It’s great to see what you Kakariki people are up to and nice to link up with others working for a positive change. I see you’ve been doing your homework on white roofs. I didn’t know about the British Institution of Engineers’ position on white roofs until I saw it on your website and this is timely because next month I’m giving a talk at the Engineers for Social Responsibility monthly meeting in Auckland.

Thanks for questioning what I said about the potential of white paint to keep your house slightly warmer at night in winter. I like good questions because it causes me to double check what I have said. There is a short video of Dr Steven Chu talking about white roofs and he mentions this point (Steven Chu is a Nobel Prize winner in physics and Obama’s Secretary of Energy) and a link to this video is on the White Roofs website front page by clicking on Chu’s image on the right hand column. In physics it is well established that matt black radiates heat at the fastest rate out of all the colours and that white is the colour that radiates heat the slowest. If you wish to read up on it you can look up “blackbody radiation” on Wikipedia. I remember this phenomenon being covered in secondary school physics. You may have seen infrared radiators installed on ceilings, and they are matt black for this reason. However, your question prompted me to check it out with an experiment this morning to gauge how big this effect is and I did observe it but it is small (I can tell you how I did it if you wish). There has not been any study that I know of to quantify whether a white roof would have a significant effect on slowing down the heat escaping through the roof in winter. So in hindsight, in the absence of a study with numbers that I can quote I think it may be best if I do not mention that effect until I have some numbers that show it how effective it might be, as I don’t want people getting the impression that it could be more beneficial than it might be. It’s my intention to deal in facts. This is one of the things that will be nice to have researched as part of an unbiased collaboration with a university later on when there is time and if the opportunity arises.

If you are painting white you might be interested in a brochure I put together, “Useful tips when painting white”, which can be downloaded from the website by clicking on the Downloads box on the right hand side of the front page and then choosing that PDF.

If I can be of assistance then let me know. In case you don’t know, Resene have helped the project through a generous discount on roof paint via a discount letter they gave me for people participating in the white roofs initiative. What you do with the white roofs idea is up to you, and full credit to you people! and I am here to help if I can.

I’ll take up your suggestion and add my previous reply as a comment on your page on White Roofs so that Kakarikians and others can read it.

Thanks
Ian

Thanks for your very detailed explanation! It’s great to have well researched arguments.
I’m curious about your comment “white paint doesn’t radiate heat (from inside the house) as fast as dark paint.” I’ve never heard of this?

A few days ago I wrote this about white roofs: http://kakarikistreet.wordpress.com/2011/06/16/the-white-roof-project
If you’d like to add your more detailed version in the ‘comments’ I’m sure it’d be appreciated by our group and other readers

There are several of us about to paint white and when we do, we’ll definitely be sending in Before & After shots!

Thanks
F

My apologies for not replying sooner, I had not thought to check for posts from time to time.

A number of people have asked me about the issue that Jeff has raised, that if you live in a cooler climate should you use a white roof? Or to put it another way, if a white roof cools your building in summer will it also cool it in winter, and if you live in a cool climate will this mean that the summer cooling savings from reduced air conditioning use are outweighed by needing to use your heating more in winter, and would a dark roof be better if you want to reduce costs and emissions? Some blogs and articles I have read are of the opinion that if the number of cooling days is less than the number of heating days, that you are better off with a dark roof, but these opinions are not backed by science. As I shall discuss, the number of cooling days versus heating days is not a measure of annual energy and emissions savings between a white roof and a dark roof.

A white roof cools the planet in two ways: (1) by reflecting sunlight back into space, and (2) by cooling the roof so that less heat comes into the building, so for buildings with air conditioning it reduces energy use and emissions, and for buildings without air conditioning it gives a cooler house that is more pleasant to inhabit in summer.

For buildings with air conditioning, studies have shown that if you have demand for cooling in summer and heating in winter, a white roof pays off in energy cost savings and in emission savings. A recent study, probably the most rigorous ever undertaken on this issue, considered commercial buildings across the USA and found that savings are greatest in the hotter parts of the country (such as near the Mexico border) and less in cooler parts (such as near the Canadian border) but still savings. Does this interest you? Then read on! I’ll talk more about this below.

But first I’ll talk about the pros and cons of white roofs on houses that do not have air conditioning. If a house is comfortable in summer and does not need cooling but does need heating in winter, and if say global warming is not an issue for the occupiers, and helping to cool the planet by reflecting sunlight back into space is not an issue for them either, then they can use a darker roof but it seems it’s not going to help them much with their winter heating bill for the following reasons.
– In winter the sun is much less intense than summer, and the difference in temperature between a white roof and a dark roof (which can peak at more than 35ºC in summer) is much smaller in winter. In many locations where the winter sunlight is typically only one third that in summer and it could be said that each summer cooling day is worth about three winter heating days, so simply comparing the number of cooling days to the number of heating days is misleading.
- In winter there are far more cloudy days than in summer, and at those times the difference in temperature between a white and a dark roof may be small or negligible, and in winter the days are shorter and there are significantly fewer sunshine hours than summer. If there is no sunshine on the house then it does not matter what colour your roof is. Again, simply comparing the number of cooling days to the number of heating days is misleading.
– In winter the sun is at a lower angle in the sky and so roofs are often in shade a lot more, and houses typically have a greater proportion of sunlight on walls and windows and a lesser proportion on the roof than in summer. This further lowers the difference in temperature between a white and a dark roof. Many houses have eaves designed to shade the windows in summer when the sun is more overhead, and designed to let the sun through the windows in winter when it is at a lower angle. It seems that in winter, the amount of solar heating coming directly through windows may be significantly more than solar heating via the roof, (but we have not found any studies to quantify this). Again, simply comparing the number of cooling days to the number of heating days is misleading.
– In winter, most of the heating is in early morning and early evening, and at these times the sun is not shining on the roof (usually because it is dark or the sun not high enough in the sky), so at the time you might need the solar heating the most it is not there and it makes no difference what colour the roof is.
- For houses in very cold climates where there is rooftop snow in winter, the roof cannot ‘see’ the sun and it does not matter what colour your roof is.
– Scientists believe that a white roof may actually make your house slightly warmer in winter at night because white paint doesn’t radiate heat (from inside the house) as fast as dark paint. (I have not found any study that investigates this for roofs).

So simply comparing the number of cooling days to the number of heating days does not indicate the effects of installing a white roof. On balance it seems that a white roof might have only minimal effect on heating cost and emissions in winter and that this is far outweighed by the contribution to cooling the planet by reflecting sunlight back into space. Research has shown that 100 square metre of flat white roof (a modest sized house without a garage) reflects enough sunlight back into space to give a cooling effect that cancels the global warming of roughly 10 tonne of CO2 emissions (the amount emitted by 2.5 average cars for one year).

If a house has a dark roof then it’s going to be hotter in the summer sun and the house hotter to live in. So a white roof gives you some free cooling in summer, which is why it has been used for thousands of years in places like the Mediterranean and Middle East countries.

Probably the most rigorous study on the energy efficiency of using white roofs compared to dark roofs was by the engineers at the Heat Island Group, Lawrence Berkeley National Lab, California, published in the Journal of Energy Efficiency, 2010 (see my website for details). The study models four types of commercial construction (new/old retail and office), as well as local energy prices and local electricity emission factors. It gives results across the US for average values of cooling energy savings, heating energy penalties, and overall energy cost savings per square metre of conditioned roof area, as well as emissions reductions. The simulation was for buildings with electric cooling and natural gas heating. The study finds that the savings are greatest in the hotter parts of the country and less in the cooler parts but that even in these cooler parts there are still savings. The study estimates that immediately retrofitting 80% of 2.58 billion m2 of commercial building in the USA would yield an annual cooling energy saving of 10.4 TWh, an annual heating energy penalty of 133 million therms, and an annual energy-cost saving of $735 million. It would also offer an annual CO2 reduction of 6.23 Mega tonne, offsetting the annual emission of 1.2 million cars or 25.4 peak power plants.

Hey “F”, if you decide to paint white, then I invite you to send a before and after photo to put on my website, as this helps spread the white roofs idea. It need not identify your property and I would respect your confidentiality. See my website (www.whiteroofs.org.nz) for examples of photos others have sent in.

(As with all my comments, they are to the best of my understanding at the time and are my opinion only.)

Could you explain your comment a little further? I’m curious as to what you mean becauss we are seriously considering painting our roof white and would like to understand all the pros and cons fully.

Thanks
F