When to Close the Windows

This tool proves suggests concedes that the old rule shuts the windows far too early a few minutes too early at exactly the right moment.

On a hot summer day the windows are a heat pump running in both directions: open at night they carry the heat out, open at noon they carry it in. This tool simulates a dwelling's indoor temperature over 24 hours and answers the one question that decides how the evening feels: at what time in the morning should you shut them?

• Two numbers from the forecast, the day's minimum and maximum, define the outdoor temperature over the whole day.
• The building side is a handful of sliders: insulation, thermal mass, the air change rate with the windows open and closed, and the solar gain.
• The main chart is a field over both decisions, when to shut in the morning and when to reopen in the evening, with the optimum, the rule of thumb and the region that is just as good marked on it.
• A second chart shows the full 24 hour trace of indoor and outdoor temperature for that schedule.
• Everything runs in your browser, with no data leaving it.