The casing of a down duvet does two things. It keeps the filling in place. And it determines how the duvet behaves between you and the room.
What a good casing must do.
Tight
It must be tight enough to let no clusters through. A good percale casing has a density of one hundred and fifty to two hundred threads per square centimetre. Below that the down will, after a time, prick through the fabric.
Breathing
It must breathe. A casing too tightly synthetic seals off the exchange with the air, and then the filling cannot do its work. Natural fibres, cotton, linen, let air through without down escaping.
Sturdy
It must be sturdy enough to bear years of daily use. Washing, plumping, using. A light synthetic fabric often wears out before the filling itself is spent.
What we avoid
What we avoid: thin casings with low thread density, and synthetic fabrics that cannot fulfil their function because they are either too tight or too open for what a good down duvet requires.