Visited my beeyard yesterday and the difference between tbh and lang ventilation was obvious. The bees were hanging out on all the lang and my combo hives. The langs are equivalent to 5 deeps and the combo has the equivalent of 5 deeps above the hive. Yet, these bees were hot and were hanging outside to reduce temps inside the hive.
The tbhs were not bothered by the heat. Bee activity was normal at the entrance with a half dozen bees fanning. I lifted the cover which is blocked up above the top bars. The top bars were cool to the touch.
I had commented on this difference last year but I've lost the thread. The difference between the two types of hives, those using frames and those with natural comb is very obvious. The combo(long) hive with frames had the same problem as the Lang hives with frames. The tbh with their natural comb attachments and very little 'free' space surrounding them are much more efficient at controlling the ventilation and temperature.
The extra 'gaps' in Lang equipment must hinder the bees somehow.
You can get the same effect on a Lang by adding an attic space. I used a 1/2" ply wood inner cover with some screened slots in it, added an empty deep with vent holes and a migratory top. The effect was dramatic, almost no bees hanging outside. The bees closed most of the slots in the inner cover, leaving just a little air flow and blocking out most of the heat from the 'attic'. Since I started using screened bottom boards, I see less need for the top ventilation. I now get by with just propping the migratory top with a twig.
I would guess the slanted sides on a TBH help some with the heat also by not presenting the vertical sides as directly to the sun.
I had wondered about those factors as well. It has been mentioned somewhere that top bar hives could dissapate more heat threw there lids making them more suitable for a hotter climate, while Langs are more heat conservative making them better for a colder climate.
Sounds logical. But my combo hive, which is just a two box horizontal long hive with standard frames, suffers from the heat/ventilation just like my vertical langs do.