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© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Reflective White Roof vs
Green Roof for Greening
Existing Building...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Greening Existing Buildings
The typical Hong Kong roof is black/da...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Greening Existing Buildings
For energy saving, and therefore
envir...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Reflective White Roof
A reflective white roof is a better
solution...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Performance Comparison
One factor that impacts energy consumption ...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
reflective white roof lowers operating costs.
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Performance Comparison
We can demonstrate the energy saving
numeri...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Performance Comparison
Q = A * U * T * alpha[1]
where:
Q = Heat Ga...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Performance Comparison
U Value
In theory adding lawn with soil lay...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Performance Comparison
Result: A green roof (lawn) absorbs three t...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
BEAM PLUS SA8 (SRI)
In the green building BEAM PLUS system,
Solar ...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
A lawn roof has higher maintenance cost
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Costs
The installation and operating costs are
important factors f...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Green Roof Operating Costs
A green roof (lawn) needs regular water...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
Green Roof vs White Roof
A white reflective roof coating does not ...
© Copyright All Rights Reserved 2019www.kelcroft.com.hk
John Herbert
@johnherbert
+(852) 2335 9830
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Reflective white roof vs green roof for greening existing buildings

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Greening existing buildings using Reflective White Roof is better environmentally than a Green Roof (lawn)

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Reflective white roof vs green roof for greening existing buildings

  1. 1. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Reflective White Roof vs Green Roof for Greening Existing Buildings by John Herbert
  2. 2. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Greening Existing Buildings The typical Hong Kong roof is black/dark colour but increasingly lawns have been sprouting around the city, an effort to green existing buildings. Improving the performance (greening) the existing building stock is a challenge, instead of a lawn, a white reflective roof must be considered.
  3. 3. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Greening Existing Buildings For energy saving, and therefore environmental performance, a reflective white roof is superior. Because, unlike a lawn, it is able to reflect more solar energy than a green lawn roof, and in doing so lowers the cooling load for air conditioning spaces beneath, and therefore lowering operating costs.
  4. 4. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Reflective White Roof A reflective white roof is a better solution for greening existing buildings, it doesn't add any significant weight, a roof coating does not require vetting, and environmentally it performs much better --- John Herbert “ ”
  5. 5. © Copyright All Rights Reserved 2019www.kelcroft.com.hk
  6. 6. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Performance Comparison One factor that impacts energy consumption in air conditioning buildings is solar gain. And the properties of the roofing material are important, because absorption of solar energy can be prevented, and that equates to lower solar energy gain, therefore lowering operating costs.
  7. 7. © Copyright All Rights Reserved 2019www.kelcroft.com.hk reflective white roof lowers operating costs.
  8. 8. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Performance Comparison We can demonstrate the energy saving numerically, referring to Hong Kong OTTV manual published by BD, page 10 provides a table of building materials and properties.
  9. 9. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Performance Comparison Q = A * U * T * alpha[1] where: Q = Heat Gain (watts) A = Area (sqm) U = U Value (w/m/k) T = Temperature difference (deg C) alpha = absorptivity [1] 1. Source: Hong Kong Building Department OTTV manual
  10. 10. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Performance Comparison U Value In theory adding lawn with soil layer to an existing roof improves the U value. In practice, for planting to survive, the 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 deemed negligible, and determined to be a constant.
  11. 11. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Performance Comparison Result: A green roof (lawn) absorbs three times more solar energy than a white reflective roof Since A, U and T are constants QG QW = A * U * T * 0.86 = A * U * T * 0.24 QG QW = A * U * T * 0.86 = A * U * T * 0.24 0.86 0.24 = 3.5
  12. 12. © Copyright All Rights Reserved 2019www.kelcroft.com.hk BEAM PLUS SA8 (SRI) In the green building BEAM PLUS system, Solar Reflectivity Index (SRI) is a referenced target. A white reflective material provides a high Solar Reflective Index (SRI) >78, and that equates to a reduced cooling demand for the space beneath, with lower operating costs for the owner.
  13. 13. © Copyright All Rights Reserved 2019www.kelcroft.com.hk A lawn roof has higher maintenance cost
  14. 14. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Costs The installation and operating costs are important factors for the building owner to consider. Installing a white reflective roof material has a lower lifetime cost than a green roof (lawn) requiring constant attention throughout its lifespan.
  15. 15. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Green Roof Operating Costs A green roof (lawn) needs regular watering perhaps an irrigation system is needed, and also additional storm water drainage provision. Just like any lawn a green roof requires frequent maintenance, including watering, tending, weeding, and cutting. Time and labour is required to undertake frequent maintenance. Often fertilizer is applied also, which could potentially contaminate the storm water run-off.
  16. 16. © Copyright All Rights Reserved 2019www.kelcroft.com.hk Green Roof vs White Roof A white reflective roof coating does not add any significant loading on the roof or the structure. For greening existing buildings, a white reflective roofing material saves energy, has a better performance environmentally, with lower operating costs than a green roof, it’s the better choice.
  17. 17. © Copyright All Rights Reserved 2019www.kelcroft.com.hk John Herbert @johnherbert +(852) 2335 9830

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