Colours for Cool Roofing
Painting your roof with heat-resistant paint can reduce the temperature on the roof by up to 50oC. This is especially true if you have a dark roof - red, dark green, grey- or unpainted galvanised iron , which can get to 109oC on a warm, still day: hot enough to boil water.
If you have insulation, houses with “cool roof” paint are typically about 5oC -10oC degrees cooler inside than houses with untreated roofs on a hot summer day. A building without insulation (e.g. a factory) can be 15oC cooler. In winter the “cool roofing” paint has no significant effect – it doesn’t make your house cold.
If your roof is hot you will find:
It’s hot at night: Trapped heat in the roof space keeps the house too hot to sleep.
Solar panels: Roofs may become too hot for solar panels to operate efficiently. They
should operate under 90 oC – not 109 oC. They may even shut down.
Water in solar hot water units boils over.
Hot roofs create a “heat island effect”. Cities are 5oC hotter than the surrounding
countryside.
If you want a quote on cool roofing please click “cool roofing” on the “Registrations”
page.
Heat resistant paints often use nano-technology – tiny ceramic beads to reflect heat. Astec Energy Star paints is codemarked in the Building Code of Australian as a heat resistant product. It comes in colours (not just white). These coatings can
- keep your house 5-10 oC cooler in summer; and save 50% on air conditioning costs.
- reflect solar radiation back into the atmosphere, before it is converted to heat.
- protect your roof from infra-red radiation, contraction and expansion, so it lasts longer.
Roof Temperatures with heat resistant paints
On a still day at 37oC the temperature of roofs with light-coloured heat resistant paint is shown below. (Colour reproduction here is not accurate – ask us for a printed colour chart.)
Broken White 48oC Neutral White 50 oC Off white 53oC
Light Cream 50oC Pale biscuit 53oC Gull grey 58oC
Sandalwood 52oC Mid biscuit 57oC Light Latte 56oC
Stone 58oC Tuscany 60 oC Merino 58 oC
The above colours comply with the most stringent requirements for heat-resistant materials: BCA Vol 1 J1.3 (b). Darker colours are CodeMarked in other sections but are not as cool.
Examples: Charcoal, 84oC; Red Iron Oxide or Carriage Green, 90oC; Mist Green, 76oC.
This is still cooler than a bare metal roof. According to the Energy Star booklet, a roof painted in Gull Grey heat resistant paint will be 58oC - still 51o cooler than bare metal.
Solar reflectance normally ranges from 0 for a black surface to 100 for white. Base White heat resistant paint has an SRI (solar reflectance index) number of 114, cooler than normal white paint. Unpainted galvanised iron is has an SRI of -7.71, hotter than black paint.
How much cooler will it be inside the house?
This depend on insulation, heat entering the house through unshaded windows etc. Heat also enters from the roof cavity, even in an insulated hyouse. A 16 year old worker recently died installing insulation, illustrating how hot it gets in the roof space.
University studies sponsored by a cool roofing paint manufacturer found that an insulated house may be 5-10oC cooler with cool roofing treatment, so instead of being 30oC inside it would be 25oC or 20oC. An un-insulated building was 15oC cooler inside when painted with cool roofing paint. Cool roofing can be more effective at cooling than insulation. The best option is to use both.
“Houses should have red or black roofs”
What we think is normal depends a lot on what we’re used to. In Australia we copy British or North European colour fashions but they don’t suit our climate.
In our “Mediterranean” climate, white or pale roofs are better. Having a black, dark red, blue, green or brown roof for the sake of fashion, when it makes the house too hot to sleep at night, is downright masochistic.
In sunny Mediterranean cities, cool white roofs keep the houses cooler.
“My roof is metal. It looks a light colour and should reflect heat”
Iron looks pale but it absorbs about 90% of the radiation. It also has low emissivity, so heat builds up. This is more important than the colour. Infra-red radiation is beyond the visible spectrum: we can’t see it, but it is there. Bare metal and black roofs are the hottest possible roof materials.
What does cool roofing cost? Cool Roofing costs about the same as painting a house roof in normal paint - thousands of dollars if done by a painter. (The paint is a little more expensive than normal paint but it makes little difference to the overall job.) A metal roof lasts decades longer if painted, so it's worth it - a metal roof costs more to replace than a paint- job! Cool-roofing paint is claimed to last 8 times longer than normal paint because it repels infra-red radiation which normally damages paint. It comes with a long warranty.
“What is the point of cool roofing? Insulation is just as good”
1. Insulation is essential in winter but most houses already have it. If they still get excessively hot, the roof cavity may be too hot for the insulation to cope. They need cool roofing as well.
2. Insulation keeps us personally cooler, but the roof still converts incoming radiation into heat creating a “heat island effect”. Cool roofs bounce radiation away, cooling our cities and our planet.
Does cool roofing save on air conditioning costs? One study in Karatha found that air conditioning costs were 61% lower in a house with cool roofing than in a similar house without it. The EPA says that cool roofing saves up to 50% of energy bills (p.7, Astec booklet).
How does cool roofing help combat climate change?
1. When hot roofs and asphalt convert solar radiation to heat it causes a “heat island” effect. Cities may be 5oC hotter than the countryside - more in late afternoon, creating ozone and smog. If the radiation hits a white surface (ice, snow, white or cream paint) it bounces off: the surface doesn’t heat up. This can cool cities and even the whole planet.
For more independent scientific explanations see the Berkeley Heat Island Group website. This group of scientists based at Berkeley Laboratories in California believe cool roofing and paving could make a significant dent in global warming.
2. Cool roofing reduces the need for air conditioning which chews up electricity. Victoria’s electricity is made from brown coal and has very high greenhouse emissions. Summer peaks of electricity demand cause blackouts. Cool roofing would reduce these summer peaks and avoid the need to build more coal-fired power plants.
Will the roof need repainting and cost more in maintenance?
No. Maintenance costs will be reduced. If the roof is metal it can last decades longer if it is professionally painted. Heat damages most paints, but cool roofing paints don’t get hot.
Astec claims heat resistant paint lasts 8 times longer than normal paint. It also keeps roofs cool, reducing expansion and contraction, wear and tear on metal or tile roofs.
I have a tiled or colorbond roof
There is cool roofing paint especially designed to use on tiles (clay or concrete), Colorbond etc.
Does Colorbond get hot? It depends on the colour and type but generally, yes. Colorbond is now advertising “Thermatech” as a new component to make their colours cooler. A “Classic Cream” or “Surf Mist” Colorbond roof incorporating Thermaguard has 31 to 32% solar absorptance (about 70% reflectivity). This is still higher than the cool colours shown above. Most Colorbond colours are classified as dark, with high solar absorptance of around 60 – 70% and poor reflectivity. Colourbond can be painted with cool roofing paints.
How do heat reflective paints work?
Many new paints contain nano particles of ceramics to repel infra-red radiation. These particles may be colored, imparting a colour to the paint. Others have been on the market for 20 years and use white membrane coatings, often suited to industrial/commercial buildings.
Sources:
Astec booklet “Energy Star Infrared Heat Reflective – cool solutions to dark colour!”
Heat Island Group:“Cool world: A modest proposal to cool the planet by cooling the neighborhood” by Allan Chen, Berkeley Laboratories (US Department of Energy)
“White roofs cool the world, directly offset CO2 and delay global warming”, Research Highlights, LBNL Heat Island Group, Nov 10 2008.
Astec, Colourbond, Heat Island group, Insultec, and websites.
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