In indoor water use efficiency, which measure is commonly implemented in existing buildings?

• A. Replace doors with tighter seals to prevent air leakage.
• B. Install rain barrels on the roof.
• C. Increase irrigation frequency.
• D. Install low-flow toilets or faucet aerators; retrofit with high-efficiency showerheads.

Answer: (D)

Upgrading indoor fixtures that use water is the most practical and widely adopted way to improve indoor water use efficiency in existing buildings. Installing low-flow toilets or faucet aerators and retrofitting with high-efficiency showerheads directly reduces the main indoor water-using components—the toilets, taps, and showers—often with a relatively low upfront cost and a quick payback. For example, modern low-flow toilets and efficient faucets can cut daily indoor water use substantially compared with older fixtures, and high-efficiency showerheads further decrease shower water consumption. These retrofits are especially feasible in existing facilities because they don’t require major remodeling and typically meet code requirements while delivering clear savings. The other options don’t fit indoor water efficiency: tighter door seals improve energy efficiency and air handling, rain barrels are about outdoor rainwater harvesting, and increasing irrigation frequency raises outdoor outdoor water use rather than indoor efficiency.