The term “breathability” is often associated with various natural building methods, however the term is often misunderstood. It does not refer to air exchange but vapor permeability. Just as our own skin is sometimes called a breathable membrane, its primary purpose is not air exchange. That is the job of our respiratory system.
A more accurate term to describe the physics in a traditional (natural) mass wall construction “is “vapor open” or “flow through”. There is no need for a vapor barrier because the clay in the wall has a very high capacity to take on vapor when conditions are humid, to store it without any damage to the wall system and then to re-release it when the indoor or outdoor climate has changed. It is this ability that has enabled the historic buildings made of clay and fiber to endure for hundreds of years all over Europe and Asia.
Whether the home is conventional or natural random infiltration is not a desirable source of fresh air. A leaky home may contribute to air exchange but it will also contribute to drafts, less comfort and more energy consumption. In the case of conventional light frame building, infiltration of moisture laden air will cause condensation and moisture problems within the wall cavity.
Traditionally air exchange has been through windows. However when it is very cold or hot and humid outside this form of air exchange also displaces indoor tempered air for outdoor frigid air! Bathroom and kitchen fans extract polluted air and make-up air will find its way into your home either randomly, or where you choose to supply it. It is safer to choose where we want our make-up air to come from. An HRV (heat recovery ventilator) is designed to automatically bring in fresh outdoor air and temper it with outgoing exhaust air without mixing the air streams.
So how much fresh air do we need? No one seems to agree on this and in fact it would depend on many factors such as number of occupants, doing what in a house made of which materials. A person would need much more air exchange in a home that is made of toxic building materials or if someone smokes, for example.
How much automation and mechanization do we wish to buy in to? In a natural building, with non-toxic materials and windows designed for cross ventilation, is mechanical air exchange necessary? Yes and no. If you live in the Arctic opening the windows to your natural home will still let in a blast of very cold air. However…..in a more moderate climate we have found that, once your mass walls are to temperature, you can open windows, exchange air and close them again without significantly changing the air temperature or your comfort. This is how our ancestors did it and it has also worked well for us in our straw clay homes. It is a lifestyle decision…unless you live in a State where HRV’s are now mandated for all new homes.