Tanking Slurry over dome vermicrete?

[PeteH]

Nearly finished the dome and am considering applying Tanking Slurry over the vermicrete layers prior to lime rendering, it’s used to prevent damp on internal and external walls prior to rendering or as a final render
Anyone tried this? It’s supposed to be “breathable” and reactivated when wet so moisture left in vermicrete shouldnt be a worry (unless in steam form)
I’ve used tanking slurry oven rendering a pond and it seems watertight

[UtahBeehiver]

There has been some use of similar products, ie chimney crown coating, etc. Check and make sure that the lime render will adhere to it though.

[david s]

You should really do the drying fires before sealing over the vermicrete. There’s so much free water in vermicrete that it’s best to let the sun and wind dry it out as much as possible, this usually takes a minimum of a week in good drying conditions for a one and a half inch thick layer (longer for a thicker layer) before attempting any drying fires. Water locked in the insulation layers can be problematic if you render over it too soon.
It sounds like the tanking slurry will do exactly that, which you don’t want.
I don’t know how much vermicrete you’ve added, but if you used a 100 L bag then you will have also added 30 L of water. Remember that one litre of water creates over 1500 litres of steam.

[PeteH]

I’ll check with their technical team I assume it will need priming first before rendering

[PeteH]

Thanks David
I put the vermicrete on over the weekend and have been keeping it moist the last couple of daysto help curing
I’m going to leave to dry at least a week, 2 now that you suggest it (2inches thick) and continue firing and cooking before I apply any slurry
However it does require wetting of the vermicrete before tanking, to draw the silicates in the slurryinwards and react to form waterproofing layer
Have put in a vent at top of dome
Have been doing the curing fires prior to and during insulating - last night cleared the dome of soot and pleased to see no visible cracks (on the inside!)
Roasted some veg as I heated it up, a bit try but tasty, have now made a temporary door with ply and tin foil

Great to eventually get cooking after countless hours of labour and costs!
Will try roasting a chicken next and then have a go at Pizza

[david s]

Hi Pete,

That sounds like a good plan. To explain my advice I will relate my experience with vermicrete.
Ceramic fibre blanket was invented during the space race and became available in the early 70's for use in other applications. It was widely used as the lining for kilns and its low thermal mass and high temperature capacity made it ideal for this application. When I began building ovens in the early 2000's there was a new generation of blanket available, thanks to the Germans who refused to allow the sale of the first generation as it was classified as a Class2 carcinogen. That is tumours had been found in lab rats but none reported in humans. Consequently, although I'd used the stuff in both operating and building kilns, I was not prepared to use it in a product offered for sale to the public. The newer generation of blanket, exonerated as a carcinogen, at that time was over double the price.
Consequently, my ovens used vermicrete as the insulation layer (at 10:1 the insulation value is about the same as blanket). I quickly found out that moist vermicrete sitting against the inner dome will get hot fast, swell and crack the layer from the steam pressure build up. If a rendered shell over the top is applied before the vermicrete has been allowed to dry slowly, it cracks the outer shell. No real drama as you can simply render another layer over the top, which is what I did. My solution was to apply the vermicrete in layers no thicker than an inch and a half at a time with a week drying in the sun between each layer.
Now that the new generation of "safe" blanket is the norm, a blanket layer(s) between the inner dome and the vermicrete layer acts as an expansion joint and protection from sudden heat to the moist vermicrete.
When the vermicrete turns white, or a really pale grey the surface appears quite dry, but the inner thick layer beneath will still be moist. If you are concerned about this you can plunge a cheap garden moisture meter deep into the vermicrete and use it in reverse to gauge the moisture content.
I hope this explains my previous advice.

Dave

[PeteH]

Thanks David
Interesting, the origins of the blanket!
I bought a moisture meter on your advice at the start of the build when I laid the 5 inch vermicrete hearth in Feb
i then gave it a period to cure and dry with heat under a cardboard box canopy before placing the ceramic board layer on top
Drilled a dozen holes through the support slab when it was curing also to allow any moisture to escape at a later date when firing
I figured the blanket would allow plenty of expansion of the oven and protect the vermicrete insulation shell
in planning on giving the vermicrete a fortnight to dry and check with the meter before applying the tanking over a moistened surface
My understanding of cement and concrete is that it never fully cures as the hydration process is continuous so hopefully any moisture left inbetween the dome and final rendering is fixed into the cement in the vermicrete and tanking layers by the chemical reactions! Tanking works by absorbing water into silicates that swell and produce a barrier
I’ve also allowed for a vent at the top of the oven
I’ll also fire the oven at least a dozen more times I know an effort to move any moisture that has permeated from the wet vermicrete into the blanket
I used about 250litres of vermiculite and 100 litres of water to make the vermicrete layers (not to mention 3-4 litres water each time I damped the curing vermicrete down) so am going to be patient before trapping any excess water in!
am planning to roof the structure at some point to allow cooking all year but I’ve a list of jobs mounting up not to mention the building site of a garden which needs addressing! (Plan to enrol some friends round for a work/ pizza party to help in this area)