Roof feature - Heat Island Effect

GBP-Keith

I'm minded to believe that the thin sedum green roofs that are becoming common in the UK would be less able to reduce the heat island effect than deep planted grass roofs that are more common on the continent. Does anyone have an opinion on this or know of any research that support or dispels my theory?

Mark Brinkley

I am very dubious about whether green roofs of any description will have any effect on the heat island effect, which is a phenomenon measurable across whole cities, not individual houses. If you really wanted to reduce the heat island effect, you'd be far better advised to paint your roof white and reflect back the solar radiation from whence it came.

GBP-Keith

Good point Mark. The yanks make much of cool roofs and white/reflective membranes but it doesn't seem to have ever been considered in the UK.

I wonder if green (sedum), is more absorbent of heat on the colour spectrum than say red tiles or grey slates.

I am also also looking for any studies that examine whether a good depth of soil absorbed heat during the day and released it slowly during the night? Though thinking about it this might actually make things hotter rather than cooler.

RMN

I have carried out numerous researches on Green Roofs over the last few years and one piece of research that springs to mind straight away would be that carried out in Canada.

National Research Council of Canada (NRC) carried out an experiment in Ottawa. The experiment called for the division of a roof into two half. One half a typical flat roof the other with a 150mm growth medium. The results showed temperatures of over 70 degrees on the typical flat roof and 30 degrees on the green roof.

Other research has been carried out in America and Japan relating to green roofs. America tried the white covering but the results where not staggering as all it did was heat the surrounding area. Add to this that the American researchers also liked the added benefit that a green roof helps with air quality where as a white roof does not.

In relation to American research you could look up the Chicago experiment between the Cook County building and City hall. These two building are very similar and are only a short distance from each other.

City hall roof (Paved) 126 -130 degrees Fahrenheit
City Hall Roof (planted) 91 -119 degrees Fahrenheit
Cook County Roof (Black Tar) 169 degrees Fahrenheit

It is the evaporation of the moisture in the green roof that helps reduce the heat gain/ emission from the roof.

I trust this helps.

greenbyname

To add to this (not wholly relevant but still of interest and topical) I discovered a journal while researching in my Uni library entitled "Analysis of the green roofs cooling potential in buildings" (1998) by Elena Palomo Del Barrio. You will have to purchase the report if you are not a student.

It uses a great number of mathematical models to calculate it's data, and is incredibly detailed.

However, the conclusion drawn was that "green roofs [u]do not[/u] act as cooling devices but as insulation ones, reducing heat flux through the roof". Of more use to designers, the report contains a number of parameters to aid the design of green roofs, eg moisture content, soil density, leaf area index etc.

James Norton

Keith,

How deep is planted grass, I it the sort of thing used on the Brighton 'Diggers', Segal self build or deeper still...?

RE the heat island, my understanding concurs with 'RMN', an if its the evaporation, I would of though the grass would have a bigger effect...?

J