Not so, The helium will be denser, but it has a molecular weight of 4 as opposed to ~29. Suppose it is 20% cooler -- thus occupying 20% less of the volume at

Not so, The helium will be denser, but it has a molecular weight of 4 as opposed to ~29. Suppose it is 20% cooler -- thus occupying 20% less of the volume at equal pressure, thus being 1.25 as dense. Lift will be proportional to 29 * 1.25 - 4 * 1.25 = (29-4) * 1.25 As opposed to lift at our reference temperature 29 - 4 = (29-4) * 1 Showing clearly that there's 25% more lift. The bigger problem is if the envelopes collapse, but clever people have already solved this by having an internal bladder filled with air to take up the slack.