A conversation on the MinnePHit website inspired me to write a quick summary of why airtight buildings are a good thing. You can find my response to the thread on the MinnePHit website. I will maintain this article on Google Knol going forward.
How airtightness is a good thing when it comes to making sure a building is robust and durable.
This knol was written in response to the common assumption that airtight building envelopes cause problems for buildings.
From the designer’s desk: Airtightness of the building envelope (that’s the walls, slab, and ceiling—basically what separates inside from outside) is essential to efficient and durable buildings.
What caused a lot of problems in the past were envelopes that were not airtight enough!
The leaking of warm moist air into assemblies during periods where conditions are conducive to condensation—combined with lack of drying potential of these assemblies—cause “early building degradation”, a.k.a. rot or mold. By making the shell very air tight, and field testing the tightness—thus eliminating potentially failure points—builders and owners can be assured that no worrisome leaking is occurring. By selecting proper building materials that allow for drying of any moisture inside of assemblies, the designer can further make the building more robust.
So in short, an airtight building envelope is the way to avoid building envelope problems.
One last thought: Airtightness does not mean that moisture cannot migrate through assemblies. A safe approach to building design is to make airtight but vapor-open assemblies. All this means is that leaking into the assemblies is eliminated, and drying potential is increased. This combination warrants safe assemblies in just about any climate zone. Whenever building with wood and organic materials this should be the preferred approach.