In a superbly sustainable home, closed windows are a passive heating system, allowing winter sun to enter and trap heat inside. This is possible because incoming solar energy arrives mainly in ultraviolet and visible light wavelengths, while outgoing heat energy radiates in longer infrared wavelengths. About 76% of winter sun that reaches a standard double-pane window enters to become free heat, according to the U.S. Department of Energy. So, as the season turns to winter, it’s time to check every window to make sure it’s tightly shut.

High-performance window glazing has coatings that allow visible and some UV light to pass through while blocking infrared light. Storm windows, either on the outside or inside of your regular windows, are another passive system that slows down energy flows, keeping homes warmer all winter. You can boost your passive solar heat systems by using window attachments, opening blinds or drapes when sunlight is shining through, and closing them the rest of the time.

To ensure you are inviting free solar heat to come in through your windows and stay for the winter, check that window systems allow only the intended amount of air and energy to flow through them. Repair defects, operate mechanisms properly and add attachments to improve performance. Here are some steps to take toward making sure you utilize the free heat:

1. Inspect every window to ensure no air flows through or around it when closed and latched.

2. Temporarily repair or permanently replace any cracked glazing.

3. Replace worn-out weather stripping.

4. If there are leaks around a window, apply caulk or low-expanding foam.

5. Replace windows that leave air gaps because their openings are out of square.

6. Remove screens from windows that receive winter sun and wash windows with a non-toxic cleaner.

7. Consider installing exterior or interior storm windows.

8. Consider installing and using insulating blinds or drapes.

9. Keep windows closed and latched, with exterior or interior storm windows in place, and operate window attachments to let in daylight and retain heat.

Buildings have windows for both aesthetic and performance benefits. In cold climates, windows facing the sun can provide passive heating but only if the windows are tightly closed. Heat loss due to airflow through even very small openings quickly swamps heat gain from sunlight entering a building.

The single most important performance factor of any window is its infiltration rate, which should be as close to zero as possible when the outdoor temperature is below the desired indoor temperature. Crank windows, which can be pulled tight against a seal, are more energy efficient than single-hung or double-hung sash windows, which slide up and down inside a frame.

During winter, screens should be removed when windows are not opened for ventilation. This will protect the screen from ice and snow damage and allow more sunlight to enter the window. Cleaning windows will likewise allow more sunlight to pass through them.

Window attachments can significantly improve the energy efficiency of windows. Removable storm windows are frames and glazing that trap an insulating layer of air. Exterior storm windows are made to be installed on the outside of a regular window. They are usually sturdier than interior storm windows, which can be made inexpensively from plastic film, but these interior window inserts can also be effective.

Insulating blinds and drapes can remain closed all winter on windows that are not needed for views and do not receive winter sun. On windows that receive winter sun, you can open blinds and drapes during the day and close them at night to benefit from solar energy for heating.

Yes, there are gains from these panes!

Fred Horch and Peggy Siegle are principals of Sustainable Practice. To receive expert action guides to help your household and organizations become superbly sustainable, visit SustainablePractice.Life and subscribe to “One Step This Week.”