[Strawbale] 5 perms / no ventilation; and top-of-wall (Mark Piepkorn)

[John Swearingen]

Just to complicate things, you also don't want vapor from your walls to go
up into an unventilated area of the roof/ceiling; moisture could accumulate
there and cause problems. If there were such places (and it's not a good
idea to have them) then Tyvek would allow some accumulation of moisture and
that would therefore be a Bad Thing.

We've seen how the sun heating a wall can be a very powerful mover of
vapor.  Janet Johnston and George monitored our vault extensively when it
was first drying out from plastering (a process that took a long time), and
they could watch the moisture move around the vault arch with the movement
of the sun and seasons.

We've also seen this under long windows, and there was a notable failure at
Shenoa where a garden wall was continuous with a wall with a roof.  The
garden wall leaked, and the sun drove moisture from the leak into the roofed
area.  Moisture hit the top of the wall and fell like rain,causing extensive
rot.

The roofed shed was unconditioned, so there was no mechanism to drive the
vapor out. That gets us back to the importance of a temperature
differential, which was noted early in this thread.

The moral of the stories (in case you need any morals) would be to allow a
mechanism for vapor to leave the building, and to avoid places where it
would collect.

John


On 3/12/07, David Neeley <dbneeley at gmail.com> wrote:
>
> A question, then, please:
>
> Would the top of the bale wall be a reasonable place to use a product
> such as Tyvek, which is advertised as being vapor permeable but water
> impermeable?
>
> With a ladder framework there to avoid the problems of a solid piece,
> a layer of "house wrap" beneath it but over the bales might prove
> useful, I'm thinking.
>
> It seems that the only way for moisture to go up would be as vapor,
> while liquid roof leaks may (if the stuff works as advertised) run off
> it and down either into the plaster or such like.
>
> What do y'all think? Has anyone tried it and then tested the result?
>
> David
>
> On 3/12/07, Sigi Koko <sigikoko at earthlink.net> wrote:
> > My 2 cents on top of wall detail...
> >
> > The top of the wall is also a rather susceptible place for moisture
> > to get into the strawbale wall system undetected -- for example, if
> there is
> > a roof leak that is not known, and the leak drains into the strawbale
> wall,
> > it would be undetected until the moisture content in the wall is so high
> as
> > to require drastic measures (replacement!).
>
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>



-- 
John Swearingen
Skillful Means, Inc.
Design and Construction
www.skillful-means.com