[javiermaestro]

Really? Check this: https://www.bre.co.uk/filelibrary/pdf/rpts/countryfactfile20...

Page 3, Summary:

Around 83% of homes in each country have some double glazing. Northern Ireland has a higher percentage of homes which are fully double glazed, 62%, compared with 32-44% in the other countries.

So, ~60% of the houses are NOT fully double-glazed. In 2007. Srsly. And sure, it's from 2007... but I highly doubt the situation has changed...

[Neil44]

Why would you doubt the situation has changed? Also, not fully double glazed would cover my current house which has one single glazed tiny bathroom window with an extractor fan in it.