Pane and Gain: A Guide to Window Performance

About 76% of winter sun that reaches a standard double-pane window enters to become heat, according to the US Department of Energy.

As the season turns to winter in the northern hemisphere, it’s time to check every window to make sure it’s tightly shut. In a superbly sustainable home, closed windows are a passive heating system, allowing winter sun to enter and trapping heat inside. This is possible because incoming solar energy arrives mainly in ultraviolet (UV) and visible light wavelengths shorter than 800 nanometers, while outgoing heat energy radiates in longer infrared wavelengths. High-performance window glazing has coatings that allow visible and some UV light to pass, while blocking infrared. 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 systems by using window attachments, opening blinds or drapes when sunlight is shining through, and closing them the rest of the time.

What Matters Most

To know how well you are inviting free solar heat to come in through your windows and stay for the winter, measure this sustainability indicator:

• How much energy do you use to heat your home?

Insulating 102

Improving window performance (winter)

Check that window systems are allowing only the intended amount of air and energy to flow through them; repair defects, operate mechanisms properly, and add attachments to improve performance.

Equipment and Materials

• Thermal imaging camera (optional)

• Caulk (if necessary)

• Caulk gun (optional)

• Low-expanding foam (optional)

• Foam backer rod (optional)

• Smoke stick (optional)

• Weatherstripping (if necessary)

• Exterior storm window

• Interior storm window

• Insulating blinds or drapes

Steps

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 weatherstripping.

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.

7. Wash windows with a non-toxic cleaner.

8. Install exterior or interior storm windows. (Optional)

9. Install and use insulating blinds or drapes. (Optional)

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

Discussion

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 due to 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.

Picture windows, which do not open, are the most energy-efficient type because they allow the least amount of infiltration. 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. 

When inspecting windows on a cold day, a thermal imaging camera makes it very easy to see airflow through or around a window. This is a great tool for checking whether a window is fully closing and latching and whether its weatherstripping works properly. Gaps in insulation around the window can also be clearly seen in a thermal image.

At elevations below a building’s neutral pressure plane, gravity will force cold air through a window. You can feel this airflow with the back of your hand. At elevations above the neutral point, warm air will be leaving. If you don’t have a thermal camera, you can use a smoke stick to find gaps that allow air to flow. Spider webs are another sign of airflow; spiders spin webs where they feel a breeze.

During winter, when windows will not be opened for ventilation, screens should be removed. 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 greatly 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.

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. 

Definitions

• Air infiltration rate: how many cubic feet of air pass through a window per minute divided by the area; industry standard is 0.3 cubic feet per minute per square foot

• Building envelope: the roof, exterior walls, and bottom of a building

• Glazing: transparent material (glass or plastic) used in windows

• Low-Emissivity (Low-E) Glazing: glazing with a thin coating that improves energy efficiency

• Neutral pressure plane: a horizontal layer of indoor air that has neutral buoyancy, being at the same pressure indoors and outdoors

• Thermal imaging: a technology that measures infrared radiation to show temperature differences

• Window: an intentional opening in a building envelope designed to let in air or light

• Window attachments: products that can be attached to a window, either inside or outside, to improve energy performance or provide privacy

Troubleshooting

1. You can’t get a window to close and latch completely:

   1. Check for debris along the side and in the bottom of the window frame.

   2. Determine whether the window frame is out of square; if it is, the entire window may need to be replaced.

2. You worry about using expanding foam to insulate around windows:

   1. Use low-expanding versions of this foam that are specially designed for windows.

3. You don’t have a caulk gun:

   1. Use caulk packaged in a tube similar to toothpaste that you can squeeze by hand without using a special tool.

   2. Borrow a caulk gun from your local library of things.

4. You are worried about your ability to replace window weatherstripping yourself:

   1. Check to see if your local area has volunteer programs that help homeowners with weatherization and home repair.

   2. Hire a professional company that specializes in weatherization and insulation.

5. You are worried about indoor air quality if you close all your windows:

   1. If you burn fuel in a furnace, water heater, stove, or other appliance, consult with a professional.

   2. If you do not burn fuel indoors, making your home more airtight and installing fans to allow you to control ventilation will improve your indoor air quality.

   3. Consider hiring a professional to do a blower door test to determine your home’s ACH (Air Changes per Hour) before making any major investments in mechanical ventilation.

Strategies and Goals

• Energy

  • Increase energy efficiency

    • Reduce airflow that wastes energy

    • Increase insulation that saves energy

  • Conserve energy

    • Keep heat from leaving

  • Solarize

    • Improve passive solar heating

Milestones

• Decrease how much energy you buy

  • Measure: How much electricity and fuel you buy

  • Method: Energy bills

  • Time Period: Year

Limitations

• Older windows are difficult to make airtight.

• Windows that do not receive winter sun do not provide any passive solar heat gain.

Opportunities

• Insulating 101: Sealing and insulating basements and attics

  • While you are looking for air flow around windows and have a caulk gun in hand, take a look in your basement and attic

  • The further below or above the neutral plane a gap in your building envelope is, the more worthwhile it is to fix

References

• Energy Efficient Window Coverings

• How to Clean Windows and Banish Smudges for Good in Just 3 Steps

• Introduction to Window Ratings

• Low-E Windows

• Visible Light Spectrum: From a Lighting Manufacturer's Perspective

• What Everybody Needs to Know About Building Science: Neutral Pressure Plane

• Window Dressers Insulating Window Inserts

• You Should Be Removing Your Window Screens Every Winter—Here's Why

Keywords

energy, efficiency, windows, weatherization, insulation, airflow, heating, cooling, passive solar