Great point Jay. All homes, natural built or not can suffer from building sickness and indoor air pollution. You bring up another point that buildings should have the ability to transpire. I think its important that we separate these two ideas because while they can effect each other they are two different things;
Ventilation and
Permeable Building Envelopes.
Homes need ventilation, or more accurately, occupants do. Indoor air is almost always more polluted than outdoor air no matter how much trouble one goes through to use natural materials or filter the indoor air. So ventilation in this respect means
introduction of outdoor air. ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupied floor area.
This is a debatable metric. On one hand, it seems to offer plenty of fresh air to occupants. Some prominent building scientists argue that this rate is too high. All that introduced air has to be conditioned which is the biggest source of most folks energy bills and environmental footprints. Personally, Iam neutral to the rate. I want to meet building codes while offering the healthiest indoor air quality to my clients so Iam willing to trust the experts on this one.
The reason we build airtight is that it gives us more control of where, when and how the air in our buildings are coming and going. Building leaky houses is bad practice because they will leak most when you dont want them to and dont leak
enough to provide fresh air when you need it. Leaky houses are more prone to mold and rot especially in humid climates as moist air flowing pass cold structural members will deposit its moisture. By building tight we can introduce fresh air exactly where, when and how we want it.
The breathability, permeability, or transpiration ability of our building envelopes (walls, foundation and roof) are not an acceptable way of providing ventilation. There is simply no way of controlling it. These assemblies should be airtight in climates that require conditioned air if the occupants care about their energy bills and environmental footprints.
It is best practice for the building envelope (with usual exception of earth connection) to be permeable. The details of this gets into some pretty heavy building science stuff. There is a pretty big difference between what is defined as a "vapor barrier" and what is designed as a "housewrap". The only place vapor barriers are used anymore (with possible exception of extremely cold climates) is under a
concrete slab.
The ability of a wall or roof to dry out is good. However, most of this concern is due to poor bulk
water management; improper flashing, leaks from rain and wind driven rain. Walls dont need to breathe but the more they can dry out between repeated rain events and poor construction details the better. These qualities however shouldn't be confused with fresh air introduction and ventilation.
