|A green roof in Norway|
Editor's Note: We continue with Tom's excellent Hotel Diaries series, learning how to turn an old dilapidated Toronto building into a fully functional green hotel - aiming at an 80% reduction in daily energy use when completed. Check out previous posts in this series via the 'further reading' section at bottom.
Well, you can’t have a Green Hostel without a green roof, right? So that’s this week’s topic: What’s the benefit, exactly, to planting grapes, greenery and grass on our urban roof? There are a number benefits — some more relevant and interesting than others — so let’s find out what they are.
First off – by “green roof” one normally means more than a couple of shrubs in a box. A full-on green roof is a roof covered in dirt, along with proper drainage, and that dirt is covered in some sort of foliage, grass or broad-leafed plants. There are, of course, intermediate stages, where parts of the roof are covered, to lesser or greater degrees. Here’s the real deal from greenroofs.net.
So what are the benefits? Well, from a energy savings perspective there are a few. The dirt, plants and evaporating water act to cool the building during the summer. The dirt is an insulator, the plants act to shade the roof, and the evaporating water suck heat out of the system. Environment Canada has put out a study showing that a one-story house will use approximately 25% less energy to cool — that’s just grass growing on 10 inches of dirt. Obviously, higher buildings with more walls and windows per unit of area of roof will not benefit to that degree. The baseline for that study is a roof with shingles (no, not that kind …).
From an air quality point of view, the greenery (like any greenery) acts to remove particulate pollutants from the air. One square metre of grass can remove around 0.2 kg of airborne particulates on an annual basis. It acts as a kind of filter. In addition, the cycle of evaporation of water — dew, rain and so on — serves to cool the city as a whole. There needs to be a whole whack of roofs done for that more global, or city-wide effect, obviously, but it can do the trick. Next time you’re in a plane looking down on Toronto, imagine the roofs all full of greenery, and you can get a sense of the thermal scale involved. Parks are cooler than parking lots, right? A modeling study at the University of British Columbia found that “Using a green roof coverage of 50%, this cooling was extended to approximately 1/3 of the City and increased the maximum cooling to 2°C” (same website site as above). So that’s a nice thing. Not saving the planet, exactly, but a nice thing. Ancillary benefits: You could grow your carrots and herbs up there, and there is some storm-water retention (easing the load on the sewer system after a storm)
The real benefit, though, is a nice, green space on the roof. That’s a no-brainer for a hostel, as the aesthetics are important for the guests’ general experience, so there is real economic value there. I would imagine any building could benefit though — a nice lunch spot overlooking the city would be a tangible bonus to any building, I imagine. So there are lots of good reasons, and I can’t imagine it’s very expensive. However, that greenery will need to compete for space with the solar thermal and photovoltaics that we’re planning, so how much space actually gets covered in dirt and grass remains to be seen. So green it we will, but those plants will have to elbow out the pipes and silicon that will be competing for the sun’s rays!