Above-roof temperature impacts on heating penalties of large cool roofs in Australian climates: final report

17 Jun 2019

This report outlines the key outcomes of research project RP1037u1 ‘Above-Roof Temperature Impacts on Heating Penalties of Large Cool Roofs in Australian Climates’, an extension to project RP1037 ‘Driving increased utilisation of cool roofs on large-footprint buildings’. The research has been focused on two key aspects of roof thermal performance that had, up until the time of writing, not been taken into account in most investigations into cool roof technology:
The condensation and evaporation of dew on the roof surface, and the effect this has on roof temperature by way of:
a. The latent heat that is absorbed and released; and
b. Any change in the effective radiative-optical properties of the roof top surface due to accumulated water.

The effect of roof temperature on above-roof air temperatures, and the influence this can have on the performance of rooftop heating, ventilation and air-conditioning (HVAC) equipment.
A review of relevant literature (included in this report) did not reveal any previous studies that had investigated both 1a and 1b, above. Previous experimental studies had shown that the apparent thermal emittance of surfaces can approach ~0.96 when covered in a film of water. The surface temperature of low-emittance roofing materials, e.g. metal-coated steel, could be influenced significantly by such a change in emittance. Furthermore, experimental data from RP1037 revealed that roof surface temperatures often fell below the dew-point temperature at night and during the early morning, which confirmed that roof surfaces were likely to often be wet with dew.
A roof condensation model was developed from fundamental thermodynamic principles and previously established sub-models, to quantify the effect that water condensation can have on roof temperatures. When implemented in a dynamic building performance simulation (BPS), the model predicts the latent heat flux introduced by dew condensation and evaporation, tracks the accumulation of a dew film on the roof, and calculates the roof surface apparent thermal emittance, taking into account the effect of the dew.
The above-roof temperature model, developed based on experimental data in RP1037, has also been revised in the present work. The model can be used to predict the actual temperature of air entering rooftop HVAC equipment in BPS, taking into account the effect of the roof surface temperature, wind, and height of the HVAC inlet duct. Both new models (the roof condensation model and revised above-roof temperature model) have been described in this report, and summarised guides on how to implement the models in simulations have also been provided for BPS practitioners.
To test the effects of dew and above-roof temperatures on a case-study 350×200 m2 two-storey shopping centre building, a parametric BPS study was conducted. Simulations were run of seven Australian climate zones, three roof types (one bare metal-coated steel roof, one light-coloured painted steel roof, and one even lighter cool roof), two HVAC systems, and four thicknesses of ceiling insulation. Each simulation was run four times: i) with the revised above-roof temperature model, ii) with the roof condensation model, iii) with neither model, and iv) with both models.
A comparison of simulation results indicated that rooftop dew and above-roof air temperature fields can affect BPS results significantly, especially in cases where multiple simulations are being compared to assess the relative effects of cool roofs. If both phenomena had been neglected in the cases investigated here, electricity savings would have been miscalculated by 11–75% (42% on average) and gas ‘penalties’ (i.e. extra gas consumption for heating of the building) would have been miscalculated by 16–46% (31% on average). When both models were implemented, calculated gas penalties attributable to the cool roof were consistently reduced and HVAC electricity savings were either reduced or increased, depending on the climate.
The operational and emissions savings attributable to cool roofs depend on the unit costs and greenhouse gas emission factors of electricity and gas, so a range of unit costs and emission factors were investigated in the economic analysis. Compared to the bare-metal roof, the cool roof provided a net saving in HVAC running costs and reduction in greenhouse gas emissions for the case-study building in almost all cases involving Darwin, Brisbane, Alice Springs and Sydney. In simulations of Dubbo, Melbourne and Canberra, running costs and emissions could be reduced or increased by the cool roof, depending on the unit costs and emission factors.
The net effect of rooftop dew and above-roof air temperature fields on predicted HVAC running cost savings and greenhouse gas emissions abatements for the cool roof varied, but was generally positive. When both models were implemented, the predicted cool roof benefits were consistently increased in simulations of Dubbo, Sydney, Melbourne and Canberra. In hotter climates (Darwin, Brisbane and Alice Springs), the combined effects of dew and above-roof temperatures were found to either increase or decrease the predicted cool roof benefits, depending on the emission factors and unit costs of electricity and gas.
The case-studies reported here demonstrate the large effect that above-roof temperature fields and dew can have on simulation studies of this type. The two models developed here will allow BPS practitioners to account for such effects in future investigations. Further research into several aspects of the phenomena would be valuable, including:
Further validation of the above-roof temperature model, in a wider range of weather conditions and on different types of building;
Investigation into the effects of uneven dew film coverage; and
Extension of the BPS parametric study to include more buildings and climates.

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