Cob RE: Vapour barrier needed?

[Will Firstbrook WCB of BC]

Hi Tim,

I talked to Ianto Evans about the questions you have asked. In answer to
your questions, no cob house should have a vapor barrier. A vapor
barrier will damage a cob wall, cob walls need to breathe.  As to frost
in a not yet dry cob wall. Ianto stated that water expands 9% when
frozen, and cob has more than 9% free space that water can expand into
thus it is not a problem. 

Personally I feel a little uncomfortable with that if the wall is still
wet inside. Paying it safe I would want to ensure the wall exterior is
dry prior to the cold of winter. I would apply enough internal heat on
the inside of the structure so that the center of the wall is protected
from freezing. In your cold climate I would build the house to be
passive solar to reduce your heating costs. Let the sun help dry out the
house. What I mean about passive solar is lots of Southern windows,
smaller windows on East & West sides and none or very small super
insulated windows on North side. The sun's heat enters the windows and
heats the mass inside your house. Cob houses can do this very well if
they are designed well. Earthships are totally designed around the
principle of passive solar and high mass.

Regards, 
Will

PS: Sorry about the delay in the response. I have been taking a lot of
time off this summer.

>-----Original Message-----
>From:	Will Firstbrook  WCB of BC [SMTP:wfirstbr at msmail.wcb.bc.ca]
>Sent:	Thursday, July 10, 1997 2:52 PM
>To:	'coblist at deatech.com'
>Subject:	RE: Vapour barrier needed?
>
>Hi Tim,
>
>Good questions! I am also very interested in Cob although we are planing
>on building in a not so cold area. I have never seen any cob houses that
>have a vapor barrier in the walls nor the roof.  I do know that some
>Mexican adobe structures that were covered in a plaster in order to
>restore them actually cased significant damage. As it created a vapor
>barrier that caused deterioration inside the wall. My understanding is
>that cob needs to breathe. It can absorb quite a bit of moisture and
>pass it through the wall. Sometimes vapor barriers cause more problems
>then they are worth.
>
>One could turn the question around, what is the benefit of a vapor
>barrier? I thought the main benefit is to eliminate /reduce drafts thus
>make the home more energy efficient. If that is one of the main reasons
>then it is not needed. I doubt that your going to feel a draft through
>an 18" to 24" cob wall. In England where there are thousands of cob
>houses over 300 years old, I would expect,  that have no vapor barriers
>in that wet climate.
>
>Regarding frost I would expect you want the cob to be relatively dry
>before the first frost of the winter/fall. At least on the outside of
>the structure as long as the inside is kept warmer. 
>
>I am attending a Cob workshop next week and I will try to get an opinion
>and some advice on these questions.
>
>Regards,
>Will
>
>
>
>>-----Original Message-----
>>From:	Tim Castle [SMTP:tcastle at sk.sympatico.ca]
>>Sent:	Thursday, July 10, 1997 9:25 AM
>>To:	coblist at deatech.com
>>Cc:	ash at freenet.calgary.ab.ca
>>Subject:	Vapour barrier needed?
>>
>>Are there any cob houses where the walls (without an interior vapour
>>barrier) are subjected to the effects of frost every year?
>>  Checking back in the archives... last august there was some discussion
>>about "Cob in really cold climates" where the issue was R-value of
>>cob.    What about moisture and frost?
>>  Where I live on the Canadian prairies, conventional stick frame houses
>>are built with a vapour barrier just inside the sheet-rock. We have
>>about 100 frost free days, and usually below freezing from November to
>>March, with stretches for a week or so when we're happy if it warms up
>>to -30 deg. C. during the day... but in the summer, can have weeks of
>>daytime +30 C and warmer.  In spite of these temperature extremes, it
>>remains relatively dry, and that's part of the problem, especially in
>>the winter. Indoor living spaces tend to be humid relative to the
>>extremely dry frosty outdoors.  
>>  What does that moisture, and hence frost, do to an earthen wall?  I
>>notice that one is cautioned against not building an earthen wall if it
>>will not dry before freezing...
>>  (Did I miss this topic in a later archive?)
>>