[GSBN] windscreen

[Jim Carfrae]

As part of my research into the moisture performance of SB in a Temperate Maritime (Damp) Climate , I've monitored two structures where a direct comparison can be made between a plain lime rendered section of wall adjacent to a section of the same wall protected by a ventilated timber rainscreen. 
(Plain lime wall = 3 x 12mm coats. Rainscreen = 1 x 12mmm scratch coat, 25mm vented void, permeable membrane, 25 mm vented void, vertical timber boarding with 5mm gaps) 
In each case the average moisture content of the straw just behind the render was between 3% and 4% lower (typically 17% behind plain render, 13% behind rainscreen)
This is an apreciable difference, especially as this was not a particularly exposed site. If there was significant driving rain I would expect the difference to be higher.

To reinforce what John Straube said - It has to be vented top and bottom.

Jim (moist) Carfrae

Room 119, Reynolds Building
University of Plymouth
Drake Circus
Plymouth
PL4 8AA

jim.carfrae at plymouth.ac.uk
07880 551922
01803 862369
________________________________________
From: GSBN-bounces at greenbuilder.com [GSBN-bounces at greenbuilder.com] On Behalf Of forum at lamaisonenpaille.com [forum at lamaisonenpaille.com]
Sent: 21 January 2011 23:05
To: (private, with public archives) Global Straw Building Network
Subject: Re: [GSBN] windscreen

Interesting topic. Last year I saw a presentation from a french sb
builder who uses 'compost fabric' (that one would pull over a compost
pile) to protect his stacks of bales on the build and, if I rember
correctly, he also uses it over his fresh plaster to have it protected
while it dries.
Another material I've heard of is used by farmers. On certain
structures, rather than building walls, they use screens that are open
to air movement with normal windspeeds but when the wind blows harder
they close up. I have no idea how esteaticly pleasing they can be.

Though I'm eagerly reading through the silanes and other 'miracle'
additives I agree with RT's worry for cracks (and the jonction between
plaster and protuding elements). I also wonder how long those products
last, how one (homeowner) would judge when it is time for a new coating
and how multiple coatings of who-knows-what aplied over the coming
decades would blend.
That's why I've come to consider plastered SB somewhat like mini-skirts:
Looks great, but offers little protection. I know for a fact that those
sturdy dutch girls still wear 'em in winter, but some of them suffer the
consequences.

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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