Single glazed windows can be responsible for up to 35-40% heat loss during winter and heat gain during summer. Juts think about it, more than a third of your electricity bill comes from poor windows!
Aluminium windows often start to rust while timber windows rot with time. They require constant maintenance, otherwise they no longer function properly. Furthermore, cheap windows are not designed to last long so unfortunately people get what they paid for.
Cheap single glazed windows allow a lot of street noise in, including road traffic, loud neighbours, road works etc.
The old windows still have 3mm glass in them, which is very fragile and can easily crack under pressure causing serious injuries. It is illegal to install 3mm glass any more.
Since single glazed windows allow a lot of heat to escape at night, your windows get condensation on the glass. It creates unhealthy environment where mould starts to grow.