Living Homes

Breathing Walls...This House is Alive

The energy crisis of the 1970’s prompted a trend towards making buildings more airtight. As a result, these “modern”, well-sealed homes have an air exchange rate of only once every five hours or longer.  Problems with stale air are compounded by the thousands of chemicals from building materials that off-gas inside the house. Building Science tells us that the human body requires between one and two air changes per hour to maintain vitality, regenerate body cells and eliminate environmental toxins.

Two styles of construction have arisen attempting to ensure fresh indoor air.  One approach uses airtight construction methods and mechanically controlled filtered air systems.  The other incorporates natural or non-toxic building materials that “breathe”.  ( By  “breathing”  I really mean vapor permeable not drafty)

My experience in airtight construction methods over the years have resulted in some important realizations.  In order for vapor barriers to be really effective, they have to be perfect.  By perfect I mean absolutely no holes, all joints taped and places like electrical outlets, switches, fixtures and other building penetrations need special attention.  A less than perfect vapor barrier allows water vapor to pass through the wall cavity.  In this case, not diffusely through the wall, but in a concentrated venturi-like way through those few cracks, seams or holes.  As this water vapor travels through the wall cavity towards the exterior in the winter, it cools and eventually condenses to water as it reaches the dew point somewhere within the wall cavity.  In many instances this often means on the inside surfaces of materials with low vapor permeability.  OSB or plywood sheathing, insulating foam panels, vinyl sidings, building papers even and some cement based stuccos have all shown the ability to trap moisture within a wall system.  When this happens, insulating materials can become ineffective and an ideal breeding ground for mold and rot spores is created.  Many long-standing older buildings have failed when retrofitted with these new techniques.

So, let’s assume the vapor barrier and other wall components are functioning perfectly.  We now need a mechanical device to facilitate air exchange and ensure healthy indoor air quality.  This usually means an air to air heat exchanger or heat recovery ventilator.  One could also open the window a crack, though this is hard to control and quantify.  Very few mechanical contractors that I have met fully understand or even have the proper equipment to regulate the humidity, the “toxic loading” of each particular building and the proper air exchange rate.  Do some solid research before investing and take up some winter sports.

A properly designed and executed breathing wall system yields some interesting benefits.  A breathing wall is one that allows outside cold air to mix with warm air inside the wall cavity, allowing an exchange of air to the interior space that is preheated by passing through the mass of the wall.  Natural materials like wood, clay, straw, cork and “Air-Krete”, with good hygroscopic ability, absorb excess moisture and re-release it when conditions warrant.  These systems allow diffusion so condensation does not occur.  Mold proliferation only occurs in extreme circumstances from some building problems like a plumbing or roof leak.  These conditions pose a problem for any building method.

Examples of breathing wall systems include adobe, cob, straw bale, straw/clay, timber/log, cordwood, rammed earth and Air-Krete.  When you think of the breathing wall as the skin or flesh of a structure, “the building can be viewed as a permeable organism interacting with the natural world and facilitating a balanced exchange of air and humidity.” 2.

Contrast with the difference in clothing yourself in a plastic bag versus natural fibers.  These are concepts espoused by Baubiologie or Building Biology.  Whether we choose a hi-tech or a passive, natural approach to deal with these building issues, a solid understanding in natural science proves helpful.  Breathe deep!


  1. Prescriptions for a Healthy House.  Baker-LaPorte, Elliott and Banta. New Society Publishers, 2001
  2. The Natural House Book: Creating a Healthy, Harmonious and Ecologically Sound Home Environment.  David Pearson.  Fireside, 1989
  3. Architectural Resource Guide : Architects, Designers and Planners for Social Responsibility.  David Kibbey (self published, 510-273-2428) 1998
  4. Institute for Baubiologie and Ecology, Clearwater, Fl. 727-461-4371
  5. Correspondence Courses, seminars, consultations and on going comprehensive work on the interrelationship between the built environment, human health and planetary ecology.