passive solar greenhouse/strawbale

YankeePerm at aol.com YankeePerm at aol.com
Wed Nov 12 09:00:32 EST 1997


In a message dated 11/11/97 1:27:03 PM, EFMonaco at compuserve.com wrote:

>First, is the use of a vapor barrier as Dan H. explained, on the humid side
>of things to keep the moisture out of the wall.  Think of your hot steamy
>kitchen and bathroom.  There you'd want it on the inside of the wall.  But
>what about days when its dry inside (maybe a fire going) and it's humid
>outside.  Then it seems as if you'd want the VB on the ouside.  Most
>conventional thinking and construction practices put it on the outside;
>they even shrinkwrap the house with a product called Tyvec.  In my
>experience as a disaster recovery consultant, I've seen rotted walls
>(conventional studs and drywall) at guess where?  The bathrooms and
>kitchens.  If it were me I'd put a double VB at the bathroom and kitchen
>(inside and out) to keep the moisture out of the wall.  But we all know the
>moisture migrates doesn't it?<<

OK, first, that is NOT what I said.  I recommended, if you really think you
can do a good vapor barrier, do it on the warm side of the wall.  I said
nothing about humid.  Warm air holds more moisture than cooler air, even if
its relative humidity (RH) is lower.  As the air moves through walls, and it
always does, even through cement walls, it often goes from warm to cool,
particularly in the higher portions of a room.  As it does so, it cools
(remember in this example it is cooler outside) and therefore moisture begins
to condense.  When the temperature difference between the interior and
exterior of the wall or wall cavity (depending on construction techniques) is
very great, the condensation is very, very significant.

No one seems to be able to construct a perfect vapor barrier, even in
laboratories.  (Accepting that someone will probably point to a study I've
missed to the contrary.)  Where vapor barriers leak, they tend to funnel
moisture and the effect of condensation is worse than usual, I've read.  I
don't see why this is so, but even ordinary condensation will be bad enough.

Again, it is the termal difference, not the moisture difference, that
predicates the best place to put a vapor barrier, if you are going to use one
anyway.  In Wisconsin or Michigan, you would alwas put it on the interior of
the wall, no more than 1/3 of the way into the wall.  Here in Florida, the
vapor barrior goes to the outside of the wall, as it would in an equatorial
desert.  Because the warm side varies with attached solar greenhouses day to
night, I recommended that the vapor barrier be left out between the
greenhouse and the rest of the living space.  This is normal--do not use
vapor barriers on interior walls.

There is a kind of construction where you have a 2x6 stud wall on the
exterior and then put up a 2x4 interior wall right inside it, giving you a
total of 8 inches (minus the usual amounts that the lumber companies cheat
you unless you buy dimensional lumber) of insulation.  In these walls, in
cold-winter climates, the vapor barrier is put on the interior of the six
inch wall, so that it is not breached when running conduit, etc, for which
the interior 4-inch wall serves.  

Tyvek and related materials used to wrap the OUTSIDES of houses are NOT
(repeat NOT) vapor barriers.  These materials are wind retardants.  While
sufficiently porous to pass water vapor, they are also more or less
impervious to air currents, so the wind does not penetrate the wall.  This
material is far more useful than alleged vapor barriers and highly
recommended.

If you are going to use plastic sheeting as a vapor barrier in straw-bale,
obviously you do not slap it up against the bales themselves because they are
prickley and will puncture the bales.  Thus you need a stud wall inside the
bales to hold the plastic away from the bales, and you probably need to cover
this with a smooth material such as gypsum board.  Add all this to the cost
of your wall.  
Stucco finish with a vapor varrier paint is probably more reasonable.  Paint
that is an effective vapor barrier is of course way more expensive than
ordinary paint.

It is nice that two buildings in the UK have made it 100 years with
strawbale.  I wonder what percent of the total buildings of strawbale in the
last 100 years that is.  There are 1,000 year old cordwood buildings, too.
 But I'd say 20 percent of that is a more reasonable expectation.

Again I am concerned that we are promoting strawbale with a zealousness that
might better be reserved for judgement day or something.  No one technique or
material serves all situations.  Permaculturists need to evaluate materials
based on experience, bioregional appropriateness, available construction
skills, other cultural preferences, and detached common sense.  I am very
enthusiastic about the potential of strawbale, but I see that potential
threatened by overenthusiasm that may apply this technique where others are
more appropriate.

Issues such as this are why it takes many long years of experience to become
a good permaculture designer.

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email.  Next live training:  Paraguay, Sept. 1998. Internships
available. Copyright, 1997, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL
32192 USA  YankeePerm at aol.com  

We don't have time to rush.



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