It’s a fact that buildings consume two thirds of all the electricity produced in North America and one third of all the energy produced in North America. While it is recognized that adding insulation under the roof surface can reduce cooling and heating costs, there is a diminishing return on the strategy of increasing insulation to conserve energy costs. This is where “cool roofing” can play a role in further reducing the energy consumed, and in minimizing the Heat Island effect created in the big urban cities. Cool roofing relies on sustainable, energy efficient, coated steel products, in a wide variety of finishes, colours, textures and roofing profiles. It conserves energy through its properties of reflectivity and emissivity.
Reflectivity
Reflectivity is the ability of the roof to reflect solar radiation back into the atmosphere. Its primary measure is solar reflectance - the proportion of the total solar radiation that is reflected back to the atmosphere. Any solar radiation that is not reflected is absorbed into the building envelope, requiring further energy to cool the building; or partially convected into the atmosphere increasing the ambient air temperature in the surrounding environment (Heat Island effect). This secondary measure is the Solar Reflectance Index, which takes into account the cooling effect of wind passing over the roof.
Emissivity
Emissivity is the ability of the roof to re-radiate absorbed solar infrared radiation back to the atmosphere. This takes place at all times, but mostly at night. Its measure is Infrared Emittance - the proportion of absorbed infrared solar radiation that is re-emitted back to the atmosphere. Most unpainted metallic coated steels have low emissivity (less that 0.12), while prepainted steels (regardless of colour) have emissivity values of around 0.9.
What is Important in Canada
In Canada, over the course of a full year, winter “heating load” conditions have a greater influence than the summer time “cooling load” because of the much longer and colder winters experienced in Canada. The degree of influence depends primarily on the geographic location, the insulation level in the roof assembly, and the cost of energy. For example, heating loads in Winnipeg, Manitoba are much higher than in Windsor, Ontario because of the greater number of heating degree-days in Winnipeg.
Calculations involving solar reflectance and emittance can be used to determine the energy savings attributable to a roof. Cool metal roofing can reduce energy costs associated with air conditioning. For cooling loads, it is advantageous to reflect as much solar radiation as possible, and to re-emit as much of the absorbed infrared radiation as possible. However, for energy savings associated with heating loads, absorption of solar infrared radiation is beneficial, and it is best to retain absorbed solar radiation, and not emit it back to the atmosphere. In other words, for Canadian climates, it is best to have roofing products with high reflectivity and low emissivity.
Conclusions
Cool Metal roofs, with both high reflectivity and high emissivity, offer significant savings in reducing cooling loads. By definition, cool roofs reflect much of the solar radiation, and that which is absorbed is re-radiated by virtue of high emissivity. This is beneficial in predominantly warm climates like the southern US. The state of California has now recognized that different climatic conditions require different approaches. California has sixteen climate zones from north to south; however, they have exempted three of the sixteen climate zones (cold climates) from their Cool Roof (Title 24) regulatory requirements in recognition of the fact that colder climates are not well served by Cool Metal roofing.
In Canada, heating load predominates even in the warmest cities. Energy savings in Canada can be achieved by selecting materials with low infrared emissivity. Prepainted, asphaltic and membrane roofs have high emissivity and therefore re-radiate much of absorbed infrared solar radiation at night - a detriment in winter when absorption heat should be retained to reduce heating loads. A 55%Al-Zn roof combines the benefit of high reflectivity, which reduces summer cooling load, with the benefit of low emissivity, which reduces winter heating loads.
In Canada, the best choice for energy savings is a roof with high solar reflectance and low infrared emittance. However, it should be noted that if a building project stipulates Cool Roofing properties, CSSBI fabricator members have products that meet the reflectivity and emissivity requirements.
