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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 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".

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



One factor that impacts the heat gain into a building is solar gain and the reflectivity and absorptivity properties of the materials. Preventing the building, and the roof absorbing solar energy, 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 = absorptivity


For an existing building where the roof area, temperature difference (delta T) and U value (see note 1) are constant, then following Building Department OTTV manual formula, green lawn with alpha value = 0.86 and reflective white roof with alpha = 0.24 means the lawn will asborb three times more radiant energy from the sun.

Note 1. In theory at least, adding soil to an existing roof improves the overall U value, in practice, for the planting to survive, that soil layer needs to be kept wet (refer to clause 5.2.11 ArchSD Study on Green Roof Application in Hong Hong) therefore the U value improvement is negligible, so determined to be a constant.

This means the maths proves a white reflective roof lowers demand for cooling in the spaces beneath the roof, and reduces energy operating cost.

Plus we must consider that a lawn provides little of the promised bioversity, the intention, to provide wild meadow to encourage nesting birds, bees, and insects is not achieved using a neatly manicured lawn.

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


Conclusions

the Hong Kong "lawn" adds little ecological value, it might look better, but in terms of bioversity its probably more negative, given the frequent cutting and fertilsers used to keep it looking green, nor is it the spring meadow envisaged providing nesting sites for wildlife.

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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