Window U-value 1.0 W/m2 = Many problems with external condensation?

[Krieng]

Hello!
I'm facing some decisions now as we're about to order windows for our upcoming house. We will be building in a slightly turn-of-the-century style and have found a window manufacturer that meets our requirements. I want a U-value of 1.0 for the windows, which would mean double energy glass + argon gas in between. The window manufacturer recommends choosing a U-value of 1.2 instead, as a lower U-value usually results in issues with external condensation, which many people complain about, he claims.

Does anyone have experience with this? External condensation occurs during clear, cold nights when humidity is high; how often is that? I think it seems obvious to choose the best possible U-value considering that energy requirements will need to be tightened in the future and that it's better energy-wise overall. There's only about a 3000kr difference between U-value 1.2 and 1.0, which isn't much when building new anyway.

What would you do?
Grateful for answers!

 

[sir_daniel]

Make sure to add vents in the window so you can ventilate

 

[verktygsgurun]

Make sure to choose valves in the window so you can ventilate

Doesn't help a bit with external condensation. 😄 you get condensation on windows with a u-value of 1.2 as well. Many manufacturers now offer fog-free glass as an option, check with your manufacturer if they have it and if it's worth the extra cost.

 

[HansR]

It likely doesn't need to be cold outside. The sky is cold.

You can counteract the condensation with e.g. awnings so the windows don't see the sky.

 

[sir_daniel]

Intended against internal condensation

 

[Picke]

We have windows with a 0.9 U-value. Have experienced condensation on the outside a handful of times in four years - not a problem, in other words. However, it is cold at least 100 days every winter on the outside of the windows - that is more important to dimension for. My advice is to get windows with the lowest U-value you can afford. You will benefit both economically and in terms of comfort.

 

[sblixten]

It is the u-value of the glass unit that determines how much/often condensation occurs, not the window.

 

[Stefan Bengtsson]

Have you calculated how long it will take to recoup the 3000 in the form of lower energy costs?

 

[Krieng]

Yes, approximately. According to my calculations in an energy simulation program, we would save about 500kWh per year if we choose 1.0 instead of 1.2, so assuming a kWh costs 1.3 SEK, it would take almost 5 years to break even. However, the window manufacturer couldn't provide the window's g-value, which also affects the calculation, so I have only used a standard value for that type of window.

 

[KnockOnWood]

In my (Polish wood) windows, the glass cassettes are stamped "ECO PLUS U 0.5", so I assume that's the glass's U-value.
They are the windows recommended for passive houses anyway.

It's really only during autumn nights, when a humid/rainy day turns into a clear night, that condensation occurs.
Maybe 10 days a year. No big deal!

And the condensation depends a lot on how large the eaves are and how many trees and other buildings surround you.
Compare it to a car that is parked out in the open or under a carport.

 

[Kyleman]

Choose 1.0 which gives you better comfort and less cold drafts. Normally, it is very rare to get condensation, as stated earlier in the thread, primarily with 0.8 windows and below.

 

[HansR]

Yes, approximately. According to my calculations in an energy simulation program, we would save about 500 kWh per year if we choose 1.0 instead of 1.2, so if you account for a kWh costing 1.3 SEK, it would take almost 5 years to break even. However, the window manufacturer couldn't provide an answer about the window's g-value which also affects the calculation, so I have just used a standard value for that type of window.

The g-value is very important when calculating for windows. It's unprofessional that they can't answer that.