Cool Roofs Hot Topic

Do you think that colour has gone out of your life recently? Is it just a matter of getting older or is it a fact?Does everything look black and white to you? What colour car do you drive? If it is black, grey or silver then you join the 60 % majority of New Zealanders who probably do not know that they are also driving the most dangerous coloured cars. Haven’t we taken the ‘All Black’ theme a bit far? Looking out a plane window years ago Wellington was dotted with Barn red and Steelite green roofs but now everything seems to be a paler shade of black. Gull Grey, Grey Friars, Thunder Grey, Shale Grey, Elephant Grey, Battleship Grey, Sandstone Grey…….

Do roofs get caught up in the fashion of the day like the black suit uniform that both men and women wear like some deathly shroud? Do designers really get the choice? Or is it a question of, women choose and men pay?

However the implication of the colour of a roof has just got a lot more serious than the aesthetic contribution it makes to the building. Building control authorities and energy agencies worldwide are now taking an interest in roof colour as a means of energy-saving.

This article started out to be a simple answer to a simple question. There have been a number of recent enquiries asking ‘Just what is meant by a ‘dark coloured roof’ in the NZMRM CoP 4.1.5 and 4.1.6?’

The NZMRM Code of Practice has put a figure on roof temperature range but this is based more on anecdotal than scientific evidence. With the range of temperatures now encountered in well insulated, unvented roofs it is also little wonder that the noise problem has been exacerbated.

What was thought to be a simple question has now become a hot topic – more correctly a ‘cool’ one. This article therefore has become a complicated answer to a complicated question of just how relevant coloured roofs are to New Zealand’s energy consumption. The white roof project in the USA has caught media attention recently. US.Secretary of Energy, Dr. Steven Chu is sold on the benefit of white roofs and has mandated that all new roofs on Energy Department buildings be either white or reflective He says if 85% of all air-conditioned buildings in the US had white roofs the saving in energy costs would be US$735 million per year.

However, like global warming worldwide there are a number of detractors who claim that painting roofs white is not a simple answer. This probably is true for New Zealand because the savings claimed for overseas are for air-conditioned buildings and the closest we get to residential a/c is the use of heat pumps for cooling in summer. Cool roofs are not a priority if you are not using much energy for cooling.

Like many other aspects of building, just how relevant cool roofs are for New Zealand is the purpose of this article. While it is not necessary to understand fully the laws of Thermo dynamics (it helps though) or to understand the difference between a absorption and adsorption, designers should be aware of the
implication following the decisions that they make in choosing the type and colour of a roof for their buildings. Do you have to know what reflectance and absorptance are- yes. Do you have to know what emissivity and reflectivity are – yes.

For these reasons some definitions and descriptions are needed.

Solar reflectance SR is a measure of a material’s ability to reflect solar energy back into the atmosphere, including the visible, infrared and ultraviolet wavelengths. (Sometimes referred to as TSR Total Solar Reflectance) Of the total energy striking the earth’s surface, infrared is the largest at about 55%, visible light is about 40% while UV amounts to only 3%. Solar reflectance is measured-on a scale of 0 to 1.0 where 1.0 represents total reflectivity, while 0 indicates that the surface absorbs all solar radiation and is expressed as a %. e.g. a surface that reflects 55% of sunlight has a solar reflectance of 0.55. Most light coloured roof materials reflect 55 to 90% while dark roof materials reflect only 5 to 20%.

A mirror may reflect 98% of the energy that strikes it while absorbing 2%; whereas a blackbody surface is the opposite, absorbing 98% of the energy and reflecting only 2%.

The US Environmental Protection Agency -EPA Energy Star rating specifies an Solar reflectance value of 0.65 or higher for roofs <10˚ and a minimum 0.25 for steeper slopes >10˚ .(which seems a pretty big jump!).

Sunlight that is not reflected is absorbed as heat and is known as Solar Absorbance (the value). Absorbance (the process) should be the reciprocal of Reflectance but sometimes things get in the way and one of these is emissivity or Thermal emittance.

Thermal Emittance TE
describes how efficiently a surface cools itself by emitting thermal radiation. It is measured on a scale of 0 to 1, where 1 indicates a perfectly efficient emitter. Unpainted Zincalume and unpainted aluminium cladding have a low emissivity of 0.1. but a relatively high total solar reflectance, 0.6 to 0.8 however the reflectance and emittance of bare metals are very sensitive to the smoothness of the surface and the presence or absence of surface oxides, oil film or dirt. Usually, bare metals are not very cool in the sun. Rough surfaces also cause the light to be deflected at different angles according to the surface profile resulting in a scattering of light away from the angle of reflection. This causes the object to appear dull or matte. The higher the degree of surface roughness, the greater the scattering of light occurs which results in a lower gloss level. Painted roof cladding is a different story as even a thin coil coated white coating “hides” the low emittance of the metal underneath, and the thermal emittance increases in most cases, to more than 0.8, regardless of the colour.

Emissivity also changes with temperature as the hotter it gets the better the emitter. Many years ago our school caretaker could not be persuaded that his painting of our matt black hot water radiators silver was not a good idea! Emittance is the critical measurement for municipalities and folk who are concerned with global warming because of the phenomenon known as the “Urban Heat Island” effect. Their research has shown that cities are at least 3˚ C warmer during the day and 12˚ C warmer at night than the suburbs.

Many of the overseas codes, standards and programmes that specify cool roofing requirements reference an additional calculated value called the Solar Reflectance Index (SRI).

Solar Reflectance Index SRI
combines the actual measured solar reflectance and thermal emittance values into a single value (1-100) by determining how hot a surface would get relative to standard black (reflectance 0.05, emittance 0.90), being 0 and standard white (reflectance 0.80, emittance 0.90) which is 100. The Solar Reflective Index is derived from ASTM 1980 SR x E (Emissivity) = SRI.

Although black roof cladding has a high emittance value 90% it has a low reflectance value 5% and becomes hot because even though the emittance is high, there isn’t enough reflectance to help cool the roof.

Like solar reflectance and thermal emittance, the higher the Solar Reflectance Index value, the cooler the roof.

While Solar Reflectance Index is an excellent indicator of solar reflectance for materials with a high infrared emittance, this value is very sensitive to a number of environmental conditions e.g. insolation, convection, roof thermal resistance air temperature, sky temperature, and inside air temperature.

For unoxidized metallic surfaces that have low infrared emittance, the Solar Reflectance Index calculated varies significantly with these variables and therefore this value is not recommended to be used for bare (unpainted) metallic surfaces

LEED- Leadership in Energy and Environmental Design is a green building rating system developed by the U.S. Green Building Council (USGBC) to provide a suite of standards for environmentally sustainable construction. The Solar Reflectance Index is used to determine compliance with LEED requirements and is calculated according to ASTM E 1980. A roof cladding must have an index of 29 or higher for roofs >10˚ and 78 or higher for roofs <10˚.

Having defined the terminology for measuring colour for roof and wall cladding we should now put these values into the New Zealand perspective. While accepting that overseas Building Control Authorities hang their hat on these values to restrict or reward designers accordingly, NZ has not reached that stage – yet. What doesn’t inspire confidence is the various methods of assessment in different states of USA, Europe and Australia and also the different published values for the same colour.

The theoretical values of solar absorbance and reflectance measure a flat surface so corrugated profile values cannot be so easily determined. They do not take into account internal reflections that occur as the angle of the sun changes during the day or the year and also they cannot take account of the deteriorating surface finish that occurs over time as the paint or material ages or it is affected by the environment and becomes faded or dirty. These calculated values however can be a good indicator of comparative performance between different colours.

While acknowledging that there has to be a colour benchmark, there are a lot of other factors that have to be considered in the overall assessment of thermal efficiency of buildings including their orientation, roof pitch and insulation.

Over the last decade, paint companies world-wide have developed IR (Infrared) pigments which considerably improve the solar reflectance value of dark colours and lighter colours to a lesser extent. In New Zealand both our coil coating companies have been researching IR (infrared reflective) pigments for some time and back in 2010 PCC announced that they were making a number of colours that use solar thermal absorption reflectivity pigments called Whitestar, Monolith, Dusk and Attic. Their stated intention was to introduce IR pigments into their standard NZ colours over time. New Zealand Steel have just released a statement about their use of IR pigments in popular colours and the good news is that they will be available from P.C.C. and NZS at no extra cost. The improvement of SRI appears to average about 12%. With this new paint technology, cool metal roofs do not have to be white or near white as they can be
of a darker hue when IR pigments are used.

Although the amount of infrared energy that is reflected is a function of the colour this new technology allows for darker colours to reflect more solar energy. As a result the solar reflectance value is higher with a corresponding lower surface temperature. Up till this time there has not been a great demand in fact there has been a ‘lukewarm reception’ because New Zealand does not have the climate or incentives to follow the US model. One could say ‘cool reception’ but the youth have taken that word and turned its meaning upside down so now that could mean ‘hot’! One downside of the new technology is that the reflectance values have to be judged after three years in service as dirt affects its efficacy which means that it is a extra good idea to wash your roof down regularly particularly the unwashed area.

Knocking white roofs for New Zealand residences will probably draw serious noises from the eco movement but then what’s new?. Passive and active ventilation of the attic space- particularly roofs of skillion or Cathedral construction can be as efficient as white roofs in reducing over heating in the summer but has the added advantage of removing excessive moisture in the winter.

Also adding insulation to a higher R value is another economic way to control summer heat but unlike Australia we do not place it directly under metal claddings.

Perhaps the most prevalent misconception concerning cool roofing is that specifying the highest possible Solar Reflectance and Infrared Emittance properties is a universal answer for energy saving worldwide. It isn’t.

Did you know we have a ‘white roof society in New Zealand? The Sylvia Park Shopping Centre, Pak ‘n Save stores and the Auckland Airport already all have white roofs but then you don’t see them very often. 75% of all Commercial and Industrial buildings built in Auckland in the last 20 years have roofs <10˚ which are all shades of white. Architects and designers like colour and when it comes to residences, but white is not a favourite designer colour.

To evaluate roof energy efficiency and heating and cooling costs, to use only roof cladding material properties, such as Solar Reflectance Index and reflectivity and emissivity, will not tell the full story. The heat gain, heat loss, cooling load, and energy usage and the performance of the whole building must be examined The ability of the roof to re-radiate absorbed solar infrared radiation back to the atmosphere takes place at all times, but mostly at night. The MRM COP 4.2.4. calls this night sky radiation.

The light reflectance value (LRV) of a colour is measured by a spectrophotometer which measures how much of the visible spectrum of light is reflected by a particular colour. NZ Steel publishes LRV values (0-100) and TSR for different paint colours within their colour range (see table).

ENERGY STAR is an international standard for energy efficient consumer products originated in the United States of America. It was started in 1992 by the US Environmental Protection Agency and the Department of Energy and since then, New Zealand, Australia, Canada, Japan, and the European Union have adopted the program. ENERGY STAR is New Zealand’s mark of energy efficiency, typically awarded to the top 25% most energy efficient products by category which generally use 20%–30% less energy. Energy Star uses Solar reflectance but does not include metal emissivity in its assessment of colour. It does however include a 3 year ageing for SR. Only light colours (L) can be used for roofs < 10˚, and medium colours (M) can be used for roofs > 10˚.

The U.S. Department of Energy (DOE) uses the low sloped cool roof definition but defines steep sloped cool roofs as those with a 3-year aged Solar Reflectance Index of 29 or greater. The reason appears to be because the majority of low slope roofs in the US are membrane and do deteriorate over a 3 year period. The result of all this is that these figures can only be used as a broad classification which is probably all that is needed for our purpose.

New Zealand has favourite colours for roofs and NZ Steel and Pacific Coil Coaters both reported recently that six colours (in order of popularity) accounted for over 50% of painted roof cladding in New Zealand. It is presumed that Titania was predominately supplied for commercial and industrial roof cladding

So how do they stack up the ‘white roof’ policies of the rest of the world.

Australia likes to be different, as we know, and as a measure they have chosen absorbance over reflectance as the rest of the world has done. The values given to absorbance in Australia use the basis of calculations by the BCA (Building Code of Australia 0-1.0) and BASIX (NSW Building and Sustainability
Index) to provide a rating for each coloured metal roof described as light, medium and dark; however they are both different! The NSW BASIX requirements include not only the roof Solar reflectance but building solar orientation, cross ventilation, insulation as well as rainwater tanks and grey water.

There are other various methods for determining reflectance values and although NZ Steel and P.C.C. may present them in different ways they generally fit the Eco template.

It is pretty obvious to any who have tramped in the hills at dusk in misty rainy conditions that the brighter the colour the greater the relief on discovery!

In some visually sensitive environments in NZ, (Waitakere and Islands in the Hauraki Gulf) there are town planning requirements which dictate that darker
colours must be used which must be below 40% reflectance (Queenstown 35%) even though this is not required for compliance with the N.Z.B.C.

Solar reflectance and glare are not the same if you measure Total Solar reflectance. Glare can only come from the visible light spectrum. There seems to be a great tug-of-war going on about colour.

There is a conflict of interest here between the eco movement who are all for green (sorry white) roofs and those people who have to put up with the glare associated with white or plain Zincalume roofs. The basis for colour comparison is BS5252 and greyness is classified A- E with D & E being bright and generally unacceptable.

On the left is the white team consisting of the advocates for white roofs and on the right the green team those that prohibit their use. While there are good reasons for each, the designer has to know about both. While there is no law governing these at the moment, there are Local Authorities that have the power under their District Scheme or Resource Consent process to tell the owner or designer what colour you can and can’t use. They also have jurisdiction over glare as a measure of nuisance to individual neighbours or the public at large.

The method adopted by Auckland Council and Queenstown District has been to designate not only the reflectance value but the ‘greyness’ of the pigment. New Zealand manufacturers offer a wide pallet of colours to choose from albeit that they are somewhat subdued ‘earthy’ ones due to the fact that they are inorganic pigments as most bright organic pigments are subject to excessive fade.

What this has meant is that there is now group of colours known as ‘Natural’, Habitat’ ‘Neutral’ or ‘Recessive’ available which take the kick out of the colour.

There is no such surface as a nonreflective one because you would not be able to see it! So glare as a nuisance is a relative thing. Queenstown have decreed that any colour >35 solar reflectance is a nono and 35 metres is the ‘distance’ so if you have a neighbour whose roof is white and glarey but 36 meters away – tuff. Such conflict of interests does not make for good neighbours so check out the houses around you and talk to your neighbours! One thing most TA’s (in NZ & Aus) are agreed on is that any unpainted Zincalume roof > 10˚ is a potential glare nuisance. Glare is related to gloss level but one mitigating factor is that it will get less over time.

Paint manufacturers like high gloss paints because they last longer!

There are many arguments that glare only occurs at some time of the day or some time of the year but designers should be aware that litigation has forced owners to repaint their coil coated roofs another colour which unfortunately voids the manufacturers warranty.

Colour makes a big difference to expansion and noise of metal roof cladding, however that is another subject for another day.

Designing white roofs for industrial and commercial buildings makes sense because if the building is air conditioned it will save energy, if it is not then the surface temperature of the underneath of the cladding will be lower and result in less radiation to the people working inside it. Because these types of building have generally long length cladding, a major plus is that the stress on the cladding and the structure will be minimised due to less expansion which will result in less noise and expansion problems.