[Strawbale] 5 perms / no ventilation;and top-of-wall

[Rob Tom]

--- In SB-r-us at yahoogroups.com, "N Leone" <nlpub at ...> wrote:
>
> 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.<<

> how do you do that in the case where the tops of the bales are pressing  
> up againstfloor joists or truss members?"   I don't think you can get  
> plaster *under* the joistsor trusses.  But if you just plaster between  
> the wooden members than you havea nice set of air leakage paths 16 or 24  
> inches on center all along your wall.

> And what should you do in the case where the bales run parallel to the  
> joistsand 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

<snip>

As mentioned earlier in this thread, I haven't had a look at Bruce, King  
of Sausalito's latest book and I stopped subscribing to The Last Straw  
quite some time ago so I haven't seen Tom's detail and I suppose I really  
should have a look at both before commenting but...

I think that there may be some confusion here.

If we are speaking of Cold Climate detailing of the air barrier at the top  
of the wall (no matter what the walls are made of) then we are talking  
about ensuring continuity of the air barrier at the joint between the wall  
plane and the ceiling plane.

In the situation that Ned (who thinks his name is "Nick") mentions where  
the joint is inaccessible or difficult to access, then one approach is to  
alter the joint (preferably at the design stage) so that it allows one to  
make connections (to ensure continuity) at a point that is accessible.

This usually means sketching out the cross section well in advance of  
construction so that one has a precise idea of what one is dealing with in  
terms of building components, potential air-leakage paths and ideally,  
where the dew points are going to be in relation to the air barrier.

As is usually the case, proactively identifying/fixing problems at the  
design stage is always better/easier than reactive fixes after the thing  
is already built. I realise this may be difficult for those who may be  
building for the first time and may not be able to "build the thing in  
one's head" first but unfortunately, that is pretty much what needs to be  
done.

Ideally, you don't want to have details like the one Ned mentions where  
you are trying to create an air seal around framing members that penetrate  
the plaster.

Framing members (be they steel or wood) and plaster have different rates  
of thermally-induced/change-in-moisture-content-induced  
expansion/contraction,  so the joint between the dissimilar materials  
needs to be able to accommodate movement while still remaining air-tight.

As mentioned earlier, this may be accomplished with flanges and gaskets  
but if one has to fit the flanges/gaskets around every framing member  
(especially if they're not flat/square) it involves a lot of needless  
futzing about.

It's simpler just to move the air barrier to some other location, but  
ideally, keeping it to within 1/3 of the total R-value of the cross  
section (warm side of) so as to avoid the potential for condensation at  
the air barrier just in case one didn't do a very good job of juggling  
vapour permeances. (ie How well do *you* know the vapour permeances of all  
of the materials that you've used in some assembly ? (Yes, they are  
cumulative)).

As a "for instance"-- where joists meet an exterior wall perpendicularly.

Instead of having the rim joist in the knee-jerk typical location at/near  
the exterior plane of the wall, think about moving it closer to the  
interior plane of the wall (50 mm of end bearing on the wall for wood  
joists is more than enough fro mthe joist's  point of view) so that  
ensuring the continuity of the air barrier would simply mean running a  
strip of Tyvek (or similar)around the outside of the rim joist and leaving  
flaps to join the upper and lower storey wall or ceiling air barriers.  
There are other details WRT the above that need to be dealt with but I  
won't go into them here just to avoid any further confusion.

I'm not sure what the context of WatJohn's statement was (quoted at the  
opening) so I may be off-base in commenting on that but "straw reinforced  
earth" or "weak mixture of lime" doesn't sound right for that joint.

If one has a look at the wall/ceiling joint even in conventional buildings  
where the wall & ceiling materials are typically made of the same  
materials (ie drywall) and the joint is reinforced with fibreglass mesh or  
paper tape, quite often cracks still develop at that joint.


As has been shown on this List and in a piece for The Last Straw (long  
ago) on the importance of air-sealing, the volumes of water that are moved  
past a crack even as small as 1/16" of an inch in width are significant.

I suppose that it's worth repeating the example for those who are  
relatively new to these lists:
===========================================================
Assume:	- interior air @ 21 degC and 40% relative humidity
	-10 m^2 ceiling of 9 mm board
	-2 coats of enamel paint
	-crack at edge of ceiling 1.5 mm wide and 1.2 m long
	-typical Moosehugger winter conditions WRT outdoor temp and humidity

Water vapour moved by diffusion in 100 days = 3 kg
Water vapour moved by air leakage in 100 days = 20 kg
========================================================
(Source:  Builder's Guide to Energy Efficiency in New Housing
	   Second Edition  Jan. 1985
	   Canadian Home Builders Assoc. )

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply