Heat, cold and condensation: what window coverings do for comfort and energy

A window covering can slow heat through a window, but only when it is used the way the studies used it, and sealing comes first. This chapter shows what the tests found, how to read an AERC label, and how closed coverings raise condensation on cold glass.

1. How heat moves at a window Understand

A window is the thinnest part of an outside wall. A 2003 Cornell Cooperative Extension fact sheet puts it plainly: a single pane loses about 20 times as much heat as the same area of well-insulated wall, and a double-glazed window about ten times as much (Energy Saving Window Treatments). A Department of Energy article says windows are about 10 percent of a home's surface but can account for 30 to 40 percent of winter heat loss (DOE on cellular shades in Minnesota); the Cornell sheet gives 25 to 35 percent for a typical single-family home. The figures differ by source and house, and they agree on the point: the window is where the heat goes. Physical principle

Three ways heat moves

The Cornell sheet names three, and each one is something a covering can or cannot touch.

• Radiation: heat passing from a warm object to a cooler one. Sitting beside cold glass, your body radiates heat toward it, which is why a room can feel cold near a window even when the thermostat says otherwise.
• Convection: heat carried by moving air. Air leaking around the sash, and air sliding down cold glass and pooling at the floor, both carry heat out.
• Conduction: heat passing through a solid. Glass conducts heat readily.

A covering works by holding a layer of still air between itself and the glass. That layer slows all three. It only holds if the covering fits closely at its edges; air that can flow around the sides, top and bottom carries the heat past it. Physical principle

Winter and summer are different problems

In winter the problem is heat leaving at night and on cloudy days, and the opportunity is sun on south glass by day. In summer the problem is sun coming in, mostly on east and west windows that take the low morning and afternoon sun (Daylight, glare and darkness covers which windows get it when). The solar heat gain coefficient (SHGC) is the share of the sun's heat a window, or a window with a covering, lets through: lower means less heat in. A covering that blocks summer sun can also block the winter sun you wanted, which is why the same covering helps in one season and costs in another.

Your setup's climate answer decides which problem is yours. A house with cold winters and hot summers has both, on different windows.

2. Seal first, then cover

The Cornell sheet sets an order: covering the windows comes after the basics. Its list, from 2003, runs from setting back the thermostat at night, through insulating ceilings and walls, to adding storm windows to single glazing and caulking and weather-stripping every crevice. It calls caulking and weather-stripping very cost-effective and says they help an insulating covering work. Convention

The Department of Energy's 2022 Energy Saver Guide calls air leaks among the greatest sources of energy loss in a home and estimates that sealing them can save 10 to 20 percent on heating and cooling bills. Its cold-weather window advice starts from the same place: put tight-fitting insulating shades on windows that still feel drafty after weatherizing.

Finding a draft

The DOE guide's test: on a windy day, hold a lit incense stick or a smoke pen near the window, the trim and any outlets, and watch whether the smoke wavers. Keep it well away from curtains and anything that burns.

• Moving air at the sash (the frame that holds the glass) or where two sashes meet: weather-stripping.
• Moving air at the trim where it meets the wall: caulk.
• A sash that does not close tightly: repair, which is a carpenter's job in an old window.

If you rent, drafts and failing windows are the landlord's to fix; report them in writing. If cost is the barrier, the DOE guide says that as many as 20 to 30 million U.S. families are eligible for the federal Weatherization Assistance Program, with services provided by the states and eligibility criteria that differ from state to state. Ask the authority

A covering over a drafty window is a covering with air flowing behind it. Seal first, and the covering has still air to work with.

3. What the studies found, and under what conditions

Most claims you will read about curtains and shades come from a few studies. Each was done in particular houses, in particular climates, with the coverings used in a particular way. Here they are side by side, with their conditions, so each figure is read with its house.

The PNNL heating finding belongs beside the 20 percent figure every time either is quoted. Both are true of their houses. Used the way most people use them, the cellular shades in the PNNL homes did not measurably cut winter heating; operated by season and time of day, they did. The ORNL figure is an upper bound from a test house and a model, not a typical result. Test it

What the three agree on: in winter, an insulating covering, closely fitted and closed at night, helps; a covering left half up all day and night may not. In summer, a covering closed against direct sun on west and south glass helps more. What none of them settles is your house: its glass, its drafts, its heating system and how you will actually use the covering.

Curtains and draperies were not tested in any of the three; the LBNL study lists drapes and Roman shades among the categories it left out. The only figures we found for them are older, from the Cornell sheet, and they are in the studies in detail.

4. Using a covering the way the studies did

The studies agree that operation matters as much as the product. The DOE guide's advice is the routine.

The night-open entry for summer is ours, not the guide's; the guide covers closing against the sun. Preference

Fit: still air needs closed edges

The Cornell sheet's rule for any insulating treatment: it has to trap and hold air between itself and the glass, which takes a close fit at every edge and a position about an inch from the glass. Its figures, from 2003 and measured against single glazing, show how much fit matters: a roller shade mounted inside the frame with no more than a 1/4 in gap at the sides and touching the sill reduced heat loss by about 27 percent, and adding side tracks raised that to about 45 percent. Physical principle

Curtains have a particular leak. The Cornell sheet describes a loop of air: warm room air enters at the top of the curtain, cools against the glass, falls, comes out at the floor and is warmed again. Its fixes are to close the top (a cornice, a board or box over the curtain heading, does this), seal the sides, let the curtain touch the sill or floor, and overlap the two halves about 4 inches at the center. It found the method of hanging mattered more than the fabric, provided the fabric lets little air through, and recommended a separate lining for the air layer it adds. Fabric, linings and fading covers linings, and Measuring for curtains the returns and overlap.

Summer sun and heaters

• The DOE guide suggests medium-colored draperies with white plastic backings to reduce summer heat gain, and closing draperies on windows in direct sun.
• Keep registers, baseboard heaters and radiators clear of curtains, as the DOE guide says. A curtain over a register sends the warm air up between the curtain and the glass; the Cornell sheet suggests a deflector to send it into the room instead. Fire distance is in Cords, children and fire.

Write the routine down. A routine kept in your head becomes "half open all week", which is the typical use that saved nothing in the PNNL heating test.

5. Ratings: the AERC label and what ENERGY STAR covers

The Attachments Energy Rating Council (AERC) is an independent nonprofit that rates window attachments (coverings, films and storm windows) for energy performance; the DOE guide names it as the place to get ratings for window attachments. A certified product carries a label with two numbers. The AERC label on Labels and spec sheets shows one and explains each line.

• Cool Climate rating: the energy improvement in a heating-dominated climate. AERC says northern U.S. readers should use it.
• Warm Climate rating: the same for a cooling-dominated climate, which AERC points southern readers to.
• What a number means: AERC explains that a Cool Climate rating of 64 means the product saves 64 percent of the energy that would otherwise be lost through that window without it. Any rating above 0.00 saves energy compared with no attachment. "Not Applicable" means AERC does not recommend the product for energy improvement in that climate.
• Max improvement: each product category has its own ceiling, for example 110 for storm windows in a cool climate and 15 for blinds, so compare ratings within a category.

A rating describes the product tested in AERC's procedure, not your window or your habits. It is the fairest way to compare two shades, and it says nothing about whether a curtain without a rating is better or worse. Convention

ENERGY STAR does not rate curtains, blinds or shades. It certifies low-e storm windows, interior or exterior, for the climate zone on their label (ENERGY STAR storm windows). A covering advertised with energy claims and no AERC label has no independent rating.

6. Condensation and mold behind coverings

Condensation is water from the room's air forming on a surface colder than the air's dew point. Natural Resources Canada explains that the risk rises as the weather gets colder and as indoor humidity rises, and that air moving across the glass keeps it warmer, which is one reason heat sources sit under windows. Deep sills, closed drapes and blinds, and even nearby furniture can block that air and let cool air pool at the window, which can lead to condensation on the glass (NRCan on condensation). Physical principle

This is the tradeoff at the center of this chapter. The tighter a covering holds still air against the glass, the less heat the room loses, and the colder the glass behind it gets. A closely fitted insulating covering on a cold night can leave a wet window in the morning. The Cornell sheet's insulating Roman shades and panels include a vapor retarder on the room side: a layer that slows moisture passing through, so less of the room's moist air reaches the cold glass behind. Physical principle

Humidity first

The U.S. Environmental Protection Agency's brief guide to mold and moisture says to keep indoor humidity below 60 percent if possible, ideally between 30 and 50 percent, and treats condensation on windows as a sign of high humidity. A 1988 University of Minnesota extension bulletin on cold-climate housing gives the same 30 to 50 percent and says to aim lower in very cold weather, using condensation on the windows as the signal (Humidity and Condensation Control in Cold Climate Housing, cited for the principle). A humidity meter (hygrometer) costs $10 to $50 at a hardware store, in EPA's figure.

The EPA's ways to reduce it: vent dryers and cooking to the outside, run the bathroom fan or open a window when showering, use exhaust fans while cooking, use an air conditioner or dehumidifier when needed, and increase air movement.

What to do at the window

1. On a cold morning, open the covering and look at the glass, the frame and the sill. Write what you see on the survey.
2. Dry any water on the glass and sill quickly, as the EPA says to.
3. Leave the covering open for part of the day so room air reaches the glass. In winter that is when the sun is on it anyway.
4. Look at the back of the curtain or shade, especially the hem. Dark spots or a musty smell mean mold.

Water between two panes of a sealed window is a failed seal, not humidity; NRCan's answer is to replace the insulating glass unit, which is a window contractor's or a landlord's job.

Mold on the window and on the covering

The EPA says a moldy area under about 10 square feet can usually be cleaned by the household: scrub hard surfaces with detergent and water and dry them completely, and fix the water source, or the mold comes back. Over 10 square feet, or after a lot of water damage, it points to its larger remediation guidance and a contractor with mold experience. Absorbent materials that become moldy may have to be thrown away.

Fabric is absorbent. The Canadian Conservation Institute's textile note gives mold growth at three months or more at 70 percent relative humidity, and a few days at 90 percent (Textiles and the Environment). A hem that rests on a wet sill every morning is closer to the second condition than the first. Wash or replace a moldy curtain as its care label allows; a lined curtain that is not washable may be one to replace. Ask the authority

8. See it See it

• Testing the performance of cellular shades in the PNNL Lab Homes: the summary's first two pages state the heating and cooling findings, including the typical-use result.
• Energy Saving Window Treatments, Cornell Cooperative Extension, 2003: drawings of the air loop behind a curtain and how to close it.
• Energy Saver Guide 2022, DOE: the cold- and warm-weather window tips and the draft test.
• Condensation, Natural Resources Canada: why closed drapes raise condensation on the glass.

9. Practice Practice

Minimum: one cold or hot window, one draft check, one written season routine and one cold-morning look at the glass. Time: about 20 minutes, plus a few minutes on one cold morning. Materials: an incense stick or a smoke pen, a cloth, a phone or notebook, the window survey, and a hygrometer if you have one.

1. Sit near each window in the room at the time of day it bothers you. Write "cold in winter", "hot in summer", "both" or "neither" in the survey's heat column.
2. Check for drafts at the sash, the meeting rail and the trim. Write what you found and who fixes it in the notes column.
3. Write the season routine in the survey's routine column: winter by day, winter at night, summer by day, from the routine table above.
4. On the next cold morning, open the covering and look at the glass, the frame, the sill and the back of the covering. Note water, frost or dark spots, and dry them.
5. If you have a hygrometer, write the room's reading beside the condensation note.
6. (Go deeper; optional) Before buying a covering for energy, look up its AERC rating for your climate and compare it within its category, using the AERC label.

10. Your windows Your windows

Your setup's climate, the direction your windows face, and whether you rent shape which steps come first.

11. Check your work Check your work

Criteria

• Does every uncomfortable window have a heat entry in the survey?
• Did you check for drafts before choosing a covering, and is each draft either fixed or reported?
• Does each cold or hot window have a written routine for winter and summer, by day and at night?
• Did you look at the glass and sill on a cold morning, and write down what you saw?
• Is any savings figure you rely on tied to its study, house and conditions?
• If there is condensation, do you know the room's humidity, or how you will find it out?

Worked example

Walt's survey has all three bay windows marked cold in winter, a draft note for each with what he found and fixed, and a routine for each. His condensation note says "inside of glass, cold mornings" and his plan names the humidity check before the new curtains go up. He has not claimed any saving, and his open items (the plaster, the track) belong to hardware, not to this chapter.

Common mistakes

• Buying a thermal curtain for a drafty window. Air flowing past the edges defeats the still air the curtain depends on. Seal first.
• Quoting the largest figure. "Up to 20 percent" was one test house and a model; typical use in the PNNL homes saved no significant heating energy.
• Leaving coverings half open all season. That is the typical use the studies found saved little.
• Closing coverings tighter to stop condensation. Closing them is what makes the glass colder. Lower the humidity and open the covering for part of the day.
• Hanging a curtain over a register or radiator. It sends the heat up behind the curtain to the glass, and it is a fire distance problem.

What is still unresolved

How much your home saves is settled only by your own bills over time, read against the weather. Which covering suits each window is in Curtains, shades, blinds and shutters, and lining choices in Fabric, linings and fading. Drafts in a rental and failed window seals are the landlord's or a window contractor's to fix; mold over about 10 square feet goes to a contractor with mold experience, as the EPA advises.

12. Knowledge check and scenario

Checklist

• The survey has a season routine for each cold or hot window and a note on condensation.
• Drafts checked at each uncomfortable window, and fixed or reported in writing.
• Glass and sill looked at on a cold morning, and any water dried.
• Registers, baseboard heaters and radiators clear of curtains.
• Room humidity read with a hygrometer. (Go deeper; optional)
• AERC ratings compared within a category before buying for energy. (Go deeper; optional)