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What the U-value means for windows

The U-value describes the heat permeability

A lot of heat is lost through old windows, Image: © Ingo Bartussek - Fotolia.com

You can find this out by consulting the U-value of the windows. The U-value (formerly k-value) describes the so-called "heat transfer coefficient". That sounds threatening, but it is very useful: the heat transfer coefficient indicates how much energy flows through a component, as well as through windows. At best, as little as possible so that the heat stays in the building.

Therefore, the following generally applies: the lower the U-value, the better the corresponding component saves energy. Below is an executive summary that sums up what you need to know if you are looking to buy new windows and are only interested in the basic requirements. Following this, we will go a little deeper into the subject in the second chapter.

The minimum requirements for the window U-value

The legislature has not remained inactive when it comes to installing energy-saving windows. In the Energy Saving Ordinance (EnEV), it stipulates that windows must reach or fall below a certain U-value. This U-value for windows is graded according to the place of installation. A facade window has to meet different requirements than a roof window. The EnEV 2014 is currently valid. You can find the full text of the EnEV with all minimum requirements here.

  • Accordingly, windows must reach or fall below a U-value of at least 1.3 (i.e. lower than 1.3)
  • Roof windows should have a U-value of 1.4
  • Skylights do not have to exceed a U-value of 2.7

These requirements do not necessarily apply in the renovation of old buildings - if you only renovate up to a tenth of the window area, you only have to meet the minimum thermal insulation - but they are still a good benchmark for a profitable window replacement. By the way, you can find out very well whether a window renovation pays off with our window calculator. Experienced window manufacturers are happy to take a look at the conditions on site and prepare a renovation schedule and profitability calculations for you. Here you can get in touch with window specialist companies in your area.

The U-value for windows in detail

Image source: SimSys, Wikipedia, license: CC BY-SA 3.0

As far as the energy efficiency of windows is concerned, a lot has happened in the last few decades. While old windows with single glazing still had U-values ​​of more than 5, the introduction of double glazing in the 1960s and 1970s almost halved the energy losses through the windows. In the 1980s, people started to fill the spaces between the panes with noble gases. This further reduced the U-values ​​of these windows. Windows with triple glazing have been around for a number of years. Current triple-glazed windows already have U-values ​​below 1.

U-values ​​of different windows
Window typeYears of constructionAverage uw value, W / (m2K)Average g-value in%
Single-glazed windowsuntil 19784,7 87
Composite windows & box windowsuntil 19782,4 76
Windows with insulating glass (uncoated)1978 - 1995 2,7 76
Double-glazed windows1995 - 2008 1,5 60
Triple glazed windowssince 20051,1 50

Data basis: VFF (Association of Windows and Facades). The u-values ​​in this table represent an average of the u-values ​​of all installed windows. Individual windows naturally deviate strongly from these u-values; a wooden window with double glazing would have a completely different u-value than a double-glazed metal window.

The heat transfer coefficient (U-value) describes the heat flow through a medium. It is expressed by the formula watts per square meter and Kelvin: W / (m2* K). Strictly speaking, the U-value for windows is made up of several individual values ​​that reflect the energetic properties of the respective components. The U-value for windows is divided into the subcategories:

  • Uw Value: The U-value of the entire window ("w" stands for "window"> window)
  • UG Value: The U-value of the glazing ("g" is the abbreviation for "glazing")
  • Uf Value: The U-value of the window frame ("f" denotes "frame")

For the calculation of the total U-value you then need the glass area (AG), the frame area (A.f) (both together result in window area A.w), as well as the length of the glass edge seal (lG) and the heat loss coefficient of the glass edge (ψG).

The bottom line is:

Uw = (AG * UG + Af * Uf + lG . ψG) / Aw

Windows with a lower U-value require a new ventilation behavior

Incidentally, with regard to window replacement, you should pay close attention to your ventilation behavior when buying new windows. In contrast to their predecessors, modern windows are practically airtight. The draft is therefore significantly reduced. That sounds quite pleasant in itself, but it can lead to a new problem: the air exchange in the house is no longer adequately guaranteed. This could then lead to strong condensation in the vicinity of windows or cold wall areas and thus also to the formation of mold (shifting of the dew point). So remember to ventilate correctly. For example, repeated ventilation is more effective than constantly tilted windows. Integrated window fans may also help. In the event of a complete renovation, you shouldn't forget to have a ventilation concept drawn up for the building anyway. In particularly energy-efficient buildings, things get complicated without additional controlled living space ventilation.

And what about the g-value?

U-value & g-value, graphic: Interpane

The U-value is not completely enough when buying a window. There is also the g-value. The g-value stands for the degree of energy transmittance, i.e. how much energy gets into the interior of the building through the solar radiation through the window. It is expressed either as a number between zero and one or as a percentage. Incidentally, the aforementioned EnEV 2014 also makes specifications here: a facade window must have a Uw value of 1.3 and a g value of 0.60 or 60 percent. The same goes for skylights.

Incidentally, you cannot generally say here whether particularly high or low values ​​are good or bad. Special sun protection glazing, for example, has an average g-value of 30 to 50 percent. A room does not heat up as quickly in summer, but less energy is gained through the windows in the darker seasons. So here you should weigh carefully and get advice accordingly.

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