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Reflective white roof is more effective than lawn for greening existing buildings

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


The typical Hong Kong roof is black/dark colour but increasingly lawns have been sprouting around the city. However, for energy saving a reflective white roof white is superior, because unlike a lawn it reflects more solar energy than a typical dark colour or green lawn roof, and in doing so lowers 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 laying a lawn on the existing structure is suspected to be the cause (the exact 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 "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".

The factor that impacts the building solar gain is the surface reflectivity, and absorptivity, if you can prevent the roof absorbing energy, that equates to lower heat gains and lower energy costs.

We can demonstrate the energy savings numerically, referring to Hong Kong OTTV manual published by BD, page 10 provides a table of building materials and properties.

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

where:
Q = Heat Gain A = Area (sqm)
U = U Value (w/mk)
T = Temperature 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 formuale, reflective white roof with alpha = 0.24 it will reflect three times more energy than green lawn with alpha value = 0.86. That means lower demand for cooling in the spaces beneath the roof, and reducing operating costs.

Note 1. In theory at least, adding soil to an existing roof improves the overall U value, in practice, for the planting to survive, the soil layer needs to be kept wet, therefore the U value can be considered as constant too. Also refer to clause 5.2.11 ArchSD Study on Green Roof Application in Hong Hong.

Also, in the BEAMPLUS system, Solar Reflectity Index (SRI) is referenced, the principle is the same, a higher solar reflctive index (SRI) equates to a reduced cooling demand for spaces beneath. and lower operating costs for the owner.

The installation cost and operating costs are lower, a lawn requires constant attention, for watering it propbably needs an irrigation system, and additional drainage, and just like your garden lawn at home it requires tending, cutting, and perhaps fertilizer too.

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


Conclusion

In conclusion, the calculations demonstrate that a white reflective roofing system is more effective, is more energy efficient, and has lower CAPex and OPex cost than installing a lawn.

About the Author

John A. Herbert is a veteran engineer with more than 30 years engineering experience, educated in the United Kingdom he has worked across Asia for the last two decades engaged by international and local companies. He is a Hong Kong Registered Energy Assessor (REA) and BEAM Professional.


About Kelcroft

Kelcroft is a built environment consultant founded in 1999 based in Hong Kong, and providing specialist services including BEAM consulting, Green building, energy modelling, and cutting building energy costs audits.



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