The Ecohome Demo House has a vapor permeable exterior air barrier, and with an ACH rate of .69 it is several 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 vapor barriers:
The job of a vapor barrier is to prevent vapor diffusion (walls absorbing humidity from the air), and the job of an air barrier is to stop air leakage through individual holes. A wall system should have one vapor barrier, but it can have many air barriers. A vapor 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 about the difference between air barriers and vapor barriers, and see this video installation of a a peel and stick airtight membrane :
Now you know more about building envelopes. Read more in the Ecohome Building guide and the pages below, and watch more green building videos here on the Ecohome YouTube channel