Final answer:
The best detail supporting the benefits of rooftop gardens is that they reduce energy costs by providing insulation and thus lowering heating and cooling expenses.
Step-by-step explanation:
The detail that best supports the idea that rooftop gardens have benefits is that they reduce energy costs. Rooftop gardens provide a layer of insulation that can help regulate building temperatures, leading to lower heating and cooling expenses.
They act like a natural thermal barrier, absorbing heat in the summer to keep buildings cooler and providing insulation in the winter to retain warmth. Additionally, having a layer of vegetation on the roof can also aid in collecting rainwater, which can be reused for watering the gardens, further contributing to environmental sustainability and reducing water usage costs.