In article , Sacha
writes

We find just the opposite. The manmade fibres are too hot and make the
duvets too heavy. We have feather pillows and duvets and pure cotton
sheets as polycotton is too warm.

The best I've found is a duvet with raw silk 'filling'. Thin, light and
good for summer and winter. (Its also been through the washing
machine).

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regards andyw