4.5 wool, year round - if it gets really cold I add a hot water bottle, used to add a blanket as well but have had problems with my knee which mean that I get very uncomfortable if the covers are too heavy. It's lovely to see other single duvet fans here, I always feel weirdly defensive about this. We switched to having two single duvets on a double bed some years ago when I was having chemo and night sweats and have never gone back to double duvet - my partner has a 10.5 feather duvet year round plus one of those M&S fleecy blankets. (I find changing the bed linen much less of an effort with singles.) (The wool duvet is machine washable/line dry, the feather one visits the dry cleaner)