We have done small stuff in hempcrete here in Brisbane. The house in Tasmania is our first 'big' build (180m3)which we are acting as the installers (DIY) with the builder helping so he learns the process - happy to share. We want to be involved to learn any site issues or if we can do things better as well as test our newly-developed hempcrete mixer.
What is different to strawbale is that you can build normal timber frames for the walls and then pour the hempcrete. On this house, we are doing the floors, roofs and walls. The builder has built the floors off ground and we are pouring next week.
Technically, hempcrete does not need to be finished (we have had a small wall unfinished in Brisbane for over 2 years with no degradation, mould or cracking). Aesthetics usually dictate that it should be finished (it is a rough texture). No doubt external or internal rendering will give it long term durability (50 years or more) as well as more insulation value - this is common in the UK where EVERYTHING is rendered 
It has to be a lime render tho so that it 'breathes' as well as self-heals any micro cracking.
There's a number of options with respect to internal and external finishes. Internally, you can use most products - we are using MgO board that has excellent finishing qualities and doesn't need plastering skills for joints. We're using it as permanent formwork cause it's tough and breathes so the hempcrete will interact. MgO board can be used externally and internally. Just remember that hempcrete needs to breathe to interact with the environment so whatever you face it with, allow an air gap or use a breathable material. You can screw your battens or cladding on later to the hempcrete surface.
The iron cladding could work as permanent formwork but to make hempcrete work to its best you should have that air gap between the iron and the hempcrete. It's not critical but it would be beneficial. Incidentally, hempcrete is fire resistant as well as termite resistant.
As to costs, hempcrete works out to being about $120/m2 (250 thick wall) for product alone. For the house in Tassie, we costed it against a number of products and it was initially cheaper than a number of thermally equivalent systems but then we need to take in account a number of factors, formwork being one of them. To be truthful, until we have done this large job, we really won't be certain about install costs and where cost savings can be made.
We are fortunate to have a client that is eco-sustainable and focussed. He's going off the grid, off the sewerage and off town water. He did not look so much at the build or material costs rather long term cost savings in energy use, maintenance issues and the environmental impact of materials used. On a long term basis, hempcrete should reduce your power consumption for heating or cooling by 30%, so even if the product cost was 20-30% more, you quickly recover that cost and gain a healthy, natural, thermally-efficient allergy-free home forever.
By the way, the client is English, has lived in damp houses all his life, has not enjoyed paying massive heating bills yet still wants to move to Tasmania - go figure. At least we can fix 2 of his hates - can't do much about the English part.
Lucas, be more than happy to talk to you. Sometimes it's just easier if you've got more questions. Call me on 0438 006 059 and I'll return your call. Being a technical engineer, I want you to have full understanding before you make a decision.
Posted Thursday 19 Aug 2010 @ 9:04:36 am from IP
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