I've been reading SANS 204 and it prescribes a total roof R value for Cape Town of 3.7. This doesn't make too much sense when comparing with the R value of 3.2 for Pretoria or the R value of 3.7 for Johannesburg.
Having spent many years in Cape Town I struggle to understand how it can require as much insulation as Johannesburg or more than Pretoria. These last two towns seem to be a lot colder in winter and hotter in summer due to their higher altitude and a lack of ocean to moderate the temperature.
If you compare a climate graphs of Cape Town to Johannesburg or Pretoria at:
You can clearly see that both are colder than Cape Town during winter (June through August) when it comes to minimum and average temperatures. Which if you are insulating for cold, as per SANS 204, would seem to be the important ones.
Maybe I'm missing the plot but it seems that Cape Town has a far more moderate climate than the Cities in the interior and thus the R value for insulation is on the high side.
Basically what this boils down to is that adding insulation with an R value of 2.5 (e.g. 100mm of Think Pink / Aerolite) to whatever the existing roof space provides should be sufficient.
Please note that the insulation chosen is an example and I couldn't care less if it makes you itch if you don't wear appropriate clothing while working with it. Discussing this would have no bearing on my question so please save it for another thread. :-)
Reasoned analysis based on facts is welcomed. Personal experiences if well written might also count for something.