ᐅ U-value of Windows - Differences

Hello,

We are currently building to KfW 70 standards, and triple-glazed windows with a U-value of 0.9 are standard for us. I have read several times that there are, of course, better windows available, for example with a U-value of 0.6.

We have many windows in our house, especially large façades facing south and west. All windows can be shaded, but that's just a side note.

I am wondering now if this makes a significant difference in terms of energy loss in winter and solar gain in summer when opting for slightly better windows, even if it might mean paying an additional cost.

Is it possible to quantify this? Or is it negligible?
What are your thoughts?

We renovated according to KfW 100 standards and installed windows with a Uw value of 0.9, which are triple-glazed with warm edge spacers, and that is actually more than sufficient. By the way, the glass itself has a Ug value of 0.6. So, it really makes a difference whether you are referring to the U-value for the glass or for the entire window. In the southern part of the building, we used a different type of glass called Solarwhite, which is said to allow more sunlight in.

Ultimately, it depends on how much extra cost is charged for switching from a Uw value of 0.9 to 0.6. I suspect that the price difference might be disproportionately high?

Hmm, we have a KfW 70 house, almost reaching KfW 55 standards (as the experts called it back then).
We have only double glazing throughout, with a U-value above 1.

How many square meters is it? You can calculate quite precisely what is theoretically saved... For example, 0.3 W/m²·K would be for 10 m² (108 ft²) of window area on a cold winter day at -10°C (14°F): 10 m² * 30 K * 0.3 W = 30 W per hour -> 0.660 kWh -> 4.6 cents

BeHaElJa schrieb:
How many square meters is it? You can calculate quite precisely what could theoretically be saved... for example, 0.3 W/m²K for 10 m² (108 ft²) of window area on a cold winter day at -10°C (14°F) equals 10 m² * 30 K * 0.3 W = 30 W per hour -> 0.660 kWh -> 4.6 cents

But in that case, you attributed the cost of the temperature change to the windows, right?

A U-value of 0.6 probably applies only to the glass itself—that is, the pane—not the entire window (the frame is usually the weak point). Currently, there are windows (triple-glazed) with a U-value of 0.84, which are among the top models in terms of frame design and are still relatively affordable. You can achieve even lower values by using special gas mixtures between the panes, but this becomes increasingly expensive.

However, you also need to consider whether this is economically sensible. Are you planning with subsidies that require meeting a certain value? If so, windows are a relatively cost-effective way to improve performance. If not, I would recommend using windows with a U-value of 0.9.

In my opinion, houses (starting at KFW70 standard) are already extremely well insulated today. Whether you need 10 kWh/(m²*a) more or less heating energy hardly makes a difference on the annual bill. The choice of heating system (e.g., heat pump with the highest possible COP) has a much greater impact on costs.