Whoa! Let's not assume that all houseplants have the same temperature requirements. Some tender tropical species will begin to deteriorate in temps below 15 C (60 F.), while others can withstand short periods of frost. It is best to know your plant species and research their native habitats so you can better determine their temperature tolerances.
Also, many plants can with stand cooler temps if those temps are introduced gradually as this is what happens in nature. Sudden drops in temperature can be devastating to plants.
