JAK, check out the threads on Wintertrekking for Anoraks, High Tech v. Traditional and Pake's Anorak Modifications in the forums and the article on Winter Clothing Outerwear.

All I know about cotton anoraks, I learned there recently. Never seen one in person.

The basic advantages, according to the guys who have them are that they are durable, breathe well and are fireproof.

But, as with any other cotton garment, once it gets above freezing, they are not good because they may get wet and then you have the usual problems of wet cotton. They are using them in below zero C weather.

I don't think a sheet would work. Look at the anoraks on www.empirecanvasworks.com Those are made out of 6.3 oz. cotton of some kind. Some of the guys on wintertrekking wear surplus CF or Swedish Army clothes because they can find them cheap.