In general, you don't want to cover vegetables when you're roasting them, and you definitely don't want to do that with these carrots. Covering (or tenting) vegetables in foil will actually steam them rather than roasting them. They'll come out a lot softer and mushier that way. And, they'll never get the delicious caramelization on them that you get from roasting them. So, skip the foil here and save it for something else (our grilled corn on the cob recipe, maybe).