Thermal insulation is important for wood windows. You can read more about the importance of wood windows and their natural advantages in this article.
Standard specifications for energy efficiency of new windows
According to current international energy efficiency standards, newly installed windows have a maximum U-value of only 1.3 W/(m²K). In energy-efficient buildings, windows should be as far below this value as possible.
Modern energy-efficient windows typically have U-values of 0.9 W/(m²K) to 1.1 W/(m²K), which is independent of the type of window. The overall U-value of windows in passive houses is usually significantly lower. Values between 0.8 W/(m²K) and 0.4 W/(m²K) are standard for passive houses and low-energy houses.
Thermal protection requirements
To keep the U-value of windows as low as possible, the following criteria are important:
the insulation of the frame
the insulation of the glazing
the best possible insulation of the frame edge
Insulation of the frame
For the insulation aspect of window frames, wood windows can show their natural advantages. Aluminum and plastic windows must be insulated at great expense, but the wood itself is a very good insulating material.
Insulation of the glazing
The insulation value of the glass (Ug) is usually the insulation value specified in most product descriptions. It does not correspond to the overall insulation value of the window. However, in most cases it is the decisive purchasing criterion and largely determines the price of wooden windows.
Insulation of the frame edge
The end area of the window frame is the critical point of every window where the window glass and the frame do not touch each other, so a well-constructed window edge (sealed against air leakage) will form a gap with the frame, which will allow it to be well insulated.
The relevant value for the frame edge is the linear heat transfer coefficient. You will not usually find this value in the product manual, you will usually need to ask the seller for it. A particularly low heat transfer coefficient is the quality standard for windows.
The narrowest possible frame profile is also an advantage. The ratio between the glass surface and the window frame surface should be as ideal as possible to achieve adequate thermal protection.
The calculated values apply only to a limited extent to wood latticed windows. You should pay special attention to the deviation values.