[GBlist] Re: Wall permeability

["Lewis J. Matt PhD" <lmatt@alltel.net>]

-----Original Message-----
From: Kirsten Flynn [mailto:kir@declan.com]
Sent: Wednesday, January 16, 2002 11:30 AM
To: greenbuilding@crest.org
Subject: [GBlist] Wall permeability

Dear Kirsten:

The concept of a wall film that is selectively permeable to hazardous gasses
is quite intriguing.

I have good results with current technology by using an 8 mil vapor barrier
on the inside and a water vapor transparent, wind resistant wrap on the
outside of conventionally framed and insulated structures with an air to air
exchanger to ventilate the interior.  The vapor barrier keeps excess
moisture from permeating the insulation and reducing its insulating
properties, and the exterior wrap allows moisture to escape the insulation
but reduces the amount of air "blowing" through the insulation which can
also reduce its effectiveness.  Air exchangers can take care of the
hazardous gasses and the moisture without creating a thermal imbalance.  Air
exchangers can be powered or rely on a roof mounted vent stack for "pulling"
old air out of the building and new air in.  As far as moisture is
concerned, I have had good results directly venting bath and cooking
moisture and odors at their source with an exhaust fan that is turned on and
off as needed.  The shower and the gas stove are the two devices in a home
that generate the most moisture - and I don't want to give up either!  :-)
Note: *unvented* clothes dryers are also major moisture polluters.

If you have adequate vapor barriers and insulation ventilation with properly
sized heating / cooling, the in-the-wall dew point becomes insignificant in
a vented home.

Lew Matt


Synergistic Solutions: Alternative Septic and Energy Systems
Lewis J. Matt III, Ph.D., C.S.E.O.



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