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 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.

.Figure optionsDownload as PowerPoint slideHighlights
► R-value should be tailored to absorptance, emittance and cost benefits optimized for medium R and low Asol.
► 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.