Are you asking about new construction? Building codes? There are certainly plenty of existing houses without double pane windows and nothing's stopping them from selling. I'm personally not a fan of double paned windows. They are bound to fail. Traditional windows can last 75 - 100 years and then with rehab last another 100. Double paned windows have turned windows into a disposable item with an extremely short lifespan. I opt for window restoration, good storm windows, and if you want them - insulating shades.