heat island effect, chicago

How Green Roofs, Green Space, and Green Design affect the Heat Island Effect in Chicago


Despite a relatively cool summer here in Chicago this year (thanks, I guess, Polar Vortex), we’ve all battled heat waves rolling off parking lots or struggled to arrive un-sweatily at our destinations more often than we’d like.  And every summer – warm, hot or baking – is warmer here in the city (and the surrounding suburbs) than it is in less developed near-by areas with the same climate and weather.

So the next day you’re sweltering through a scorcher like Tuesday think about how much all our lives can be improved by fighting the good fight – design-wise – against the evil forces of the Heat Island Effect.

city hall green roof

The Heat Island Effect is …

pretty obvious to anyone who has ever stood on an asphalt parking lot in the sun on a hot day.  The effect is baking – the black pavement absorbs and re-radiates the sun light so you feel like you’re getting it from all sides.

If you step off the edge of the parking lot – even into a small grassy verge – its cooler.  If you walk away from asphalt altogether, into a soccer field, park, garden or other green space the difference will be pronounced.  Heat Islands are the aggregate of that effect – when much of the square footage of a city is made up of paved roads and dark roof tops each on of those surfaces adds to the heat and the result is summer temperatures 2-5 degrees warmer in the day time and up to 20 at night (when you could be cooling off your living spaces with open windows).

Not only does that added heat make life in the summer less pleasant, heat islands increase need for power for cooling (adding cost and greenhouse gasses), increase ground level ozone formation, warm up storm water run-off which can have bad effects on local waterways, and up the risk of heat stroke, heat exhaustion and heat related deaths in at-risk populations!

In a city like Chicago, the combination of parking lots, dark roofs tops and … can be overwhelming and uncomfortable, even when we have the lake effect helping to balance the effect.  But fortunately for us there ARE steps we can take to cool our city down.  And many of them are related to design and to architecture! You can imagine how happy this makes us!

Mitigating a Heat Island

Fortunately Heat Island effects can be counteracted in a very obvious way – cut down on the area of heat absorbing hard surfaces exposed to the sun.  This can be accomplished by

    • Using permeable and more reflective surfaces for necessary paved areas.
    • Planting Trees to shade both paved areas and roof tops
    • Using lighter colored (more reflective) materials on roof tops (white roof membranes instead of black)
    • Adding green roof systems or roof decks to deflect the sun from roof surfaces

Green Roofs and Chicago

Chicago’s precise heat island effect was studied as part of an EPA pilot program and in 1999, Northwestern University used
data from the National Climatic Data Center (NCDC) to determine where Chicago’s hottest areas are.  In Chicago, because of the lake effect coolness, the hottest area is in the western suburbs (our sketch is adapted from their tiny pixelated diagram) but that doesn’t mean we can’t work on mitigating the effects here in the central city.

heat island profile

As a result of the city Chicago amended its Landscape Ordinance to encourage more green cover, and its Energy code to require reflective roof surfaces in many cases.  Read all about the project and its results at the EPA website.

The green roof at city hall was a part of that pilot project.  Not only does it reflect heat and create a cooler microclimate but it “can retain 75% of a 1 inch rainfall before there is stormwater runoff into the sewers.”  It is a lovely demonstration of the possibilities of green roofs to improve urban quality of life and of the city’s commitment to keeping Chicago cool.  In 2010 there were 359 green roofs in the city – 5.5 million square feet – but we still need more.

We always look for opportunities to place green or alternate roof elements in our projects.  Replacing bare black roof with wooden roof decks can also serve to knock down the heat absorbing effects of our buildings.  And wherever possible we advocate for minimizing hardscape as the controversy of last week’s Montrose Beach post demonstrates.

What’s your take on green roofs?  Let us know in the comments!