There is almost an endless choice for duvet casings. Cotton, silk, polyester etc.. Traditionally the Chinese used a very course threaded casing for their silk duvets. This was to allow body heat to escape through the casing and help in heat regulation. Today, largely due to modern manufacturing techniques a much tighter and finer casing can be woven which has similar properties. The choice of silk casing is largely down to individual preference, however there are a few points to consider.