Assessing the insulation performance of windows

Hello,

I registered anew because I don’t know where else to place my question.

It is not a house, but a condominium in a multi-family building from the 1930s (worker’s housing). For years I had the original wooden box windows and in autumn 2015 I decided to have new plastic windows installed. These are double-glazed windows with a 5-chamber system and values of Ug=1.0 and Uf=1.3 (according to the contract).

As far as sound insulation is concerned, it really is a difference like night and day. However, I am not quite sure about the thermal insulation.

My apartment is heated with a gas boiler, which I can control via a panel in the bedroom that also measures the temperature. During the heating periods I have noticed that the boiler keeps turning on again and again, as it did before. So the heat apparently is not effectively retained. Today I deliberately checked the loss and according to the panel the temperature dropped by 1.5°C from the end of the heating period at 9:30 a.m. until 12:00 p.m. Outside the heating period my minimum temperature is 19°C, during the heating period 21°C. The outside temperature here today is about 2°C. I find the loss somehow too high. I also would have expected the boiler to stay quiet after heating up at first – after all, most of the energy is lost through the windows and I would have thought that the new windows insulate many times better. I have little experience with the technical details, so I wanted to ask if there are simple ways to check the windows for their insulating performance (laser thermometer on the glass etc.).

My apartment is on the first floor and borders apartments on four sides, i.e. only the front and courtyard sides are exterior walls. Of course, I don’t know how well the apartments above and below heat. The windows were installed according to [RAL-Standard].

I would be very grateful for tips...

How big are the windows? If they are small, the effect is relatively small - in other words, is there more wall or window?
How and when the heating cycles can have many reasons. Is it controlled purely by a thermostat?
Do you have consumption values? Your heuristic methods are all well and good, but it is quite possible that they are meaningless... especially since you don't even know how long the temperature drop lasted with the old windows.

P.S. if the windows are really airtight, hopefully you have been informed about the importance of ventilation - or better yet, have considered a decentralized controlled residential ventilation system!

I never really gave the old windows much thought because I didn’t expect anything from them. That is different now, of course, I just pay more attention. Whether my doubts are justified or unfounded is exactly what I want to find out.

The windows are typical for old buildings and large, but there is definitely more wall than window. Except in the bedroom, where a two-leaf, fully glazed balcony door leads to the balcony. The dimensions there are 1120x2410cm+520x1410cm (window next to the balcony door). In the living room, it is 1650x1470cm+600x1470cm (three-leaf window). Both rooms have about 12-14 m² – the entire apartment about 44 m².

I air out one to two times a day for about 5 minutes each time – I was told that this should prevent mold, etc... or what do you mean by Kontrollierte-Wohnraumlüftung?

Yes exactly that... depending on the wall structure, that is too little - although the risk is probably not that high with an absolute sandwich layer.
12 sqm windows, so - AT of 2°C -> 19 °C difference. If your U-value improved from 2.5 W/sqmK to 1.2 W/sqmK, you save 1.3 W/sqmK * 19°K * 12 sqm = 296 W or in 4 months 296 W * 120 * 24h = 850 kWh heat -> 85 cubic meters of gas * 70 cents = 60 euros in annual heating costs.
100 assumptions, but roughly it is correct. If you save 40 euros, it is probably still somehow plausible. If you have higher annual costs than before, then not.
Just roughly for your assessment.

Hm, do you now mean that it would still be plausible regarding the bill at 40 euros or concerning the acquisition? Because at 60 euros per year, the windows will practically not pay off within their own lifespan.

Exactly - energy renovations often just barely pay off - that's how the prices are calculated (I guess quite boldly). 60 € per year energy savings - but of course additionally comfort gain, increase in value, and functioning windows. Feel free to critically review the calculation again, but it shouldn't be entirely wrong.