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    • CommentAuthorCrawfW
    • CommentTimeApr 28th 2021
    I would welcome some views on a slightly unusual wall-design I’ve been dreaming up…..

    The essence of this would be a constructional timber-frame, with straw-bales wrapped around the frame and the whole lot finished inside and out with lime renders.

    More specifically, the frame would be C16 grade timber, and probably built on-site to avoid moving heavy prefabricated panels around a place with limited access. This part of the approach is well-established and perfectly conventional.

    The less conventional aspect is that this wouldn’t be straw-infill insulation: the bales would be larger than the frame. One reason for this is that Helston’s measuring 500 mm should yield a U-value of around 0.30 W/M2/K (based on thermal conductivity of 0.15 W/mK).

    Crucially, the whole thing should end up with a visual and physical “heft” similar to that achieved with cob – whilst being quicker to assemble and having better thermal properties. And quite cheap too.

    I was thinking of trying this out for a 4-bay garage with upstairs office area and, if this works out, using a similar approach for a house extension in the next year or so.

    There are obviously lots of design details to work through, and both planning and building control officers to convince, but thought I’d see what experts here thought.

    What am I missing? Why is this the worst idea yet posted of this forum?!
    • CommentTimeApr 28th 2021
    Posted By: CrawfWThe less conventional aspect is that this wouldn’t be straw-infill insulation: the bales would be larger than the frame. One reason for this is that Helston’s measuring 500 mm should yield a U-value of around 0.30 W/M2/K (based on thermal conductivity of 0.15 W/mK).

    A conventional timber frame wrapped with straw is a fairly normal way of building straw bale, especially in places like Germany where loadbearing straw is not a legal construction method. There have also been loadbearing straw buildings built using Hesston bales.

    I'm not sure what size bale you are referring to as a 'Helston' with some dimension of 500 mm though? There are lists of the usual bale dimensions at https://www.hay-straw.co.uk/bale-sizes I also don't understand your U and k values? The thermal conductivity of the bales used in my PHPP calculations was 0.052 W/m.K but that depends on details of the bale density, type of straw and even the type of baler used to make them. There are various European test values for the conductivity, and also for fire resistance of plastered walls etc, that were valid in UK building regulations. I've no idea what the current situation is regarding relying on European ETAs when building (anybody?)

    I'm not sure about the comparison with cob either. I don't know what it adds? The outside will look like rendered straw bale :bigsmile: Inside you will need to think carefully about how to achieve fire resistance.
    • CommentAuthorCrawfW
    • CommentTimeApr 29th 2021
    Goodness djh, thank you for coming along to straighten me out!

    I meant Heston - but that was the wrong size anyway! I was thinking of a "conventional" as shown on the site you linked.

    It looks as though - in my second error - i got a decimal place wrong in my thermal conductivity, and - third error - was also using the wrong value anyway. And then didn't keep my reference. Strong work so-far!

    Having not been able to find any to look at, i am going off internet images - but my sense was that there was a broad similarity in the external appearance of straw and cob. I guess i was struck by the similarity of the rounded reveals. At the risk of asking an impossibly subjective question, are there actually more striking differences on close inspection?

    I am finding it quite hard to find anything written about fire resistance - lots of rather partial summaries. Will keep looking, but expert pointers welcome in the meantime.

    And, asking one last question, do you know of any key words i should use to track down German description of this?!

    For now, thanks again for the comments.
    The straw-bale theatre at the Centre for Alternative Technology (CAT) is 'wrap-round'. It has had some issues, but they were about construction phasing, I think, not the basic method. If I understand correctly it was built frame, bales, roof, when of course the ideal is frame, roof, bales. Nothing wrong with the principle. As for fire resistance I guess you plaster as normal behind all 'sticks' which 'stand off' the bales, and you may have to apply intumescent paint to any posts in contact with the straw (a guess).https://www.google.com/search?q=CAT+straw+bale+theatre&tbm=isch&source=iu&ictx=1&fir=MxeLLlAXSoWjqM%252Cd_EKC0VwUAlpgM%252C_&vet=1&usg=AI4_-kQvzZNd_cAk_nblrQ7jY1iwF6mNJQ&sa=X&ved=2ahUKEwiF4N-ZsaXwAhXCQkEAHVWwDPUQ9QF6BAgMEAE#imgrc=MxeLLlAXSoWjqM

    Edit: Only 3 pics in there are of the SB theatre.
    • CommentTimeApr 30th 2021 edited
    CrawfW quoth: "Having not been able to find any to look at"

    Where are you based?

    With straw there's a lot of choice about finished appearance, somewhat constrained by practical requirements that can depend on where you are. IMHO, direct rendering of both the inside and outside with lime is essential in the English climate for both water resistance and fire resistance, as well as airtighness. In some exposed parts of Wales and Scotland and maybe some parts of England too it would be wise to consider a rainscreen external to the render as well.

    The render can be finished very flat, with right-angled corners, or can follow the surface of the bales pretty closely and corners can be carved, as can be niches. It can be pargetted (sculptured) with any kind of design you can think of. It can be polished, or made to look like rough stone. In so much as the same is true of render on cob, they can look the same, but the same is also true of render on blockwork.

    In the UK, Barbara Jones' book is the best known and is useful.https://strawworks.co.uk/ There are many other books on straw bale building that are also interesting; I'd particularly mention Jacob Racusin & Ace McArleton's Natural Building Companion. https://newframeworks.com/meet-us There are lots of other US and Canadian builders out there.

    In Germany there are several interesting people. Gernot Minke has experience of both earth and straw building and has written about both - you can find some online - maybe start at http://gernotminke.gernotminke.de/projects/ Straw and clay is Stroh und Lehm for searching. Google translate is your friend. Austria, Fance, the Netherlands, Czechia etc etc. Jakub Wihan is another helpful chap.

    There are various test results online. A lot of mine seem to have bitrot but some still work http://amper.ped.muni.cz/jenik/straw/tests/FiretestStrawCZ.pdf

    There's also some interesting Australian experience including official government support https://www.yourhome.gov.au/materials/straw-bale and a web forum https://ausbale.org/ also New Zealand https://www.ecodesign.co.nz/strawbale.html

    There are some pointers to the various official tests at https://en.wikibooks.org/wiki/Straw_Bale_Construction/Resources/Technical_Studies but I don't know how complete the links are.

    Nick's link is interesting - the Straw Theatre by Salto Architects is one of the loadbearing Hesston projects I was thinking of. As Nick says, building the roof first is the best strategy if you can arrange it. Barbara uses a technique where the roof is built and held up on posts a little taller than its finished height; the walls are then built and finally the roof is lowered on top of them. Check out the Sworders auction hall for one such example. https://www.sworder.co.uk/award-winning-saleroom/

    Good luck with your project.
    • CommentTimeApr 30th 2021
    Incidentally, it might be worth mentioning that the best solution depends significantly on where you are. Here in most of the UK, lime-rendered bales externally, with lime or clay render inside works well (but consider airtightness with clay - much more prone to cracking). In the US in some drier and more earthquake-prone areas, cement render with embedded chicken wire works better, and in some other even drier places, external clay render works well. In some very exposed European sites, even lime render isn't enough and a rainscreen is necessary.
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