To be honest we've not worked out the costs exactly, as we still have one wall to complete, but my gut feeling is my original estimates etc are about right.

Its drying out nicely now the weather is getting a bit warmer, though its still obvious (visually) which bits we did at which stage, the curing process is quite steady and although the surface is nice and dry I suspect the core is still quite wet.

mixing in a normal mini-mixer is fine, and LT will give you the proportions you need in bucketfulls, for the various "grades" - its just a matter of honing the technique. I suspect that the exact method will be mixer-dependent, but this is how we did it:
Add the hemp shiv to the mixer - we used 3 buckets of shiv, as 4 was a bit much for our mixer, then the correct amount of binder (depends on the "mix" - we used two different mixes for different parts of the building), then attach dustbinlid over the mouth of mixer with bungees and then mix for a minute or two to thouroughly mix binder and shiv. Then remove lid (oh, and this is when you realise you need gloves, breathing mask and eye protection - the binder dust is vicious and light) add water, about half the total and spread evenly over the mix, add in a couple of large stones, about the size of your fist, then mix for a couple of minutes. Stop mixer and dig out the dry mix from the back and sides, add the rest of the water, mix until all consistent, do another dig around for dry bits, mix again. You will eventually recognise what the mix should look and feel like, not too wet, but evenly wetted. Chuck into your shuttering. Tamp down if its standard mix. Repeat over and over again...

Availability - they seem to have plenty and didn't talk about shortages, the hemp grows quickly and they seem to have it coming in regularly.

Effect, well at present the barn isn't finished, so there's not a complete insulation envelope, so its difficult to make any judgements, but, it certainly feels warm to the touch, much warmer than, say a plastered wall, even an internal one. Ours probably isn't dried out much either as its not been that warm.

Its time consuming, but a strangely satisfying creative process.

I think you could get away with nothing, but probably best to email Mark Patten at Lime Technology for advice. I think anything that will tie the layers together would be ok, and then its a matter of minimising the cold bridge effect.

Lime Technology have done experiments using the fixing sometimes used on plasterboard - the thing that is like a very coarse thread screw, which you then fix a smaller screw inside. They used a test wall at their Abingdon warehouse. They kept loading a shelf with more weight, but the experiment ended abruptly as the wall itself fell over as it wasn't fixed to anything! However, I think they'd be reluctant to publish anything about loading as the exact mix achieved does seem to make a huge difference to the friability and the overall strength is linked to the structure. We're not planning on hanging anything beyond small shelves. In our bathroom we've put in permanent shuttering using multipro boards (breathable and water resistant) and we will be hanging sink etc from plywood fixed between the uprights, behind the board. Behind we're using a lighter hemcrete mix, so the overall insulation is about the same, thermal mass slightly lower.

This is the bathroom wall, 150mm low density hemcrete mix, behind multipro boards:

Looks amazing. Truly an inspiration. Will you be trying to get it really flat when plastering, or go with the organic/rustic feel it seems to have at the moment?
Can't wait to get to the stage you're at - just held up by lack of funds at the moment.

Also your stairs look quite nice...!

Time for an update - quite a lot has happened since my last post here: Our LCCC project funding came through, so Hook Norton Low Carbon (www.hn-lc.org.uk) is now live and spending money. I finally decided on ASHP, and its now installed and running, we've had the hemcrete walls plastered, and we've discovered that lime plastering multipro is interesting, you need to pva it, and even then, the plaster drops off... eventually we got it sorted. The lime plaster over the reed cracked along the reeds, but a bit of remedial plastering sorted that out. Lime plaster takes really well on hemcrete without any base coat, though we chickened out and employed a professional plasterer. More photos soon, or find me on facebook.

I don't think the lime plaster has dried completely (and its still cracking in places!) but no feeling of damp or any condensation on the single glazed window. Its also amazingly warm in there, even though there is no heating on..

and upstairs, now plastered..

Yes, it turns out that a third of the barn was very old, and was probably a traditional hayloft, the rest (foreground in the picture) is relatively recent. The very old boards were like weetabix. We had to replace them but got some nice (if expensive) old pine to match the rest... main job now is to develop a finish that is similar to the original ones. Potassium permanganate, pigments and oil all ready, just need to get the lime out of the boards!