Optimized cool roofs: Integrating albedo and thermal emittance with R-value

For cool roofs the combined effect of the three parameters that define heat gain and loss from a roof, namely solar albedo α, thermal emittance E, and sub-roof R-value, must be considered. An accurate contribution of night sky cooling, and hence humidity and total down-welling atmospheric radiation...

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Bibliographic Details
Published in:Solar energy materials and solar cells 2011-12, Vol.95 (12), p.3207-3215
Main Authors: Gentle, A.R., Aguilar, J.L.C., Smith, G.B.
Format: Article
Language:English
Subjects:
Albedo
Applied sciences
Buildings
Buildings. Public works
Cool roof
Cooling load
Cost benefits
Demand
Emittance
Exact sciences and technology
External envelopes
Heating
Marketing
Peak demand
R-value
Reduction
Roof
Roofs
Winter
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Summary:For cool roofs the combined effect of the three parameters that define heat gain and loss from a roof, namely solar albedo α, thermal emittance E, and sub-roof R-value, must be considered. An accurate contribution of night sky cooling, and hence humidity and total down-welling atmospheric radiation is needed. A systematic analysis of the contribution of a roof to average cooling load per day and to peak load reductions is presented for a temperate climate zone over 6 cooling months using an hour-by-hour analysis. Eighteen 3-parameter sets ( α, E, R) demonstrate the over-riding importance of a high α, while sensitivity to R-value and E drops away as albedo rises. Up-front cost per unit reductions in peak demand or average energy use per day always rises strongly as R rises unless albedo is low. A moderate R∼1.63 is superior to high R unless a roof is dark, or winter heating demand is high. We indicate briefly why the roof typically does not present a dominant influence on average winter heating needs in most temperate zones, enhancing the benefits of cool roofs. [Display omitted] . ► R-value should be tailored to absorptance, emittance and cost benefits optimized for medium R and low A sol . ► Peak and total heat load reductions with “cool roofs” are substantial ► Sensitivity of heat gains to R-value and emittance decreases as albedo increases ► Night sky cooling is important ► Winter heating loads in temperate climates are dominated at present by air exchange rates.
ISSN:0927-0248
1879-3398
DOI:10.1016/j.solmat.2011.07.018