximated according to the surface colour (see Table 1 below). The amount of solar energy absorbed also depends upon the angle at which it arrives (See Table 2). 

The emissivity and absorption coefficients for some common roofing and building materials can be found in Table 3 below. For the purpose of maximising the cooling effect, choose a low ratio between absorption and emissivity. (The coefficients for some products varies with the temperature. As a guideline, the figures are based on a temperature of 300^oK. )

The amount of solar energy absorbed also depends upon the angle at which it arrives, as shown here.

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David Thorpe is the author of Passive Solar Architecture Pocket Reference, Solar Technology and Sustainable Home Refurbishment