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

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




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