[Strawbale] 1. Re: Europe's largest strawbale building (Sport Hotel, Jure Pozar)
derek at unm...
Thu Nov 25 22:13:04 CET 2010
I share Max's views and endorse his comments, especially on fire
safety. I am convinced that for fire safety, structural, and
moisture reasons, both the inside and the outside surface of the
bales in the walls should be plastered. Plaster applied directly on
the bales. It isn't sufficient to put plaster on OSB or wood, that
is in contact with the bales. The straw and plaster need to be an
integrated unit, with continuous plaster on both faces of the walls.
In general, I am not a great fan of OSB. I have seen it mold in the
dry climate of New Mexico, USA, on vertical surfaces of shed walls,
exposed to the exterior, hence no moisture build-up was possible.
Even so, mold covered at least thirty percent of the OSB surface
after a few years. Perhaps these sheds were built with bad OSB, but
how do we know the properties of the OSB that we buy?
There is no need for either OSB nor wood covering the interior wall
surfaces. As mentioned above, plaster must be worked directly into
the bales. If you choose to add wood strips (separated by spaces),
on top of the interior layer of plaster, that may be acceptable. It
adds to cost and complexity. Properly applied plaster wouldn't
crack, but I would worry about moisture build-up in the wall, unless
the wooden strips were narrow, and the spaces between them wide
enough to offer significant open area. Continuous sheets of OSB
would certainly interfere with moisture control in the wall.
On item 2 of Jure's list, a 10-meter high wall is very tall for
strawbale. I have never seen any testing of a wall of that height,
and I know of few examples over 3 meters in height, unless jumbo
bales were used, or there is an intermediate floor. I might expect
70-100mm compression on a 2.5-3 meter tall wall.
On items 2 and 3, I believe there is value in pre-compressing a wall.
But after a wall is plastered, the plaster skins take the roof loads,
on a load-bearing structure. For a post and beam structure, there
are no significant loads on the load-bearing elements. In either
case, the plaster skins hold the bales in position. There is
unlikely to be further compression or settling of the bales after the
plaster has cured.
On item 4, most people that I know advise against placing bales in
the floor when those bales are below grade (below ground level).
Moisture problems are very likely for bales below grade. However, I
have visited several houses built to meet or exceed Passiv Haus
standards, which use bales in the floor above grade. Most of these
houses were built on posts, so that there is free air circulation
between the earth and the floor.
On item 5, Werner Schmidt and others have used bales in the roof. It
offers challenges with moisture, fire resistance, avoiding insulation
gaps and thermal bridges, and weight. It's easier to insulate a roof
effectively with cellulose.
I'd recommend reading Bruce King's The Design of Straw Bale
Buildings, as well as The Last Straw Journal. We've learned a number
of important things in the last twenty years of the straw bale
building revival. I think it is worth following the consensus of
strawbale builders, at least for the first few buildings that one
builds. Trying to do a number of things in new and original ways, on
one's first strawbale building, is asking for trouble. Below are a
few links that might be useful.
Perhaps others can provide additional European resources and links.
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm...
--On Thursday, November 25, 2010 7:08 PM +0100 Max Vittrup Jensen
<Max at PermaLot...> wrote:
Dear Jure and others,
For several years I've been opposed to using OSB on the inside of SB
walls; what I (neutrally) refer to as 'the Austrian approach', (as
that's where I've seen it applied the most).
My reasons being that I'm aware of the fact that OSB is
more susceptible to fungi growth as compared to particle board and
plywood, that there's been cases of completely rotten houses where
everything looked fine inside, but the interior was filled with fungi
spores (fault was due to poor craftsmanship/water detailing not the
OSB itself), and none-the-least the famous case of post-hurricane
Katrina trailer homes where some people died in their toxic OSB
boxes, but apparently this was due to the poor production quality of
the particular Asian OSB boards used.
Which leads to another issue: Global conscience; 2 years ago I read a
research that 60% of OSB on the European market came from Asian rain
forests. (Relative good news is that the new Kronospan OSB from Czech
appears to be leading in eco parameters; the factory is only 1,5
But most of all it's due to my non-scientific gut feeling that I just
don't believe in the research saying that such an apparently solid
plate can breathe enough!!! My aim in wall/roof design is to let the
moisture out as well as possible, and to condensate beyond the SB's,
whether it's a wall or a ceiling.
In our new house this means an earth plaster on reed mounted on
wooden boards which are spaced with about 10 cm in between on the 45
degree roof construction (simply to stop the SB from dropping
inside!), we've likewise covered the top of the bales with an earth
slip (fire and moisture protection), have 5-10 cm ventilation gap
before there's a breathable roof foil (Brand: Jutafoil; Please be
aware not to use 'Tyvek' brand, due to the company's (Dupont)
environmental record), then another gap to insure ventilation of the
In the case of a roof we currently are applying for (and where you
may get to sleep for the ESBG, if funded), we need a more effective
(fast) solution than SB, and have opted for 30 cm. blown in cellulose
insulation under a green roof. We face the same interior issue
regarding OSB, and our Architect was very surprised when I blankly
refused the interior OSB. Rather than using OSB we'll likely end up
using one layer of common (gypsum) drywall mounted on wooden boards
every 40 cm, and interior clad with reeds and spray plastered. This
solution also makes for a lot better fire-safety value; the OSB
option only had 15 min!!! (I've woken up in a burning building once,
and would never design a 15 min. fire safety for any of my
To satisfy our OSB literature/marketing indoctrinated architect (!),
we're adding a breathable paper membrane above the drywall, under the
insulation, to allow for better diffusion of vapor, though personally
I'm not convinced of the need...but hey, I've never studied that
I hope this adds to your question 1 and 5, and hopefully will spark
some discussion (maybe a good topic for a consensus finding workshop
at the ESBG? I trust some of you others to have better answers than I
on the others. Only like to add that I'd advice you to look at
www.atelierwernerschmidt.ch for answers in regards to SB in the
floor, and perhaps also for solutions to reduce the timber frame
And best of luck with your very ambitious aims! Looking forward to
your presentation at the ESBG?
1. Re: Europe's largest strawbale building (Sport Hotel, Jure
Dear fellow straw balers,
I appreciate help of all who replied to my previous posts so far,
have helped me a lot with planning my first straw bale house. I have
been able to put together a house plan for post & beam with straw
outside and now when it is finished I would like to ask you for an
opinion. I am attaching wall intersection and want to hear your say.
you consider that your replies are too valuable to share freely,
give me number and I am willing to pay a certain amount if it will
me with my planning (reasonable answers with theoretical explanation
should it be done so and so)
The questions that are still crossing my mind are:
1. are OSB plates really better than wooden boards on the inside of
post & beam structure. Timber guys told me the wooden boards breathe
much which make the clay plaster break. I know wooden boards would be
cheaper and more natural but I don`t really want to have cracks on my
walls. They tell me OSB plates breathe enough. Is it true? The same
question with the OSB plates on the outside of the structure: do they
breathe enough or will they make moisture on the straw bales when warm
air will go from outside to the inside? Or is the lime plaster enough
suck up all the moisture from the straw bale? And how much moisture
really go through the lime plaster? If it is low, than I guess it
shouldn`t be a problem. What happens when warm air goes through the
from the inside to the outside? Will it condense?
2. Another question is what percentage of straw bale can be pressed
every meter? If the straw bale wall is 10 meters high, how much should
it be compressed that it won`t fall down/compress any further in the
3. as far as I know it is necessary to compress the straw bale wall,
avoid settling/compressing in the future which would also make the
plaster crack. How do I compress the walls where I have windows and
doors? Is it better to use board frames or are boards above the
4. I am planning to do floor and roof isolation with straw bales as
well. I can remember a discussion from Belgium that it is not best to
put straw bales in the floor, but can`t remember the explanation why
not. I think it was professor Minke who opposed the straw bale floor
isolation but there wasn`t enough evidence to prove this theory.
5. I am also a bit concerned about the straw bales on the roof. We are
planning to do the continuous straw bale layer with only screws in the
middle to fix the roof laths so there shouldn`t be no problems with
thermal bridging. The thing that concerns me is the classical vapor
which will be laid above the straw bales. It is supposed to be
breathable, but I am afraid it will make condensation in the straw
bales. The foil is necessary if the rain gets under the roof tiles -
it slips down the foil. Would it be maybe wise to put another foil
the straw bales, or would it be better to put dip the straw bales in
clay before putting them on the roof, so it would such the moisture
I appreciate your knowledge, as there are only a few people in my
country who know something about straw bale building and they have
little experience as there are only a few straw bale houses in
and they are all done with infill design which is suitable for certain
regions, but not for where I live where we have big temperature
differences. If everything goes right the house will be standing till
the next ESBG, which I plan to attend. I will start working with straw
bales professionally next year in September and will also initiate
bale association of Slovenia to share the knowledge of straw bale
With best wishes to you all,
Jure Poz(ar, SLOVENIA STRAWBALE BUILDING
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