[GSBN] windscreen

[jfstraube]

A couple comments in the design. People like Bruce King have said to me that we should call SB "plastered strawbale" because people often forget how important the plaster is.
It is not just there to stop rain: it is critical to stop air movement and insects and makes a big difference in structure and fire.
Hence, I have some serious reservations whenever someone dispenses with the outside or inside plaster layer.
Andre, your wall assembly would be vastly improved in my eyes if there was a clay plaster directly on the exterior of the SB.  Pressing the bales to the back of the DWD board (which for north americans is like the fibreboard like we used to use for sheathing in the 60s/70s) will leave many small gaps and air flow pathways.  The wonder of plaster is that it is liquid applied on site and hence can fill all the gaps in irregular bales.  Plastering over the board is much better than not at all, but it is not nearly the same as plastering right to the bales.  The boards dont offer much insulation value either.

The vapor permeance ratio of 5:1 is a really rough rule of thumb that does not apply very well to SB.  Your interior plaster like has an Sd value of around 0.2, maybe a bit less.  This arrangement does not worry me in the least because the board will easily allow vapor to penetrate and move to the outside.  I am pretty sure that such a design will work very well in your cool maritime climate.



On 2011-01-21, at 6:05 PM, forum at lamaisonenpaille.com wrote:

> 
> Anyway, for some years I've pondered about the 'best' way to make a cladded SB wall and came to the folowing concept (that I intend to build this year): wood cladding (douglas fir), battens/vented airspace, AGEPAN DWD* boards, strawbales (placed vertically and pushed tightly against the boards), earth plaster. The posts (between the bales) would be placed at 40cm on center.
> What I wonder is: as these AGEPAN DWD boards have a sd value of 0,2 m, would I get enough vapour barrier on the inside with a 5 cm straw-clay plaster? The golden rule says that the outside should be 5 times more open than the inside, but considering there is a vented rainscreen my gut feeling (or rather my desire) suggests that it should work. Anyone want to shoot this down (or confirm)?
> *for those unfamiliar with these boards, they are structural and act as a drainage plane.
> http://debelux.ahk.de/fileadmin/ahk_debelux/AHK_Projekte/Praesentationen_Energieeffizienz/Glunz_AG_01.pdf
> 
> André - it's hip to be square - de Bouter
> France (ex-dutch)
> 
> 
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John Straube
www.BuildingScience.com