These lids fit TIGHT. I can't even get the lid on unless I open the slider on top. That's how air tight they are. I haven't yet tested how well they insulate, but I've tipped my cups over at my desk enough, I'd sacrifice a little insulation for not spilling when I tip them over. That said, I have no reason to expect these are not decently insulating. They actually are so tight, that it also meters how much liquid comes out while drinking. Not sure if I like that yet or not, but it means if it tips while open, it won't just gush out.
Rating:
[5 of 5 Stars!]