Is the dreaded Vermicrete really necessary?

[adm]

Still planning my build, but.....

Do I really need to use Vermicrete to insulate my dome? I was thinking of just using 100mm of ceramic blanket covered in chicken wire or mesh of some kind and then just rendering over that? Would the blanket give enough insulation? If so, then there's really no need for the Vermicrete is there?

I was also thinking about maybe building a steel frame over the dome and then tiling that directly instead of render ( I used to be a roofer in my younger days) as that way there would be no cracking due to thermal expansion given that the blanket would act as an expansion gap anyway.

Would anybody know if wooden battens would be an absolute "no-no". I know on the US forums, they say you MUST build any enclosure with metal studwork, but I would have though that if the dome was insulated well with 100mm of blanket there wouldn't be enough heat able to get through to even warm a wooden frame much...

Any thoughts? Is the blanket and vermicrete a bit of a belt and braces approach, or is it really needed?

[bookemdanno]

I always think that the Vermin is an intermediate layer, light to sit upon the blanket, but hardens enough to support the render. Being insulation too, it adds to that property brought by the blanket.

CB is relatively stiff and holds against compressive force reasonably well i believe?
I think there might be one or two who have forgone the Vermincrete?
It'd probably be just as awkward to render to chickenwire over blanket, than apply Vermicrete.
Perlite is another option, and is a more granular material which may create a more workable mix? Not used it, so cannot say for definite.

100mm of CB should be enough.

Personally....I'd just be worried about the render being strong enough to withstand a football strike, if only set onto blankey. So then its about creating a method to build a strong enough shell i suppose.
Maybe adding polypropoline fibres, or mesh, to the render and getting closer to 50mm overall in thickness would provide enough of a shell? Again, not done it, so can't definitely say.

Is wooden studs over 100mm Blanket, any different to a timber stand under Thermolites?

The FB lot are taking things to a whole new level, and worry about the minute details of most aspects of a build. Obviously timber is combustible, but one Cob build on here has a rather lovely timber shelter. Could you call it calculated risk?

How many WFO are situated close to wooden fences or overhanging trees?

Have a look on the Ebay for secondhand metal studding (gypframe) from office buildings?

If you want a built enclosure, why not brick, and loose fill vermiculite/Leca/Perlite between dome and the walls?
How about internally line the wooden enclosure with Cementfibre board, like Hardibacker or Aquapanel, and loose fill between the blanket and panel with vermiculite? I think there is an element of fire resistance there in cementfibre board?
Any enclosure needs to be free standing and not fitted to the oven in any fashion or you'll get heat soak.

If you want a visible dome, drop to 50mm CB, and 50mm Insulating "crete" with render over?
Or 50mm CB, 75mm "crete" and ceramic tile directly onto that?

Decide on your desired oven style, and i'd say there is a multitude of solutions that can be created with a bit of research and "calculated risk" by yourself. You'd definitely be able to gather plenty of info from here or FB.
I'd say a wooden chimney might be a bit on the risky side... but if you can stay near to "tried and tested" simple methods, they usually work out the best.

Your in the planning stage? So where possible, plan to follow a tested route unless there is no way possible due to space constrictions. Then it may be better to reduce oven size then to accommodate insulation, etc.

Vermincrete isn't that bad really, it just needs a different and personal touch!
Its certainly not that bad to have to take on a new risk just to avoid using it.

Ask for help as much as you like, as you've got a massive pool of various WFO experience and other trades on both here and FB.

Good luck with the planning, and as one wise man on FB once said..."don't over think it!"

[cannyfradock]

No further comments on Danno's reply...brilliant informative reply!

I would like to say that if you are capable of building a roof or housing for your oven, that would always be the best option. I've always wanted to build an enclosed wood-fired oven. Out of interest, this is how I would do it...

I would build a double wall. Interior insulation blocks tied in to outer brick wall. The interior wall would support the slab and the outer wall would be continued to support your housing. I used this system on Rob's build...



The outer wall would then continue up from this stage...



....about 3 bricks high? on the side's. On top of this would go the wall plate and trusses. The back gable would be full brick and the front would close into the arch. As mentioned...loose vermiculite infill would fill the void straight over the blanket.

Calculations would be very important as you would want a minimum of 4" of vermiculite between blanket and roof structure. Myself (with all that insulation and vermiculite fire-proof barrier)...I would use normal timber. The dome would have to be fully cured and dry before enclosing it.

Terry

[jemb]

spent most of yesterday mixing vermicrete, & was suprised that it wasnt as bad as i beleived. really dreading it after reading the reviews on here, but to be honest mixed fine in a cement mixer, yes it clogs but nothing a spade cant sort out. stick with it & use this

[spinal]

I think part of the reason people consider devil's porridge so hard to use/mix is that (according to the bods on fb) you need to hand-mix it trying to disrupt the fragile vermiculite as little as possible. The second issue is that water is quite finniky - too much and the vermiculite collapses, too little and the mix is too dry...