# Roof window exchange - when do new windows pay off?

Especially roof windows cause high heat losses in case of poor insulation. How a roof window exchange pays off, and what measures can be taken instead of a window exchange, you can read in this article. How to calculate savings.

### Meaningful roof window exchange

An exchange of skylights makes sense only in inhabited attics. If the attics are not heated, it is sufficient to insulate the upper floor ceiling. Thus, heat losses through the skylight fall anyway not significant.

Before the roof windows are exchanged in heated attic rooms, a roof insulation should be done in any case. Only then do you know the actual heating demand, and can set a meaningful U-value for the skylights.

### U-values ​​for skylights

Modern skylights may only have U-values ​​of maximum 1.3 W / (m²K). This is set by the EnEV for all windows equally. A substantial underrun of these values ​​does not always make sense.

Depending on the size of the roof window surfaces (usually smaller than the window areas on the other floors), different U values ​​make sense. Windows with U-values ​​below 0.9 W / (m²K) are usually disproportionately expensive and no longer pay for themselves properly.

### Calculate heating cost savings

Unlike many window manufacturers want to make believe, the savings in heating energy through the roof window replacement can not be calculated exactly. For the possible savings in heating energy many factors come into play, which one would have to include:

• the position of the windows and the sun's rays on the skylight
• the sunshine duration in the respective area
• the ventilation heat losses
• the efficiency of the roof insulation
• the type of heating, the efficiency of the heating and the heating costs
• the exact values ​​(U-value and g-value) of the previous windows

### Observe heat permeability from outside

A particular value that is often overlooked is the g-value of the skylights. It indicates how much of the heat radiation from the sun can enter the room from outside.

For starters, the idea of ​​summer heat would sooner be to choose low g-value windows. Especially in winter, however, solar radiation is a very effective means of compensating for thermal heat losses.

From a purely mathematical point of view, solar radiation in our latitudes brings in a multiple of energy than is lost through the window. If the walls and the roof are well insulated (U-value of 0.20 W / (m²K), the solar radiation can provide a positive energy balance even on many winter days, or almost completely compensate for the losses the expert necessary to determine this.

### Meaningful measures

Before a window exchange, there are also other meaningful measures that you can take yourself:

• Test window to find out the actual U value
• Apply protective foil, if necessary also heat protection foils
• Installation of a window ventilation with heat recovery

These measures are cheaper than a window exchange and also highly effective. They can also be used in addition to new windows. In many cases, however, they are sufficient to replace the skylights in order to achieve sufficient effects.