Friday, 14 March 2014

Roofs in black and white

The current issue of  the Australian on-line magazine The Fifth Estate features a Special report: the case for white roofs by Cameron Jewell, and the byline:

With externalities including an increase in the urban heat island effect, peak electricity demand and climate change, is it time to ban black roofs?

The best thing about the article is that it draws together in one place most of the conflicting considerations in choice of roof colour, and even to some extent, roof material. The worst thing is that it is missing perhaps the most important variable: the construction of the roof.

The issue with roof colour is the absorption of solar radiation. In any climate where summer overheating is an issue, the solar radiation absorbed rather than reflected has the same effect as raising the outside temperature, and therefore the temperature difference between outside and inside with which any insulation under the roof then has to deal.  The equivalent rise in temperature is known as the SolAir fraction, and can be very significant in the total cooling loads.

But this is only true if the roof is mainly relying on reducing the heat flow by conduction. It becomes less important if the roof system has low emissivity cavities which are effectively ventilated to the outside.  In an ideal version of such a roof, the ‘inside’ face of the cavity would be barely warmer than the temperature of the ventilating air, and therefore the effect of the solar radiation would be effectively neutralised, regardless of the roof colour.

In some other countries, such ventilated roofs are commonplace, even usual. It is important to understand that we are not talking about a ventilated attic of the kind to be found on a typical Australian pitched roofed house. Rather, the ventilated cavity is directly under the roofing material, with the low emissivity achieved by a downward facing foil faced membrane. The cavity is then formed by a rigid sheet lining, supporting the second of the ‘double sarking’ layers, this time with upward facing foil. The secret to the arrangement is to leave an unobstructed air path that will turn into an efficient buoyancy driven air stream, inducing intake at the low point and exhausting hot air at the ridge. This is achieved by the primary battens supporting the roofing running up the slope of the roof, rather than across the slope.  And of course it works best with roofs that are simple gable shape.

Anyone who has followed the description so far will realise that the system, ironically, works best if the roof sheeting is dark, because that will provide the necessary heat to drive the ventilation.
So what about winter, when it would be good to benefit from the extra heat absorbed by the dark roof? An additional elaboration of the system switches over to capturing the warm air in the cavity, and pulls it into the house interior. Admittedly, this may require a fan, but the fan power is small compared to the heating obtained.

OM Solar system Japan
In case anyone thinks the description is fancifully theoretical, it is in fact essentially the type of roof construction employed in Japan, parts of Europe, and of North America. Only the last bit, the winter heat capture is a relatively recent development, as exemplified by the Japanese OM Solar system, which gathers so much heat from a typically small Japanese house roof, that it also puts a heat exchanger in the loop to provide the prodigious amounts of hot water needed to support traditional Japanese bathing habits.
Summer daytime operation

So why don’t we build such roofs in Oz? Because we traditionally used to think it’s for those wankers overseas, while we can save heaps doing away with the plywood sheeting, and even sarking under the tiles. Well, the time has come to look again at what those wankers are doing, why they are doing it, and to spend a little more on building energy efficient smaller houses, rather than the world’s biggest poorly built homes.

Oh yes. And then you can have your black roof and feel good about it.


Chris_S said...

You can have your black roof and feel good about it and I understand that there is a certain appeal to specifying black roofing. But it seems to me that this somewhat ‘extravagant’ roof construction specification, for Sydney in particular and anywhere north of here, seems a little over the top to harness any energy it may provide in winter when there is only 2-3 months of the year that it would be required.

Therefore that’s a fair amount of pressure placed on the correct construction and specification of a system that has to work for 9-10 months of the year, or risk serious overheating and rise in energy costs due to having to cool your home because it doesn’t work.
Local builders aren’t familiar with it and clients probably wouldn’t understand any benefits it may have. This argument can also apply to the specification of fireplaces in new residential construction; wouldn’t you only use it for 8 weeks of the year? The rest of the time it gathers dust.

Perhaps we should definitely look at different construction methods for roofs in our Australian climate, but perhaps the 'energy' would be better spent on client education?

bronte.doherty said...

After conducting further research on the OM Solar Systems being used in Japan, it shocks me Australians could be so ignorant to the benefits, (well, actually no it doesn’t). Yes, it was developed by the foreigners and yes, we are a little to proud, but enough is enough. It became clear to me, this roofing system is the way forward.

Slightly more expensive in construct and more difficult to do, but I would not describe it as ‘extravagant’, when it’s long term effects are far greater and beneficial to all. If homes are designed properly and carefully in Australia, we could say goodbye the air-conditioning and heating all together, with an understanding of thermal mass and natural ventilation.

So, it’s really a win, win situation. “The under floor concrete slab, which as pure thermal, mass saves heat in winter for release at night and on cloudy days, also serves to cool the interior in the hotter summer months.”1 So, wouldn’t it be suffice to say, this system could potentially work in almost any climate in the future? Maybe excluding countries on the equator with little to no season change. Black roofs, white roofs, whatever you fancy, its all about the construction type.


Anonymous said...

Anonymous said…

As an Engineering student, this kind of posts on the blog interests me as they are something that we, engineers, never could have thought of. This very simple question of “Why are the roofs black?”, is not something that we could have spent time to think about as we are only used to solve problems that somebody else have thought about.

So I started to research which one is better option, black or white. But I ended up in the endless “conflicting considerations in choice of roof colour”. It was a problem that could differ depending on which environment this roof is going to be installed. So I found it was very hard to say this is better than this. In your blog, you have mentioned that “the worst thing is that it is missing perhaps the most important variable: the construction of the roof”, but I could rather say that the ideas were missing perhaps the most important variable: the people. I believe no matter how good it looks or how cheap it is, if it harms people in any ways, or if it has any possibilities of harming people in any sort of the ways, I think it should be reconsidered. As I read through The Fifth Estate online magazine article, I found more information about which colour reduces “annual cooling energy costs” and the how the colours and the design of the roofs should “blend into surrounding environment” but there was almost none talking about which actually is better option for the people.

You have mentioned that “the effect of the solar radiation would be effectively neutralised, regardless of the roof colour”, but after a research I could not agree with that. From the new paper in the Journal of Climate, Mark Jacobson, a professor of civil and environmental engineering at Stanford and his colleague found that ‘while white surfaces cooled houses, they also reduced cloudiness, allowing more sunlight to reach the ground’ which is one of the largest factor of global warming. And since ‘none of these [roofing] studies had ever looked at the climate feedback effects, how clouds and pollutants would be affected’, I could not say ‘effect of the solar radiation [due to the colour of the roofs] would be effectively neutralised’.

So I thought what would be better. As reading throughout your blog, I found that you were a great fan of ‘change of direction for renewables’. As I read through your post on 31st of May 2012, on “Germany Sets New Solar Record By Meeting Nearly Half of Country’s Weekend Power Demand”, I thought this could bring a better option. As solar energy engineering student, this part was one of the most interesting part of BIPV. These days, technology develops so fast that solar roof tiles are already available and are in use around the worlds. []

It cools the house by absorbing the sunlight and produces electricity that could be used to both warm and cool the internal house when needed that could solve the conflict we had in black and white roof. For your interest, these days, even solar windows are being developed that could produce electricity from both artificial lights and sunlights. []

From Ayn Rand’s The Fountainhead, a young architect says “I don’t build in order to have clients. I have clients in order to build”. I believe this mind of putting people in priority than any others will make myself to a greater person hence a greater architect.

1. Problems with white roofs:

2. Better option than white roof:

Steve King said...

Excellent point about 'climate feedback'. I have to look it up.

But you misunderstand the role of colour within the 'system boundaries' of the building itself. If you build a roof such that it has a cavity which has two low emissivity surfaces, and is clear for laminar air flow that is exhausted at the ridge, you can make the roof assembly on the inner side of such cavity subject to a temperature difference in which the absorbed solar radiation plays almost no part. This is because the air in the cavity will not be appreciably warmer than the air temperature outside.

To drive the convection in the roof cavity, you actually benefit from a darker colour.

But while some houses in North America, many in Europe, and especially many in Japan are built with such roof plane cavities, in Australia it is not considered a necessity.

Jiwen Yu said...

This article really gives me a new perspective about the roof system, which the heat absorbed by the roof could work on the ventilated air for other uses. As the diagram of OM Solar shows that, the solar heat gained by the roof in winter could warm the air in the roof cavity and used to warm the entire house. For summer, the heat is used to heat water. In both the seasons, this system could reduce the energy consume. In addition, the cavity of air could also work as an insulation material. Especially in summer, the heat of the air is absorbed by the water, which could make sure the air would not affect the building temperature directly, which is very rational in summer. Therefore, in this case, the dark roof might work better than the roof which light colour, because it could absorb more solar energy.
However, there are also some studies demonstrated that, the colour is not the reason of the solar gain. Material is the main factor to affect the amount of solar gain. A green roof could reduce a large amount of energy used for cooling down. It also pointed that dark roof might not very useful for heat gain because the angle of sun is low and the daytime is short.
For my own opinion, the ventilated roof system is really interesting and could be applied if the climate is suitable. The colour of the roof could be picked to fit with the material of the roof as for some material, certain colour could work better.

Rena Yang said...

After I have read this article, I have a strong understanding of roof system, how it works for ventilation by using temperature differences within the ventilated cavities. The OM solar system works well in Japan might have two reasons. One might is the solar radiation is not that strong as Australia, so the temperature of roof is not that high (different climate). The ventilated cavity works better. The other reason is Japanese people have traditional bathing habits. Therefore, during the summer they can effectively use unwanted heat to get enough hot water.
Although this system is works in summer to cooling down some of the room temperature, I still think the black roof might bring more issues in summer. Dark surfaces have higher solar absorptance than light colours,3 also Australian have very high solar radiation in summer. During the summer, the temperatures under dark roofs can exceed 60°C. That means that is unwanted heat, we perhaps to solve by using air conditioners. The federal government published a report in 2008 entitled,Energy Use in the Australian Residential Sector: 1986-2020. The study noted that "the penetration of air conditioners has more than doubled in the past 10 years to about 65 per cent,". Australia has the highest per capita greenhouse gas emissions of any OECD country, and more than 4 times the world average.1
Why many of the Australia houses still use black roof? 1,2
• Dark roof blends into the surrounds better.
• Neighbours roof colour, we want to “fit in” to our community.
• Building industry. “Dark roof come about from the same beliefs. Builders install them because thay think customers want them, and customers want them because the building industry always uses dark roofs, so they must be the best option.”
• Local government. The majority of the housing already has a dark roof, so new buildings have to blend into some degree.
I have also found some aspects from articles how to solve dark roof problem to make it more efficient/ lower building thermal loads. 1, 4
• Use powdered additive mixes with household paint to creat a heat reflective coating that might make surface 20 to 40°C cooler.
• Change materials. Reflective paint steel roofs up to 18°C cooler than old silvery zincalume roof.
Finally, from my point of view I think use light colour roof in Australia might a better choice. That can reduce use of air conditioning (saving more dollars), also slash greenhouse gas emissions.


Yang LIU said...

The choice of roof color actually affect on the absorption of solar radiation on the particular roof area. Therefore, it will affect the entire house heating or colling system.

In terms of the black roof, the black roof material would absorb most of the solar radiation, and then warm up the internal temperature of the house, it is a good idea in some extents. However, in the tropical climate zone, the temperature of the outside is extremely hot, this black roof would lead to the overuse of the air conditioning. Also, it hard to say that it has no effect on the global warming.

To think about the white roof, this sort of roof material would reflect more solar radiation, rather than absorb it. Thus, it means there are less heat transmit into the interior of the house. So the white roof is much better in the tropical climate zone. Also, from an life-cycle analysis,“white roofs cost $9 per square foot less than green roofs over 50 years, or $0.30 per square foot each year.” therefore, the white roof, to compare with black roof, would be much cheaper sort of green roof in the world.

In this“black or white”question, there is an interesting situation that in some locations in North America, also the Northeast of China( heavy snow in winter). Because of the solar absorption and the climate condition, the black roof is more popular. None the less, after few times of snow, the black roof tuns white as there would be covered by snow. Then, the internal heat transmit outward through the roof and then melt the snow. But, due to the extremely cold temperature, the melted snow becomes ice on the roof surface. Unfortunately, the original black roof start work as the white one which reflect the solar radiation and transmit much less heat into the internal house. To release this serious problem in a more sustainable way( instead of air conditioning), the solar panel is installed on the top of the roof for the water heater or shower( I am not quite sure if they use the sort of OM Solar system).

Overall, the major issue in the decision of black or white roof should well consider of the climate zone, orientation, durability of materials, sustainability and other aspects, basically, all we want would be cooling in summer and warming in winter.

Anonymous said...

Firstly, I am agree with several comments above. For using choice of black or white colour roofs, needs to be considered which kind of climate zone they are in. Look at Australian climate, which tends to be more months in hotter days rather than cooler. For this sort of OM Solar system with black roof is suitable for Japan rather than Australia, because Japan is located in cold and temperature climate zones. In the winter, this OM Solar system could works through absorbing more solar radiation in black roof to make air warm, and trapping warm air in cavity to heat the house interior temperature. In the summer, it is easy to turn unwanted heat to heat water to satisfy Japanese traditional bathing habits. They are why black colour roof is more efficient in this system, it could absorb more solar radiation to turn into useful aims. But, in Australian, we could not pay this system just in order to satisfy people’demands in around one month coolest weather. And it seems to be nothing to heat for in summer. It seems to be not economical. And this is why Japan gets more benefits from this kind of OM Solar Systems with black roof rather than using in Australia.
For walking around Australian suburbs, we could see more typical Australian pitched roofed house with darker roof tiles, and they have more traditional construction which put cavities in to get better ventilation in roof and take heat away. It seems that Australian traditional roofs keeping with traditional darker colour. Actually, Australia tends to be more tropical climate, has more summer days than winter days, I think we could try to modify more darker roofs into lighter ones in some extend, in order to reflect more unwanted solar radiation in summer days.

Anonymous said...

It seems in most of the home built in Australia has a dark coloured roof. The choice of roof colour is not as easy as what we pick what to wear in different season.
I am really into this topic is because I have seen an old house renovation video online before, and it is introducing the OM system in Japan. The problem this house had before was too cold and there is no insulation material under the floor. Also the old house have thick dark roof tiles (like traditional Japanese temple or housing) the architect reused these thick roof tiles as absorption materials to gain solar heat. The new system is the cavity underneath the dark roof system conducts down the warm air to the ground floor. I would not say it would be high in cost and difficult to do because in the video the architect simply used a fan (ventilation system) and a plastic round pipe that leads straight underneath wooden floor layer. The video did an actual test and the result is the floor had immediately increased up to 2 degrees. This would show a great impact on heating needs during winter. Not only the cost down during winter but also the solar heat gain can also be the pre-heat of the household water supply.
So why not in Australia, having a win-win situation that during summer the stored heat gain can pre-heat water supplies and during winter the roof system could be a overall heating system and reduce heating costs?
To be honest in regions like Sydney where cooling needs is much higher that heating needs, I don’t think that dark roof systems would be a solution.
I did some research on issues of the “urban island heating effect” which is talked several times in the “Cool roofs versus dark roofs” article. The effect is about cities that have the dark asphalt roads that absorbs heat can easily gain a temperature up to 50 degrees during summer. Each year summer in Australia seems to break numerous heat records. To be in contrast with the dark roads, shouldn’t light roofs be a solution to these issues? However, in the Journal of climate “if all roofs were changed to white roofs, while temperatures would reduce in urban areas marginally, they would cause more overall global warming because the cooling of cities could reduce cloud formation.” In Australia, that we have more UV radiation and no protection from the cloud… we do need a second thought of having white roofs.

Perhaps these all leads back to the roof construction type and the different climate issues. Also it depends on people and how architects built with more consideration.


Video of the simple OM system 17:53-24:58: (only in Japanese unfortunately):