[Strawbale]Re: Strawbale digest, Vol 1 #474 - 7 msgs

Sat Jan 8 19:04:05 CET 2005

Herbert -

I'd love to see the results of an in-situ test for U-value for your 
standard Austrian design.  The heraklith and timber layers will 
certainly result in a better figure than for some other sb 
constructions, though I also see scope for convective air circulation 
around the bale layer if the bales are not particularly well packed.

Hot-plate lambda-value testing is not the only option when it comes to 
internationally-recognised standard procedures.  Obviously, each 
government can to some extent pick and choose which international 
standards it approves, and I don't know the situation in Austria, so I 
can only note the British situation.  Here, hot-box U-value testing is 
recognised in our building regulations - the relevant test here is BS EN 
ISO 8990: 1996.  (From a cursory reading of the standard, I think some 
modifications would be necessary for a full-width strawbale wall, but 
that's not a major point.)  The By og Byg testing programme in Denmark 
included this same procedure.

Now, the relevance of all this to strawbale construction is that the 
U-value tests which have so far been carried out give somewhat worse 
values than those calculated in the standard manner from lambda-values.  
Calculations from lambda-value tests (yourselves in Austria; By og Byg 
in Denmark, FSD in Germany, abd el-Fattah Ashour in Germany, McCabe in 
the US, Sandia in the US) give U-values ranging from 0.08 to 0.16 for a 
standard bale wall with 20mm lime or clay plaster on each side.  Apply 
the official 20% fudge factor which you mention to the tests to which it 
is relevant, and that range changes only slightly:  0.10 to 0.16.  Now 
compare that to the wall-assembly U-value tests which have been carried 
out (values adjusted for same standardised bale and plaster dimensions):
  Ship Harbour, Nova Scotia, 1995:  heat flow monitoring:               
                U=0.20  (bales laid flat)
  ORNL, Tennessee, 1996: guarded hot box, to US standard ASTM C236:     
     U=0.32  (bales laid flat)
  California Energy Commission, 1997:  guarded hot box test, again to 
C236:     U=0.26  (bales laid flat)
  California Energy Commission, 1997:  guarded hot box test, to 
C236:             U=0.19  (bales on edge)
  ORNL, 1998: C236 again, but better-built 
wall:                                              U=0.21  (bales laid flat)
  By og Byg, Denmark, 2001:  hot box test, to ISO 
8990:                                  U=0.19  (bales laid flat)
  By og Byg, Denmark, 2001:  hot box test, to ISO 
8990:                                  U=0.22  (bales on edge)
Both the first ORNL test and the first CEC test had well-documented 
problems, so they can be disregarded for our purposes.  That leaves us 
with virtual unanimity around U=0.20.

This represents a big difference from values calculated from 
lambda-values, and we do ourselves no favours by pretending otherwise.  
Jeff Christian (ORNL) and Joergen Munch-Andersen (By og Byg) have both 
made useful comments on the subject, and I expect to make some of my own 
when I've done a bit more data analysis.  But, however you work it out, 
I am convinced there is something happening here besides straight 
conduction through the wall.

Yes, as you say, there will be disparities between theoretical and 
actual U-values for more "conventional" types of building construction.  
If your interest is in being at least as accurate as the "competition", 
you're probably right.  Similarly, if you just have to convince building 
officials, you should be able to do so with lambda-values.

But if we want to understand how the material works, or predict building 
performance (for ourselves, so we know we are doing the best we possibly 
can), or merely do calculations to size components of the heating 
system, just working from lambda-values is, I think, going to be misleading.

atb,
Mark

asbn wrote:

>Dear Mark
>
>If you want to know the difference between e.g. cellulose, sheep-wool, flax,
>fiberglass and strawbale, our tests give a relevant result (all are 0,045),
>even when the thickness of the measured probe is smaller than that of a
>strawbale. All of these insulation-materials act different in reality or
>when wet. But for all organic insulation-materials there is a 20% addition
>in value by way of calculation for that reason (not so for fibreglass). And
>all are measured under the same conditions.
>
>When you have to prove the insulation quality to officials to get better
>financial supplies when you build, these tests are relevant, because this
>paper is the only thing that matters and all common building-engineers use
>these tests for energy-analysis.
>  
>We all - networkers, builders, architects, officials - know, that reality is
>a different thing, even one (passive)house doesn´t act as the other one. But
>as long as we have to compete with cheap prefabricated-houses or expanded
>polystyrol-passive-houses, truth is on our side, when we rely on our tests.
>
>Strawbale-(passive)houses in Austria are usually build with this
>construction / wall-system:
>1,5  2 cm limeplaster on facade
>3 - 5 cm magnesiabound Heraklith-plates (woodchips)
>Wind-protection-folie
>2,4 cm diagonal wood-boards
>35 cm strawbale with 6 x 35 cm construction-timber (sometimes in a
>dual-sandwich-system, isolated with 5 cm cork)
>2,4 cm wood or OSB-boards*
>3 - 5 cm magnesiabound Heraklith-plates (woodchips)
>2 - 5 cm clayplaster (thickness depends on wall heating-system)
>
>*If OSB-boards are glued, wind-protection-folie is not necessary.
>
>Blower-door-test is obligatory if you want a financial (eco-)supply by the
>government in Austria.
>
>This wall-system has many advantages (e.g. easy to use installation-level in
>Heraklith-boards). And I´m sure, this is a passivehouse-system, U-value
>around 0,12 - 0,14 W/m2K. For sure it is a safe (and therefore accepted)
>system (post-and-beam-construction) for countries with much and cheap
>wood/timber (not so for Denmark).
>
>Best wishes from Austria
>Herbert Gruber, ASBN
> 
>
>08.01.2005 15:35 Uhr
>
>  
>
>>Dear all,
>>
>>The low lambda results in Germany, Austria and Denmark are indeed based
>>on official test procedures, and appear to be reliable.  However, in
>>these procedures, (i) the thickness of bale used is smaller than that
>>for a wall assembly, and (ii) only very dry material is used.  Hence, if
>>(a) there is any heat transfer effect which isn't proportional in its
>>extent to the thickness of material, or (b) if moisture movement makes
>>any difference, these tests will not be directly applicable for U-value
>>calculation without some sort of adjustment factor.  There is reason to
>>believe that convective effects, including moisture transfer, do make
>>some difference in a real-life wall assembly.
>>This indeed appears to be the case when we look at the whole-wall
>>U-value tests carried out by ORNL and by the Danish testing programme -
>>both to variants of official test procedures in the countries
>>concerned.  These give significantly higher U-values than can be
>>explained by the lambda values.  There is some useful discussion of this
>>in the Danish summary document:
>>Munch-Andersen, J & Møller Anderson, B (2004),   Halmhuse: Udformning og
>>materialeegenskaber,  By og Byg resultater 033, Statens
>>Byggeforskningsinstitut, Hørsholm, Denmark
>>which is available online in .pdf format at the By og Byg website.
>>Realistically, I would not assume a U-value of less than 0.2 W/m2/K for
>>a 2-string-bale wall laid flat.  The results seem to me to suggest that
>>you'd get much the same for the same bales laid on edge.  (Though I
>>personally have big doubts about on-edge bales for other reasons...)
>>
>>Mark
>>    
>>
>
>
>
>____________________________________________________
>    European strawbale building discussion list
>
>Send all messages to:
>Strawbale at amper....muni.cz
>
>Archives, subscription options, etc:
>http://amper.ped.muni.cz/mailman/listinfo/strawbale
>____________________________________________________
>
>
>
>
>  
>