Yesterday I was cleaning a house using wfp and the elderly Dutch owner was chatting to me while I was working.
He was impressed with the results using pure water and told me that when he was a child in 1940s Holland the cleaning ladies would scrub the windows then rinse them off by dipping a smaller container into a bucket of rain water collected off the roof and throwing the water at the window.
So I presume these cleaning ladies had worked out that rain water was superior to tap water.