Our double-glazing is about 16 years old, and in the last 6 months three
units have failed with condensation inside. All the failed units are
south-facing - presumably the more extreme heating cycle weakens the seal
eventually.

1) What's a reasonable lifetime for a DG unit?
2) Would K glass help to extend the life of a south-facing window?
3) Can I replace the units myself like for like, or does a FENSA bod have to
do do (or approve) the work?

--
Reentrant