What’s that overhead? Is it a bird? Is it a plane? Nope – it’s a roof! That thing that keeps our buildings protected from rain, snow, wind, and sunlight. If you are in UHN’s Bickle or Rumsey Centre, you are under a cool roof! Roofs are a key component of the building envelope consisting of structural members, insulation, a water-proofing mechanism, and some method of affixing the roof to the building. Typically, commercial buildings have used built-up roofing systems or bituminous/rubber membranes consisting of dark materials that absorb sunlight. Many systems are also ballasted (i.e. held down on the roof) with gravel stones of various shades of grey.
Over the last year UHN’s infrastructure department replaced two roofs at our Bickle Centre and our Rumsey Centre and chose to go with white roofs as an energy savings measure. White roofs are also known as cool roofs or eco-roofs.
Why a White Roof?
A white roof reflects about 80% of incident solar radiation compared to only 5% for a dark roof. By reflecting more heat back to space, the following benefits are produced:
- Reduce the surface temperature of the roof by up to 30 degrees Celsius. Since a large portion of fresh air supplied to buildings is from the roof, this significantly reduces our cooling costs
- Reduce heat conduction into upper floors of building, again reducing cooling costs
- Reduce heat flow to local area, mitigating the heat island effect often experienced in cities
- Reduce heat flow to atmosphere, helping to mitigate global warming impact
Now I know what you might be thinking – wait a minute, it gets really cold in Canada! Won’t this increase our heating bills?? There are several reasons why the heating penalty is not as large as it seems.
- The sun is at a much lower angle in the sky in winter and, thus, there is much less incident solar radiation on the roof to begin with
- Cloud cover is much more frequent during the winter months, further reducing solar radiation
- Days are shorter, reducing the hours of sunlight compared to summer
- We often have a temporary white roof during winter anyways in the form of snow
- Heating utilities (natural gas) are significantly cheaper than cooling utilities (electricity)
Roofs at Bickle and Rumsey
Now that we have had a quick background on the benefits of white roofs, let’s take a look at the projects at Bickle and Rumsey. As seen in the pictures below, there are many air intakes that are directly adjacent to the roof. With the new white roof, we will see lower air intake temperatures during the hot summer months.
The picture below shows the final white roof at Rumsey Centre. As you can see, we opted to go with white gravel rather than a white membrane or coating. Gravel is easier to maintain because it doesn’t have any flat surfaces for dirt to collect and in most cases the material is kept clean by rain, except for larger debris such as branches or leaves which can be removed during routine inspections. Flat membranes and coatings often require regular pressure washing to maintain performance, increasing maintenance cost. The roofing materials are similar on the Bickle project and additional insulation was added where possible to improve the thermal performance of the building
UHN worked closely with the City of Toronto on the two projects through the Eco-Roof Incentive Program and received rebates totaling $45,590.
The projects are expected to save UHN a relatively modest $5,000 in cooling costs annually, but it is important to note that, with the help of the rebates, the white roof option came at no extra cost to UHN. We are also contributing to the reduction in local heat island effect and decreasing the global warming impact of our buildings. I would like to thank the project managers Regina Rahmilov and Franz Thompson for supporting the implementation of white roofs on these two projects.