An efficient window is one that lets as little heat as possible out in winter and as little as possible in in summer. It does not depend only on the material: it depends on the whole set of glass, frame, seals, installation and ventilation. If one of them fails, the rest loses strength.
This guide explains, in plain words, where a window loses heat, what Uw, Ug and Uf mean, and what you can do, from a cheap adjustment to replacing everything. We give no percentages or promised savings, because they depend on the home.
What an efficient window is
Think of a thermos flask. It keeps heat because it has good walls and a good lid. If the lid does not close, the walls are no use. A window is the same: the glass is the wall, the frame and the rubber seals are the lid, and the installation is what fits it all into the house.
An efficient window also gives comfort. Next to it you do not feel as cold, there are no draughts and the glass does not turn icy. That lets you use the whole room, not only the corner of the sofa far from the window.
Where a window loses heat
| Where | What happens |
|---|---|
| In the glass | It is most of the window, so the biggest loss, especially with single glazing |
| In the frame | Frames with several air chambers insulate better |
| In the seals | Worn or badly fitted, they let air through |
| Around the frame | Gaps or missing insulation between the window and the wall let the cold in |
| In the shutter box | If badly insulated, it is a hole above the window |
The area around the frame is the most forgotten. It creates thermal bridges: spots where cold passes more easily. An excellent window with a badly treated frame edge loses a large part of what it promised.
Three abbreviations, one line each
- Uw: how much heat passes through the whole window. The lower, the better.
- Ug: how much heat passes through the glass only.
- Uf: how much heat passes through the frame only.
To compare windows, what matters is the Uw of the complete window, with your sizes. Good glass in a weak frame does not make a good window. Ask for the Uw in writing in the quotes and compare numbers of the same kind.
The role of the glass
Double glazing has two panes with air or gas between them. Triple has three. Treatments such as the Low-E coating (almost invisible, it reflects heat back into the room) and argon gas (it insulates better than air) help keep heat in.
More glass does not always mean better for you. It depends on the climate, the sun and what you want to solve. See Double or triple glazing.
The role of the frame
PVC insulates well by nature. Inside the frame there are air chambers, and the more chambers and the better designed, the better. Wall thickness and class also count. See Thermal insulation of PVC windows and Profile classes.
Seals and installation
The rubber seals close the sash against the frame. If they are worn, dry or badly fitted, air gets in even with the window shut. The installation closes the gap between the window and the wall: if it is careful, with fixing and sealing all around, the window delivers what it promised. If it is badly done, even the best glass is wasted. See PVC window installation.
Summer: the other side of heat
A window also works the other way round. In homes facing south or west, the sun heats a lot. The type of glass, the colour of the frame and, above all, shade count: shutters, blinds and awnings outside the window stop the sun before it reaches the glass. Curtains inside help less.
Ventilation: the other side of the coin
A very closed-up home, never aired, collects humidity. Drops appear on the glass and, later, mould. New, well-sealed windows need more attention to air: open them a few minutes a day, preferably when the home is warm. See Water on the glass.
A more efficient window does not mean a home with no fresh air. It means air comes in when you decide, and not through gaps.
Before replacing everything
You do not always need to replace. Try these simple tests:
- Paper test. Trap a sheet of paper between the sash and the frame and close it. If the sheet slides out with no resistance, the seal is not pressing.
- Hand test. On a windy day, move your hand slowly around the window. Feel air? There are gaps.
- Touch test. On a cold day, touch the glass and the frame. If the glass is icy and there are drops, the glass is weak or the home has a lot of humidity.
A sash out of adjustment or a worn seal can be fixed with an adjustment, without replacing the window. If the windows are old, single-glazed, and you have cold and a high bill, replacing usually makes a difference. How much you will save depends on the home, the climate and how you use it.
An example: the cold living room
In a room with old single-glazed windows, the cold can be felt even with the heating on. The glass turns icy, the curtain moves in the wind and the sofa next to the window sits empty. After replacing them with better windows, well installed, the glass no longer gets as cold, the draughts disappear and the whole room is used again. But if the installation leaves gaps, part of the gain is lost. It is the whole set that counts.
The most common mistakes
- Thinking “PVC” is enough. Performance comes from the whole set.
- Changing the glass and forgetting the installation.
- Closing everything and never airing. Humidity and mould appear.
- Comparing only the material. Compare the Uw of the complete window.
- Expecting miracles. Better windows help, but they do not replace insulation in the walls and roof.
Mini-glossary
- Uw / Ug / Uf: heat passing through the window / the glass / the frame. Lower is better.
- Thermal bridge: a spot where cold passes more easily.
- Low-E: a layer on the glass that helps keep heat in.
- Argon: a gas between the panes that insulates better than air.
- Airtightness: the window’s ability not to let air or water through.
- Chambers: the hollow spaces inside the frame.
What to do next
- Read about the thermal insulation of PVC windows and about condensation.
- See what changes the cost in How much do PVC windows cost and use the calculator to estimate the cost of changing.
- If you have heard about grants, read about the CLASSE+ label and check with ADENE.