Vapour barrier needed?

[Shannon C. Dealy]

On Thu, 10 Jul 1997, Tim Castle wrote:

> Are there any cob houses where the walls (without an interior vapour
> barrier) are subjected to the effects of frost every year?

Lots of them.  I am not aware of any cob buildings having a vapor barrier,
it would serve no useful purpose and might even cause damage by trapping
moisture in the cob.

>   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?
[snip]
>   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?)

I don't recall if this was discussed further or not (I definitely need to
add a search engine to the archives).

The main reason for giving the wall some time to dry before a freeze
occurs actually applies to virtually any building material.  When water
freezes it expands, so if a building material containing water is frozen,
the water will expand and cause damage.  Often the damage is microscopic
because there are little bits of water distributed throughout the
material, but through repeated freeze-thaw cycles this will slowly break
down the material (wood often suffers from this type of damage if it is
left unpainted).  Much of the damage often suffered by concrete and
asphault roadways in Northern climates is also caused by this (water gets
into the little cracks in the road surface and then freezes).

If you live in a climate subject to hard freezes, your cob wall WILL be
subject to freezing before it is completely dry (it will take a year or
more for full drying), but keep in mind that only the exterior couple of
inches of the wall are likely to freeze (assuming you are building an
enclosed space and depending on how hard it freezes in your area), so
this is really all that needs to have enough time to dry.  Even if there
isn't enough drying time, I think it is extremely unlikely that any
structural damage will occur, though the outer couple inches of a wall
surface that froze might be subject to a greater degree of cracking and
flaking then normal.

Shannon Dealy
dealy at deatech.com