Vapour barrier needed?

[Will Firstbrook WCB of BC]

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?)
>