Cob RE: Vapour barrier needed?

[Will Firstbrook WCB of BC]

Hi Tim and others on the list,

In the Cobber's Companion Ianto Evans has a list of the most common
mistakes people make with Cob construction. One of the problems is not
allowing enough time for the cob to dry out before frost. Thus I must
conclude that it is a judgement call, the cob should be dry enough prior
to the frost so the water has enough space that it can crystallize into.
Obviously the drier the better.

Regards, 
Will
>-----Original Message-----
>From:	Will Firstbrook  WCB of BC [SMTP:wfirstbr at msmail.wcb.bc.ca]
>Sent:	Monday, August 18, 1997 3:14 PM
>To:	'tcastle at sk.sympatico.ca'
>Cc:	'coblist at deatech.com'
>Subject:	Cob RE: Vapour barrier needed?
>
>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?)
>>>