[Strawbale] (no subject)

Bruce Glenn bruce at strawbalehomes...
Mon Dec 6 20:46:18 CET 2010


Hello fellow natural builders,


This has been a interesting thread concerning two different issues on
plastering straw bale buildings, so indulge me.


The first is applying plaster directly on the bales with proper separation
with different building material wood post etc. is the key to allow that
expansion and contraction of different materials through the heating and
cooling cycles.


Here in the USA where I have designed over 20 straw bale homes in Colorado
and Washington State, and built 14 of them since 1993 I have had zero stucco
failure, abet the first home is only approximately seventeen years old in
Colorado. So far I have had no call backs concerning rotten bales or failed
flashing details etc. I have also inspected dozens of sb homes  through the
years, finding moisture issues failure based on poor construction habits
from inexperienced home owners or builders. I believe that the success in
any type of building is in good design & detailing whatever the building
envelope is used. 


There's are many moisture studies for different plaster types for sb walls.
One of which I was in the issue # 46 of the last straw journal by Candace
Gossen a during a 7 year study in raining Portland Oregon where moisture was
never higher than 12 percent located in the exterior 4" of the lower bales
during summer where the delta T was the highest. Clay sand base coat, lime
sand finish plaster on the exterior and interior were used. 


The Canada cmhc http://www.cmhc-schl.gc.ca/en/index.cfm  has documented many
plaster failures in SB homes.


The imporatance of applying plaster directly to the bales is demonstrated in
the fire testing by the EBN group at this link.


A 2 hr fire rating for cement /lime plaster and a 1 hour fire rating for
clay sand plaster.




I feel using any type of sheathing for lateral loads losses that fire rated
protection and one of the major benefits of building with straw bales. Here
in the states conventional wood framing with plywood shear has a 20 mins
fire rating, which I believe is a hazard & liability when building in fire
prone areas or when it takes more than 5mins for the fire department to
reach your home.


Building in Washington State on the waterfront we have horizontal rain
conditions with wind driven rain up to 85 mph (exposesure D =125 mph lateral
loads) begs for protection for all types of building envelopes. I don't care
what type of siding is used it will get through it with that amount of
hydrostatic pressure.


An old solution has been to install a rain screen between the siding and
felt paper, to mechanically allow the water seepage to drain down the air
space safely out the bottom at the water table. This works in all type of
construction and is especial good detail for a severe weather location
allows the siding to dry out.


An SB wall would have a scratch and brown coat of plaster flush with
vertical exterior pins, 

( interior and exterior pins tied together stabilizing the straw bale walls
transversely, vertical 1x2x3/4" 16" o.c. nailed to the top girder and bottom


I then install an emulsified impregnated paper (15 lb felt, installed
horizontally with 6" laps attached to the vertical pinning.


Add another 1x2x3/4" vertical pinning ( rain screen) over the SB exterior
pinning and felt paper, creating a ventilation space.

Siding can now be nailed to the vertical pinning. If plaster is to be used I
would use a drainage plane mat (instead of the vert. furring ) with 2"x2"
welded wire furred approximately to key the plaster.



In areas where there will be a high amount of rain or snow that may contact
the base wall, especially clay & sand, I always suggest an exterior 3-4'
tall wainscoting of stone or brick for protection. It protected all of the
Clay sand plaster on my Colorado Cottage, except a small 4"x12" area on a
conventional wall with no eves.


I'm curious about using SB for the roof insulation without an 1" air space
to vent the heated moisture can anyone comment on this. I have seen this in
europe, have the roofs been inspected for rot ?



Thank you have fun building your straw bale home. Yahoo!



Bruce Glenn


Passive Solar Designer Builder


27+ yrs Healthy, Passive Solar, Energy Efficient Home Environments


Straw Bale Pioneer since 1993, Twenty homes and counting!


Creative Solutions for our Living World


Terra Sol Eco Homes LLC The Green Builder



Ask about our winter & summer straw bale, passive solar, workshops!


bruce at strawbalehomes...

web at strawbalehomes...

wk 360-385-5477

cell 360-301-3150
























Message: 1

Date: Mon, 06 Dec 2010 11:52:46 +0100

From: asbn <asbn at baubiologie...>

Subject: Re: [Strawbale] OSB or Not? (rendering versus sheeting) (dirk


To: European strawbale building discussions

            <strawbale at amper....muni.cz>

Message-ID: <C9227F0E.1016A%asbn at baubiologie...
<mailto:C9227F0E.1016A%25asbn at baubiologie...> >

Content-Type: text/plain;       charset="ISO-8859-2"


just an addition...

> The question comes back to the basics; too much protection against 

> moisture movement sounds dangerous.


No, I don't agree to this. This is building physics standard and can be
calculated easily.

You can eventually plaster directly on the inside but the problem is, that a
direct plaster on the outside without a folie or other prevention against
water & humidity (rainfall) on the facade often leads to problems within the

The reason is, that (in our areas) limeplaster is used, and this plaster
sucks the rain, gets wet and so the insulation layer, if you have no really
big overhangs in the roof.

Earth-plaster on the outside in our rainy/snowy areas leads to a wash-out of
the earth-plaster, as it can be seen on the loadbearing house in St. Georgen
/ Carinthia.

First there are small hairy cracks which freeze in the winter and grow to
big cracks, where the water runs in...

So if you limewash your plaster every year (as you often see it in Greece)
and have big roof-overhangs, you maybe will get no problems. But if you want
to be sure and safe, that nothing happens especially in our areas (and
especially on the weather side), you have to install a rainproof layer
(folie or Agepan board or similar) on the facade under your plaster (or if
the facade is shadowed by trees better use a ventilated wooden facade). You
need this folie and board also for your windproofness...


Mit lieben Gr??en

Herbert Gruber


asbn - austrian strawbale network

?sterreichisches Netzwerk f?r Strohballenbau 3720 Ravelsbach, Baierdorf 6

Email: asbn at baubiologie...




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