Heat-proofing your roof
From Mvca
Cool Roofing
We know that ice, snow and white roofs reflect light, while black surfaces absorb solar radiation, becoming very hot. Bare galvanized metal also absorbs radiation, much like a black roof, although it looks almost white in the glare.
Hot tin roof – 109 degrees C!
A galvanised iron roof can get to 109oC on a still, sunny day, hot enough to boil water. It gets very hot in the roof cavity - a 16 year old insulation worker died of heat stroke in 2010.
Heat trapped in the roof space diffuses into the house throughout the night, making it stuffy and hot.
Solar panels heat up and become inefficient. They work by converting solar radiation to electricity. When very hot, they physically cannot absorb any more energy. Cool roofing makes solar panels more efficient in summer.
Heat reflective paint treatments
Heat reflective roof paint makes houses significantly cooler. One company, Insultec, sponsored a study which found that a metal-roofed building without insulation was 15oC cooler when it was painted with Insultec white membrane paint. Even a well-insulated house may be 5-10oC cooler with heat-reflective roofing paint. Insultec claims the paint is more effective at keeping buildings cool than insulation. Obviously the best option is to use both.
Coloured cool roofing paints
Energy Star heat reflective paint, made by Astec paints in Australia, comes in colours as well as white. Pale colours work best: roofs painted Pale Cream, Light Biscuit or Sandalwood are only a few degrees warmer than the surrounding air. These light colours reduce the roof temperature by as much as 50 degrees C. They are CodeMarked in the Building Code of Australia and comply with the most stringent requirements for heat resistant materials - BCA Vol 1 J1.3 (b). Intermediate colours such as greys, and pink terra cotta colours work quite well, but dark Energy Star paint colours such as charcoal, red oxide and carriage green may get to 85 or 90oC, cooler than bare metal or ordinary paint but still hot.
The heat of the roof is determined by its colour, the material it is made from and the type of paint. Pale roofs are cooler than dark roofs. Tiles are cooler than metal. There are insulating paints specially designed to paint on tiles and Colorbond, as well as on galvanized iron roofs.
FAQs
Does it work?
Speaking personally, yes. I painted our flat-roofed family room myself. It used to be the hottest room in the house in summer. Now it’s the coolest. On a hot summer day the difference is probably 6 degrees (comparing before and after) but there is no difference on a normal coolish day. (source Barbara Hutton, Essendon.)
How does cool roofing work?
Heat resistant paint reflects infra-red solar radiation back to space. The surface of the painted roof on a hot day stays cool enough to walk on in bare feet, (unlike a bare metal roof, which will burn your feet). The reflective paint prevents infra-red radiation even entering the roof material, so it is not converted to heat. Energy Star coatings use nano ceramics to reflect the infrared light. Other products such as Insultec use membrane coatings.
Will “cool roofing” make the house colder in winter?
Insultec insulates significantly and will keep the house significantly warmer at night and in the winter.
Astec coloured Energy Star paint doesn’t insulate. It may cool the roof slightly on a sunny winter’s day but makes no noticeable difference inside. There is relatively little difference between the winter temperature inside the house, and out on the roof so minimal heat diffusion occurs. But in summer when the roof is 109oC and the interior is 22oC, the difference is a massive 87 oC. Even with excellent insulation, there will be heat diffusion. Cool roofing paint then makes a major difference.
I have insulation already. Will cool roofing make a difference?
Yes. Insulation is essential in winter, but most houses already have it. If the house still gets hot in summer it needs cool roofing as well.
We already have air conditioning
Cool roofing will cut the amount of electricity the air conditioner uses, saving greenhouse emissions and money. Insultec commissioned studies in Brisbane and Karratha. One study found that air conditioning costs were 61% lower in a house painted with Insultec, compared to a similar house without it. Astec, too, quotes the EPA as saying that cool roofing saves up to 50% of energy bills for cooling.
How does cool roofing help combat climate change?
Insulation cools you personally, but your roof will still absorb solar radiation and convert it to heat, which heats up the surrounding neighborhood. When many houses and roads do this it causes a “heat island effect”. Cities can be 5oC hotter than the surrounding countryside.
Reflecting solar radiation away not only cools your own house, but the surrounding area. For more independent scientific explanations see the Berekely Heat Island Group website. This group of scientists based at Berkeley Laboritories in California believe cool roofing and paving could make a significant dent in global warming. Google Bekeley Laboratoroes’ “Cool World: A Modest Proposal to Cool the Planet by Cooling the Neighborhood”, 11 Dec 2008.
Cool roofing reduces the need for air conditioning which creates summer peaks of electricity demand. Victoria’s brown-coal-based electricity produces 1.31 tonnes of greenhouse gas for every MWh of electricity (among the highest emissions in the world). Peaks cause blackouts, requiring more coal fired power plants to be built so we especially need to reduce summer peak demand.
Will the roof need repainting and cost more in maintenance?
No. Cool roofing paint extends the life of the roof by reducing expansion and contraction. Heat damages most paints, but cool roofing paints don’t get very hot – thus, Astec claims its paint lasts 8 times longer than normal paint. A professionally painted galvanised roof lasts decades longer than an unpainted one, and roofing iron costs tens of thousands of dollars to replace, so in the long run cool roofing should save money.
Does Colorbond get hot? Yes, especially in dark colours. Colorbond claims that “Classic Cream” or “Surf Mist” Colorbond roofs incorporating Thermaguard have 31 - 32% solar absorptance (about 70% reflectivity). But most of the colours are classified as dark, with high solar absorptance of around 60 – 70% and poor reflectivity. (See Colorbond website.) There are cool roofing paints for clay or concrete tiles, or Colorbond.
Insultec and Astec websites are on the internet.
'Astec: phone Mike Bailey 1300 789 686 (northern Melbourne and country) to organise supply or painting, or the Bayswater supplier on 9729 8400. Astec has coloured paints to suit residential roofs. Insultec is a white insulating membrane suitable for flat roofs. Its main offices are in Sydney. Other brands of insulating paint also are available. Ask your paint shop to order it if there is none on the shelves. If possible get the application done professionally: preparation is difficult.
