Cool walls refer to coating exterior buildings walls with solar reflective materials like light coloured or specially pigmented paints which reflect the incoming radiation from the sun. Similar to cool roofs, which reflect heat from roof tops, cool walls minimise heat absorption into the structural constitution of the buildings.The American Society of Heating, Refrigerating, Air Conditioning Engineers (ASHRAE) considers Cool walls to be a more comprehensive solution to heat management in conjunction with cool roofs.
How does it differ from cool roofs?
It has been found in several studies that sole reliance on cool roofs is an adequate strategy for comprehensive heat management. Drawing on evidence from building simulations and real world applications, incorporating cool roofs with cool walls delivered better results.
Cool roofs, while being effective in reducing roof top heat accumulation, fall short on addressing the full spectrum of solar heat gain (SHG) of buildings. Roofs usually receive more direct sunlight throughout the day, but walls especially in densely populated urban areas contribute significantly to overall heat transfer. Usually walls have less insulation as compared to roofs,particularly in buildings constructed before the implementation or bypassing of modern energy codes. According to a study conducted by Lawrence Berkeley National Laboratory, it was found that despite lower sunlight intensity, wall surfaces can lead to considerable heat ingress,often comparable or even exceeding roof surfaces, into the interior of buildings.This was mostly accounted for larger surface area and poorer insulation of wall surfaces compared to roofs.
In urban heat islands( UHI), where buildings shade each other, walls absorb reflected heat from other buildings, direct sun exposure, pavements and adjacent structures thereby amplifying the total heat ingress in the interior of buildings. In such a scenario, mere cool roofs are found to be inadequate in heat mitigation, in fact the efficacy of cool roofs get drastically reduced if not complemented with cool walls in these scenarios. Thus, a holistic approach which integrates both is essential.
Cost of savings
Obviously an important metric while considering its application. Cool walls deliver substantial savings with a minimal upfront cost.Building simulations run by the California Energy Council ( CEC) indicate that raising wall reflectivity from conventional levels to cool standards can yield annual HVAC energy cost reductions by 25-30% for both commercial and residential buildings.The overall decrease in energy load due to the combined effect of cool walls and roofs can lead to substantial energy savings over a long term average.For a typical residential premise, this translates to 35-40% lower whole building energy use, with peak demand cuts during summers ,reducing utility bills. Retrofitting existing structures like schools, hospitals, hotels with cool walls could potentially save substantial amounts with zero payback periods since light paints cost the same as dark ones during routine maintenance. In the broader Indian context, such as the tropical zones of north India, savings can have tangible benefits, allowing for down sizing of air conditioning systems, qualifying for rebates. These benefits further enhance grid reliability, lowering overall societal energy expenses.
Environmental implications
Similar to cool roofs, cool walls are also an important arsenal in curbing greenhouse gas emissions and reducing urban heat island effects. By reducing solar heat ingress and absorption, cool roofs help slash CO2 equivalent emissions by up to 100 tons in large scale applications. The CEC report highlights comparable reductions in greenhouse gas emissions, pollutants like nitrous oxides, almost similar to cool roof impacts but extending them to wall dominated heat flows. In cities, its widespread application can have a significant bearing on lowering ambient air temperatures which helps counter urban heat island effect that exacerbates smog and heat induced risks. Berkley Lab shows cool walls helping reduce dispersion of pollutants in California and southern states. Cool walls also increase the shelf life of buildings. If PU based paints are used for wall surfaces, it acts as a hydrophobic layer, preventing against weathering effects.Thus, cool walls also help in reducing construction and demolition waste by increasing the resilience and shelf life of buildings. This is particularly important for a country like India with ever increasing urbanisation and constant surmounting of associated environmental challenges. Cool walls help in passively reducing fossil fuel usage by decreasing energy loads,aligning with sustainability goals.
In conclusion,while cool roofs are efficacious, their inadequacy without cool walls underscores the need for integrated plans. As urbanisation intensifies, prioritising cool walls could help transform building dynamics, design and resilience for a sustainable, cool and comfortable future.