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?
Cheers,
Max
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,
Jure Poz(ar, SLOVENIA STRAWBALE BUILDING
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