[hrudham]

Different South African here, but I grew up in Durban and now live in Cape Town, with regular visits to Johannesburg (2-4 times a year). Single-glazing is used in all three by default, despite the climate being quite different in all cases. Durban is tropical; winter (we joke that it doesn't have one) is dry, and summers are wet, but its warm year-round. Temperatures can vary quite a bit between seasons in Joburg and Cape Town. Cape Town has a Mediterranean climate; summers are warm and windy, while winters are cold and wet, often getting to 7°C at night (it's a cloudless 25°C summers day today though). Joburg can get a little colder, although its a much more dry kind of cold.