ᐅ Window Replacement in a Renovated Older Building

We don’t care about the SFP at all; I only need the U-values. The (obvious) replacement of windows and doors is the only measure we will take.
I’m very grateful for your responses, but somehow the thread has gone in the wrong direction and unfortunately none of my questions have been answered, or I don’t understand the answers. As already mentioned, the SFP is irrelevant to me and I haven’t asked any questions about it. Maybe my question wasn’t clear enough, so I’ll try to put it differently:

A hypothetical standard multi-family house has 4 residential units and a stairwell. The apartments are all insulated from the inside (with a U-value that allows triple-glazed windows). The stairwell is not insulated and has a U-value that allows double-glazed windows. All windows date from 1970, so they are very old. The residents tend to leave their front doors open for a long time. The windows in the apartments will definitely be replaced with new ones. In this case, would it also make sense to replace the stairwell windows with the best possible double-glazed ones, or does this make no sense from an energy or building physics perspective? Is the heat loss, for example, negligible because the heating is inside the apartments and keeps the warmth there? Or could it even be counterproductive because it might promote mold growth, since old windows allow constant air exchange? Should all windows (apartments and stairwell) be kept at the same performance level (e.g., all only double-glazed), or is it possible to mix them?

Questions in a single block paragraph are hardly noticeable because they don’t stand out. I also don’t believe there are enough construction experts here to answer these questions. If you haven’t already, I would suggest visiting another forum where more experts gather. Maybe it’s more of an insulation issue after all?

danny80 schrieb:

The apartments are all insulated from the inside (with a U-value that allows triple-glazed windows). The stairwell was not insulated and has a U-value suitable for double-glazed windows.

All U-values allow for both double and triple-glazed windows. Even in completely uninsulated older buildings, triple-glazed windows are not a problem if properly installed (considering reveal insulation) along with appropriate heating and ventilation practices.

danny80 schrieb:

We don’t care about the SFP at all; I only took the U-values from it. The apparent window and door replacement is also the only measure we will undertake.
I’m very grateful for your answers, but somehow the thread went off in the wrong direction and unfortunately none of my questions were answered or I don’t understand them. As mentioned before, the SFP is irrelevant to me and I didn’t ask any questions about it. Perhaps my question wasn’t clear enough, so I’ll try it differently:

I have underlined the crucial part in the quote.
Your question was clear enough, and I understood it immediately. You did not ask any question about the refurbishment roadmap, and it is only irrelevant insofar as you do not recognize large parts of it as matching your priorities. But the consequence of dismissing it is wrong. Use it as a structural template for creating your own plan. Because you will need a plan. The attitude "I only know the one measure I want to do in the first step, so I don’t really need a proper ‘plan’" is, from my professional experience, wrong and would cost you roughly 30% more (or about 25% longer / less efficient overall). I’ve already given you suggestions on how to structure your individual plan (very important: besides the columns “measure” and “budget,” you must also include the schedule!). I seriously mean all the differentiations of the window measures. Don’t forget that you’re reading this from a former window manufacturer. Full replacement (for each single window) as the only measure is nonsense, and full replacement (for all windows) even more so. That would only literally realize the saying “throwing money out of the window,” nothing else.

danny80 schrieb:

Or is that even counterproductive, as it promotes mold growth because old windows allow for continuous air exchange.

Even the much-mocked rebate ventilation is still more sensible than simply leaving old windows in place. This is NOT a qualification of my recommendation not to shoot every sparrow with a cannon!
A “colleague” in educating homeowners, whose videos I highly recommend, is the “energysaving commissioner” (if you have no allergy to the South Hessian dialect).

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Our house from 1921 was insulated in 1993 with 6 centimeters (2.4 inches) of insulation. I don’t have the exact wall thickness at hand, but they are definitely quite thick. Except for the basement, we have had triple-glazed windows everywhere for 7 years (previously double-glazed) and have had no problems at all. We ventilate generously, mainly for comfort.

Regarding the window in the hallway, I would probably avoid installing a completely airtight window or one with a ventilation groove or something similar. But that is just a layperson’s preference.

danny80 schrieb:

Is the heat loss negligible, since the heating systems are inside the apartments and they retain the warmth? Or could this actually be counterproductive, as it may promote mold growth because old windows allow continuous air exchange. Should all the windows (apartments and stairwell) perhaps be kept at the same standard (e.g., all double glazed), or is it possible to mix different types?

The relevant factor is the building’s thermal envelope, which should be defined during the building assessment and the creation of an ISFP (Individual Sanitation and Fire Protection Plan). In simple terms, this is a continuous “line/surface” around your building meant to keep the heat inside.

Usually, it makes sense—as is the case here—to include the stairwell of the multi-family building. You can therefore disregard the airtightness and thermal protection of the apartment entrance doors and focus on the losses from the stairwell to the outside air.

So: include the hallway in the thermal envelope → the windows should be of good quality. The wall here can also accommodate triple glazing.

To prevent mold growth and moisture issues in the only indirectly heated stairwell, you have several options.

A cost-effective solution is to install window frame vents, which have the disadvantage of uncontrolled heat loss and effectively reduce the benefits of replacing the windows, but they come at almost no additional cost.

Another option is to heat the stairwell by adding a radiator with fan assistance. This creates no temperature difference between the apartments and the corridor while promoting air circulation. Of course, the existing piping must support this.

The overall heating demand of the building will not increase significantly, since the heat is no longer provided by the apartments.

Alternatively, you could install a ventilation system with heat recovery in the stairwell (a unit about 30cm x 40cm (12 inches x 16 inches) with a 180mm (7 inches) wall penetration), which dries the air without additional heating. This probably costs about the same as the radiator but should be eligible for subsidies in connection with the window replacement.