Air leakage is responsible for a large portion of the heat loss and moisture damage in walls.
The Ecohome Demo House has a vapour permeable exterior air barrier and with an ACH rate of .69 is many times more airtight than the average new home. Air leakage can be responsible for up to 1/3 of the total heat loss of a home and cause significant moisture damage in walls, as one small hole can be responsible for depositing countless litres of water inside walls. Houses should always be built as airtight as possible, don't let anyone tell you otherwise.
The difference between air barriers and vapour barriers:
The job of a vapour barrier is to prevent vapour diffusion and the job of an air barrier is to stop air leakage through differences in air pressure between inside and out. A wall system should have one vapour barrier, but can have many air barriers. A vapour barrier can act as a very effective air barrier, but an air barrier does not (and should not) always stop vapour from difusing.
Understanding the difference between the two opens up a world of options for material choices and wall designs that can make walls perform much better and last longer, and it doesn't have to add a ton of cost or labour to a project. Read more here.