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Reflective white roof is more effective than green roof or lawn for greening existing buildings
27 May 2016

John Herbert, hong kong energy saving expert, BEAM expert
John Herbert
Director



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. Allegedly, overloading, a result of laying a green roof (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 says John A. Herbert
Reflective white roof is more effective in Hong Kong climate
credit: John Herbert




John Herbert, a Hong Kong green building and energy expert, said "a reflective white roof is a better solution for existing buildings, it does not add any weight, does not require vetting, and environmentally it performs much better".

reflective white roof more effective said John Herbert
Reflective white roof is more effective than green roof
credit: John 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 gain and therefore lower energy costs.

We can demonstrate the energy savings numerically, referring to Hong Kong OTTV manual published by BD, page 10 it provides tables with 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 has alpha value = 0.86 and reflective white roof alpha = 0.24, that means the green roof (lawn) will absorb three times more radiant heat 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 biodiversity, the intention providing wild meadow to encourage nesting birds, bees, and insects is not achieved with a neatly manicured lawn.

Also, in the BEAMPLUS system, Solar Reflective Index (SRI) is referenced, the principle is the same, a higher Solar Reflective 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 too, a lawn requires constant attention, for watering it probably needs an irrigation system, and additional drainage, and just like your garden lawn at home it requires tending, cutting, and often fertilizer too.

reflective white roof better than green roof lawn
Innocentre reflective white roof Hong Kong
credit Google Maps


Conclusion

White Roof or Green Roof?

The Hong Kong "lawn" or green roof adds little ecological value, it might look better, but in terms of biodiversity its probably more negative, given the frequent cutting and fertilsers used to keep it looking green, and its not the spring meadow envisaged providing nesting sites for wildlife.

It also avoids the contamination of run-off from green roofs dosed with fertilizer, and saves water, green roofs need watering to keep the roof green.

In conclusion, the calculations demonstrate that a white reflective roofing system is more effective, is better environmentally, saves water, its more energy efficient, and has lower CAPex and OPex cost than installing a green roof (or lawn).

About the Author

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


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 cost cutting building energy audits.

white roof is better than green roof



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