why not replace the bathroom window completely, as it's borked, then just replace sealed units as and when. What makes you think windows now are any better than they were then? Its plastic!.. Only difference will be that later ones have a bigger air gap. But if you replace them all with new ''''efficient!!!!!!!!'''''''' windows, you will never recoup the cost, via the savings in thermal efficiency. Just do as above, then spend your hard earned, (or willfully borrowed) dosh on other things.
* Yes, the air gap is bigger on modern double glazing (typically 16mm), but there are also options for the gap to be argon filled to reduce heat transfer and condensation.
* modern double glazing also offers options of a range of film coatings. So if you have a south facing room that overheats in the summer, you can get a coated glass that reduces solar gain, whilst retaining heat from the other direction to remain efficient in the winter. It can work very effectively (my conservatory says so
)
* The insulating benefits of double or triple glazing do not outweigh the cost compared to a simple single glazed solution. However, you take the benefits in other ways such as reduced maintenance, better comfort and reduced operating expenditure on heating bills. The home will also heat up quicker and stay warm longer providing a more even temperature. Environmentally, good double glazing (e.g. argon filled, with energy retaining coating) may well have a lower lifecycle impact than triple glazing once embodied carbon is considered.
Wood vs PVCu vs aluminium is probably a personal taste issue. We installed decent engineered wood frames 5 years ago and they show no sign of needing to be painted yet. Possibly in 2021. PVCu is environmentally harmful to produce and difficult to recycle, and when it is recycled it is typically mixed with virgin polymer to produce yet more plastic products. Aluminium is fully recyclable. Aesthetics aside, the most important issue is to ensure the design avoids thermal bridges between different window elements otherwise you'll get cold spots where condensation can develop, and where heat escapes.