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

[N Leone]

Says the Good Doctor (by way of Mark):

>>The top seal of the wall needs to be an air barrier,
and not a vapor barrier. This air barrier can best be formed by straw
reinforced earth, or a weak mixture of thick lime plaster (applied in
several layers to avoid cracking). ... Building paper over a thin plaster 
skin would be darn fine.<<

OK.  I understand it conceptually.  But today I was staring at my house 
design (as I have been for 5 or so years...  Sigh...) and thinking "So how 
do you do that in the case where the tops of the bales are pressing up 
against floor joists or truss members?"  Since the bales ought to be in a 
state of compression between the top of the foundation and the bottom of the 
roof/second story (even in a infill design...), I don't think you can get 
plaster *under* the joists or trusses.  But if you just plaster between the 
wooden members than you have a nice set of air leakage paths 16 or 24 inches 
on center all along your wall.  Should you tack Tyvek or something on the 
under side of each joist and then plaster over that?

And what should you do in the case where the bales run parallel to the 
joists and you can't really reach the top of the bale at all once its in 
place?

Page 35 of Issue 34 of The Last Straw shows a top of wall detail that Tom 
Lander used (uses?) that involved "a plastic vapour barrier over strips of 
drywall" for the bales to press against.  (It's hard to tell from the 
picture, but I'm assuming that the 'over' means 'farther away from the 
ground' and not 'applied after'.  I'm also assuming that this vapor/vapour 
barrier would be sealed into the plaster on the sides of the bales...)  This 
would certainly work for the situations I describe above (not to mention 
allowing me to use up all those drywall scraps I've been salvaging...), but 
would it do what Dr. John suggests needs done?

Thanks,
Nick



----- Original Message ----- 
From: "Sigi Koko" <sigikoko at earthlink.net>
To: <strawbale at listserv.repp.org>
Sent: Monday, March 12, 2007 12:18 PM
Subject: [Strawbale] 5 perms / no ventilation;and top-of-wall (Mark 
Piepkorn)


> My 2 cents on top of wall detail...
>
> Having moisture tested numerous strawbale walls, I have always found that
> the moisture is higher at the bottom of the wall, not the top of the wall.
> (I can only assume this is due to the play of gravity.)  I'm not saying 
> that
> condensation at the top of a wall is not a valid concern, just that over
> time any moisture there seems to work it's way down, not stay up in the
> wall.  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!).  On the other hand, if a 
> barrier
> over the top of the bale wall detours any leaks to the sides of the wall, 
> it
> will create a stain in the plaster, indicating to the building
> owner/occupant that there is a moisture issue to be dealt with.  They know
> to fix the roof, and at worst need to refinish plaster.  The bales go
> uneffected.  I use 2 layers of roofing felt over the tops of bale walls 
> for
> this purpose.  (The second layer is to provide a safety net.)
>
> Thanks, Sigi
>
>
>
>
> Sigi Koko
> be the change you wish to see...
>
> DOWN to EARTH DESIGN, inc.
> natural building design
> environmental consulting
> www.buildnaturally.com
> 215-540-2694 (PA)
> 202-302-3055 (DC)
>
>
>
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