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Reflective white roof is more effective than lawn for greening existing buildings
27 May 2016
John A. Herbert

The typical Hong Kong roof is black/dark colour, and increasingly green lawns have been sprouting around the city. However, for energy saving the white roof reflective white is king, because it reflects more solar energy than typical black or green lawn roof, lowering the cooling load for air conditioning spaces beneath and lowering operating costs.

Green roofing has been brought into focus because the roof of the Hong Kong City University roof collapse in May 2016. Overloading, a result of a lawn on the existing structure is suspected to be the cause (the cause is unknown at the time of writing the investigation is still on-going).


Reflective white roof is more effective in Hong Kong climate photo credit John A. Herbert



John A. Herbert, a Hong Kong energy expert, said that a reflective white roof is a better soluton for existing buildings, it does not add any wieght, does not require vetting, and it performs much better.

One factor that impacts the building solar gain is the reflectivity of the outer surface, if you can prevent the roof absorbing energy, that equates to lower heat gains and lower energy costs. To demonstrate the energy savings, refer to Hong Kong OTTV manual published by BD, page 10 provides a table of materials and properties.


The OTTV formulae for Heat Gain Q = A * U * T * alpha (A)

where:
A = Area (sqm)
U = U Value (w/mk)
T = Temp difference (deg C)
alpha = absorbtivity



For an existing building where the roof area, temperature difference (deltaT) and U value( see note 1) are constants, then following Building Department OTTV manual formulae (A) a reflective white roof with alpha value = 0.24 will reflects three times more energy than green lawn with alpha value = 0.86, thus lower cooling load for the spaces beneath and reducing operating costs. 1. In theory at least adding soil to an existing roof might improve the U value, in practice the soil layer needs to be wet for the planting to survive, therefore the U value can be considered as constant too.

Also In BEAM PLUS, Solar reflectity Index is referenced, the principle is the same, a highest reflctive index equates to a reduced cooling load and lower operating costs.

The installation cost and operating costs are lower too, a roof lawn requires addition installation work, for example, irrigation system for watering, and drainage system should be installed, and just like your garden lawn frequent tending and fertilizer is needed.

reflective white roof beats lawn
Innocentre reflective white roof Hong Kong credit Google Maps


About the Author

John A. Herbert is a constuction veteran with more than 30 years construction experience, educated in the United Kingdom he has been working across Asia for the last two decades. He is a Registered Energy Assessor (REA), and BEAM Professional.




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