Modular Build In Deepest Lancashire

[downunderdave]

Hi Lance,

I've just read your whole thread for the first time. WFG have obviously not provided sufficient or appropriate instructions for your build. A couple of things I noticed. 1. When building form work for the outer decorative arch, you must put some wedges under it so when you remove them the formwork will drop a little for easy removal. Failure to do so almost always results in damage to the arch.
2. The drying out fires need to be done after the vermicrete layer over the dome, but before the outer render is applied. Otherwise the large amount of water left in the vermicrete layer gets locked in. (This is even more problematic if you have added waterproofer to the outer render layer.) The swelling steam filled vermicrete can then swell and cause the outer render to crack.Also, if you use wet vermicrete under the oven floor, this will take months to dry out properly with fires, best to allow a couple of weeks drying in sun and wind, if weather permits, before covering it with firebrick floor.
3. You discovered yourself that a gap around the flue is required that should be filled with high temp silicon. Because the stainless steel is so much more conductive, it wants to expand well before any surrounding render can.

i'm sorry you've had problems with your oven, they could all have been easily avoided had you been given appropriate advice. I've built more than a hundred ovens and have learnt the hard way.

Go back to WFG and tell them that they need to give their customers this info with decent instructions with the kit.

[albacore]

Hi Dave, I think you are being rather hard on WFG; the outer arch design was mine and nothing to do with them. I did have wedges under the former, perhaps you can't see them in the photos. The fault was in the design, insofar as there was insufficient support for the side pillars, until I rectified it - obvious when you know!

I'm pretty sure my vermicrete was dry enough; the problem with the render was a lack of expansion gap round the flue. If you look on this forum, a lot of people have had problems with cracked render, which is why many have moved over to K-rend, which is a bit more flexible. It seems to me that rendered ovens are really based on unsound building principles: you are applying a hard shell to a soft background of insulation blanket, netting and vermicrete. Add in British weather and oven heat cycles and no wonder they are prone to cracking. On the other hand, they look nice!

In my posts, I try and point out not only the good things, but also where I went wrong and how I would do it next time. No first build is ever going to be perfect and many people on this forum will only ever build one oven. You are lucky in hsving built so many ovens, so I am sure you have learnt from your early mistakes and incrementally improved your designs.

Thanks, Lance

[downunderdave]

Vermicrete insulating slab.doc (84.26 KB)
Hi Lance,
my apologies re the former wedges. You are correct I didn't see them at first viewing. Regarding the slow heat up of your oven and the high fuel consumption, it is probably either moisture trapped in the insulation layer or the extra floor bricks you placed over the kit floor.
Re wet vermicrete, it contains way more water than you would think (around a third of the total volume is water and probably less than half of that is used up in the hydration process. Although the surface may appear dry after around three days it will still be moist below that, even in ideal conditions. (see attached experiment on vermicrete floor slab), the same applies to wet vermicrete applied over blanket. Driving this water out by fire is slow especially if the outer shell has been made waterproof, although even without an escape route it will find its way out eventually. Some builders provide a special vent for this, my own design utilises a vent that surrounds the flue pipe. Regarding the outer shell, it is only a weatherproof coat to hold the insulation in place, so any cracks are really only cosmetic, although they will provide entry for water, so should be sealed up if they appear. K-Rend or its equivalent acrylic render is a good solution over the cement dome shell after all the moisture has been eliminated. If the outer shell does not contain reinforcing it is more prone to cracking. The usual is to use chicken wire, but it is difficult/time consuming to get it to conform to the sphere, for this reason I prefer to use random AR glass fibres.

Dave

[richieh]

Id be very proud of your build , you did a great job i hope that mine is just as nice ! thanks for your help.
Rich

[albacore]

Thanks Rich; good luck with your build. If you need any advice along the way, I and all the other helpful members on the forum will do our best to provide.

Lance