Well… since you’ve asked for thoughts regarding the comment, “Cool roofs reduce urban heat; green ones, not so much”, here you go:

Short response: Incorrect. Green roofs are quite effective for urban heat reduction.

Longer version: Both green and white roofs are “cool roofs”, since the term is used as contrast to dark-colored conventional roofs (aka “hot” roofs?). Ample research has demonstrated green vegetated roofs substantially reduce roof top temperatures, thermal loading through roof structures, building energy consumption, and urban heat island effect at levels similar or exceeding white reflective roofs.

The *only* relative advantages of reflective roofs over green roofs are white roof cost may be roughly equivalent to conventional roofs and no additional structural load-bearing capacity is required.

The many advantages of green vegetated roofs over white roofs are comparable thermal benefits PLUS concurrent stormwater management, pollinator habitat, absence of reflective glare / heating into adjacent buildings / windows, improved aesthetic sight lines, mixed-use amenity roof space, particulate and greenhouse gas mitigation, double or greater roof lifetime extension (offsetting all or part of the higher green roof cost), etc. The extended roof lifetime alone gives green roofs many additional benefits by avoiding more frequent roof replacement costs, workplace disruption, materials consumption and landfill waste.

White reflective roofs definitely offer a cost effective urban cooling option for restricted weight roof structures or in climates where low-maintenance vegetated roofing is not yet a viable alternative. For most temperate climate cities, well designed green roofs achieve comparable heat reduction and a substantially broader range of ecological and municipal benefits at modestly higher initial costs, whose investment is returned by the longer functional lifetime.