In ether case modern doors build out an insulating section around the seal, and the seal itself creates a second seal against that so it's air gapped inside the freezer as well. The seal itself of course is a hollow tube which is somewhat of an air gap with the outside too. They're pretty good, I would be surprised for their small surface area if they caused a huge conductive loss.
Chest freezer seals are far better than upright freezer seals because:
* The length of the seal is smaller compared to volume,
* The height of the seal is not constrained by the closing magnets (as mechanical latches are not legal on consumer freezers),
* The temperature differential at the top of the freezer is smallest.
The specific heat capacity of air at constant pressure is ~1 kJ/kg K and 1 m^3 air has a mass of ~1.2 kg, so 1 kJ/kg K * 1.2 kg = 1.1 kJ/K. Even a freezer that exchanges 2 m^3 of air for outside air at say a 70K differential (the difference between -40C and a 30C room) for each opening only has to remove 1.1kJ/K * 2 * 70K = 154kJ, or 0.05 kWh, which is the standard unit of electricity.
An unit of residential electricity in the most expensive US State (Hawaii) is 41.2c / kWh, so opening our enormous freezer in the worst possible conditions and completely exchanging the air using the most expensive electricity costs us 2¢. It's a complete non-factor.
Source for prices: https://www.energybot.com/electricity-rates-by-state.html
closewith (https://news.ycombinator.com/item?id=32890571) is saying that:
> Chest freezers have better insulation and door seals
I'm not claiming anything either way, just pointing out that the seals are for sealing heat, not (primarily) air.
I think the idea is that the seals can conduct heat, and do so much better than whatever abitrarily-sophisticated thermal isolation is in the walls of the freezer, not that the airflow through the seals is carrying any significant amount of heat.