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Breathing
Walls…This House is Alive!
By Thomas
Hirsch
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”.
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.
Air exchange rates (the entire volume of indoor air) of 1.5 to 3
times per hour can be achieved. 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.
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!
References:
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
Correspondence
Courses, seminars, consultations and on going comprehensive work on the
interrelationship between the built environment, human health and
planetary ecology.
Harmony
Home Construction, LLC.
8122 Barney Road
Traverse City,
Michigan 49684
Ph: 1-877-45-GREEN Fx: 231-932-9193
info@harmonyhomeconstruction.com
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