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

asbn asbn at baubiologie...
Thu Nov 25 22:25:50 CET 2010

Dear Jure & Max 

First I have to say, that in Austria we use OSB as a structural board and
airproof layer just on the inside, never on the outside of a building.
OSB is not a "breathable" material (because of the amount of glue in it, up
to 7%), the better term would be: open to diffusion.
So the goal is to keep the (warm) air inside a house and just let the
waterdamp go out through a wall, the outer layers have to be more open than
the inner layer, because the water(damp) comes normally from the inside. Its
like in double windows with a space between (Kastenfenster): You make the
dampproof insulation on the inner window not on the outer one to prevent
water coming down the outer glass (and destroy the window).
For Austrian passive houses OSB on the inside is nearly a standard, because
of the structural capacity, the low price and the double-function as an
airtproof layer on the inside of the walls.
You can use wood instead, if you make a diagonal bracing and add an airproof
and diffusion-reducing layer (which is normally resin-paper or plastic), but
its much more work and therefore a higher price than OSB.

Second, Kronotec is not the best product (as tested by Ökotest in Germany),
its cheap, but not formaldehyde free. If we want to build an ecological
house, we use Egger Eurostrand OSB 4 Top, which is a product made in Austria
from Austrian wood (we have enough of it).

For the outside layer of a wall you need a windproof, yet "breathable"
layer, which means, that diffusion of waterdamp can go through, but rain and
wind can`t come in. For these boards we use Agepan or DP50-boards (Egger),
it is important to find a material which is open to diffusion yet windproof!
Never, please never use OSB on the outside, every building physician can
easily calculate you, that this is a high risk in our climate.

Mit lieben Grüßen
Herbert Gruber
asbn - austrian strawbale network
Österreichisches Netzwerk für Strohballenbau
3720 Ravelsbach, Baierdorf 6
Email: asbn at baubiologie...

> 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,
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