I'm using a duvet that's so old that they didn't even have Tog ratings when I bought it! It's 38 years old and I've never put a cover on it because it's so lightweight (possibly around 2 Tog?) that launderettes regard it just as if it's a very thin blanket (meaning that it can go in with a normal wash, rather than having to pay for specialist duvet cleaning). I've not put any heating on yet and, despite only using that ultra-thin duvet, I was still very warm in bed last night.