@witty ,
You can think of air walls in legacy as a perforated sheet of plywood. Air can flow easily though them but they block all radiant exchange. In the new LBT plugin, they are a whole new type of object that allows radiant exchange across them. This text that’s copy/pasted from the LBT Plugin Release Noes includes more detail:
True AirBoundaries
Throughout the development of Legacy, EnergyPlus had no way to specify true AirBoundary surfaces between Zones and, for this reason, legacy air walls were more like a sheet of plywood across which air could mix but sun and radiant heat could not pass. All of this changed with EnergyPlus 9.3, which introduced the Construction:AirBondary and we are happy to say that HB-Energy fully supports these AirBoundary constructions.
Now, in HB-Energy, you can simulate two rooms connected by an AirBoundary and know that the sun can pass from one to another and that surfaces can radiate heat to one another across the boundary. This will end up being particularly helpful in improving the accuracy of microclimate maps (currently under development) but it also has a range of immediately-usable applications. For example, you can now model a large indoor space like an atrium or a gymnasium as several rooms joined by air boundaries and this will help account for the differences in air temperature throughout the space without compromising the accuracy of the radiant calculation.
This section of this video also helps explain the difference:
