[Strawbale] 1. Re: Europe's largest strawbale building (Sport Hotel, Jure Pozar)

Max Vittrup Jensen Max at PermaLot...
Thu Nov 25 19:08:42 CET 2010

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 years old).

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 tiles.

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 clients!!!)
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 field...  ;o)

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 construction.

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 Pozar)
> Dear fellow straw balers,
> I appreciate help of all who replied to my previous posts so far, which
> 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 bales
> 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. If
> you consider that your replies are too valuable to share freely, please
> give me number and I am willing to pay a certain amount if it will help
> me with my planning (reasonable answers with theoretical explanation why
> 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 the
> post & beam structure. Timber guys told me the wooden boards breathe too
> 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 to
> suck up all the moisture from the straw bale? And how much moisture can
> 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 wall
> from the inside to the outside? Will it condense?
> 2. Another question is what percentage of straw bale can be pressed for
> 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
> future?
> 3. as far as I know it is necessary to compress the straw bale wall, to
> avoid settling/compressing in the future which would also make the lime
> 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 openings
> sufficient?
> 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 the
> thermal bridging. The thing that concerns me is the classical vapor foil
> 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 - so
> it slips down the foil. Would it be maybe wise to put another foil below
> 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 out
> of them?
> I appreciate your knowledge, as there are only a few people in my
> country who know something about straw bale building and they have very
> little experience as there are only a few straw bale houses in Slovenia
> 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 straw
> bale association of Slovenia to share the knowledge of straw bale building.
> With best wishes to you all,
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