A few questions before I start my build

[magicmike]

Hi all, sat in my garden trying to plan the next stage of my first time build. Just completed my base structure out of wooden sleepers and have a few questions before I purchase any materials to build my oven.

Any input at this stage is very welcome. I've grasped the basics I think what with insulation being important, fire bricks being preferred but not necceassarily a must and the magic 'home brew' formula.

My first questions are as follows:
1. In a 10:1 vermicrete mix what type of cement needs to be used?
2. Is a 75mm slab of vermicrete a sufficient insulated base if 25 mm Fire bricks are to be placed on top to create the oven floor?
3. Are the 25 mm Fire bricks layed loose on the base or are they cemented in?
4. In the home brew mix, what type of cement is it?
5. Where should there be expansion gaps?
6. What is the recipe for the final render?
7. What are the various drying procedures throughout the build?

That's all I've got for now lol. There will be more but any advise is gratefully received. Thanks.

[downunderdave]

Jun 5, 2016 15:47:04 GMT magicmike said:

Hi all, sat in my garden trying to plan the next stage of my first time build. Just completed my base structure out of wooden sleepers and have a few questions before I purchase any materials to build my oven.

Any input at this stage is very welcome. I've grasped the basics I think what with insulation being important, fire bricks being preferred but not necceassarily a must and the magic 'home brew' formula.

My first questions are as follows:
1. In a 10:1 vermicrete mix what type of cement needs to be used?
2. Is a 75mm slab of vermicrete a sufficient insulated base if 25 mm Fire bricks are to be placed on top to create the oven floor?
3. Are the 25 mm Fire bricks layed loose on the base or are they cemented in?
4. In the home brew mix, what type of cement is it?
5. Where should there be expansion gaps?
6. What is the recipe for the final render?
7. What are the various drying procedures throughout the build?

That's all I've got for now lol. There will be more but any advise is gratefully received. Thanks.

1. General purpose or Builders Portland cement.
2. 100 mm thick  5:1 vermicrete mix is usually used for under floor insulation because it needs some strength here. 10 :1 is ok over the dome.(see attached doc)
3. Laid loose allows for easier removal and expansion 20 mm of 50/50 dry sand and clay works ok as a leveller (some builders  apply it wet)
4. As for 1.
5. Some builders leave a 5-10 mm gap between the floor and the first course of dome bricks. Also do a search for heat break which leaves an expansion gap either between the inner oven and entry or between the entry and decorative arch. If you can isolate the inner oven completely from the outer shell and decorative arch, it can expand and contract freely without stressing the outer parts of the oven.
6. Lots of alternatives, but 4:1:1 sand, cement,lime is pretty good, or go for a proprietary mixed render.
7. I like to leave the vermicrete slab a few weeks to dry before building over it as there's lots of water in that layer. A week for the inner oven to cure and dry, likewise for the vermicrete dome coat (depending on its thickness)

Vermicrete PHYSICAL PROPERTIES.docx.zip (61.2 KB)Vermicrete insulating slab.doc (84.26 KB)

[magicmike]

Thanks Dave, just one issue I may have is I only have 100mm recess in my wooden base to build the slab. The slab will sit on 100mm of solid wooden sleepers. Is there a way to maximise insulation but keeping all layers within 100mm?

[magicmike]

[downunderdave]

Yes,
If you use cal sil board instead of 5:1 vermicrete you only need 50 mm as it is around twice as good an insulator as 5:1 verm. The downside is that it's expensive, but it's also dry and you don't have the problem of drying it out. I suggest you drill some holes through the sleepers if using either verm or cal sil so any water build up can drain out. This also allows steam to be forced out from the fire, as it will travel away from the fire.

[magicmike]

Took your advise Dave and changed the base design. Going to lay a 100mm vermicrete slab at the weekend, but ordered perlite instead. Is the ratio the same, 5:1? also how is best to mix it?
Thanks

[downunderdave]

Perlite is fine. I mix the cement dry with the perlite (careful the fine dust is an irritant if breathed in vermiculite doesn't seem to have this fine dust) with the cement dry first. For every 10 litres of perlite add 2 litres of cement. I also add a handful of powdered clay to make the mix a little stickier. Then add 3 litres of water and mix well. If you end up with water pooling in the bottom of your barrow you've added too much water, throw in a little more dry material. Too much water simply washes the cement off the grains of perlite. I've tried mixing this stuff on a mixer but found it unsuccessful because it sticks to the paddles and sides of the mixer and tends to abraded the grains resulting in a loss of volume. You will need around 20% more dry material than the volume required. Eg 100 litres of perlite wil make 80 litres of wet mix.

A vermicrete or perlcrete slab takes a while to dry. Read my attached experiment on drying a slab of this stuff to get an idea.

Vermicrete insulating slab copy.doc.zip (73.2 KB)

[magicmike]

Thanks for the document, interesting to know and very useful. I'm guessing that a 4" slab that's contained within the wooden sleepers will take months to dry which is disappointing. If I made the slab in 1" layers allowing drying time between each layer do you think that would speed the drying process up a little?
Is it possible to fill some of the slab with another insulation medium like glass for example to reduce the amount of water?

Thanks for your help

[downunderdave]

Yes doing it in layers with a week to dry after each layer would help a lot. That's what I do when applying vermicrete over the dome. Because the under floor insulation is not surrounded by air it can take months to dry out by fire alone. This is not really a problem because you can still use the oven for cooking while doing this, but you notice its performance improving as it gets progressively dryer.

[magicmike]

Cheers Dave.

Trying to get some materials together but struggling to find hydrated lime without paying high delivery costs. However I did come across some hydraulic lime, firstly can this be used in home brew and secondly does anybody know the difference.

Also I've come across some storage heater bricks and they're a fraction of the price of fire brick. What are people's thoughts on using storage heater bricks for either the floor, the dome or both?

[oblertone]

Hydraulic lime will set under water, non-hydraulic lime won't is the answer to your question. Most builders merchants or Wickes will stock lime, just say you need it for making lime mortar.
As to heater blocks, I used them very successfully for the floor of my oven, I am aware that folk have used them for the dome but it'd be a lot of cutting because they're quite thin; I used reclaimed 'wire cut' bricks for my dome, total cost for the heater blocks and bricks was £5.14p

[downunderdave]

Yes they are different. I tried to get hydraulic lime,but it's ridiculously expensive in Australia so I gave up and use hydrated lime only.
Read here
nmnikhilmishra.blogspot.com.au/p/wanna-know-difference-between-hydrated.html

Regarding storage heater bricks there are quite a number of uk folk who've used them successfully, but be aware that they have been designed to withstand a much lower temperature and rate of increase than a WFO will subject them to. Some may be ok but others from a different manufacturer or different mix used may fail. If you can get them cheap enough and are prepared to risk a possible failure and rebuild down the track, then go for it. Maybe go for real firebrick in the floor at least, because that's where they get a beating.

[sparkles]

Natural Hydraulic Lime is Calcium Oxide also known as Quick Lime and it's good for disposing of bodies... I'm told )Reacts with water to produce Calcium Hydroxide or Slaked Lime. The reaction is exothermic and it can get hot enough to boil the water. I have used NHL 3.5 in a render. Sets rock hard but can take a while to go off.

[richied]

...and hydraulic lime burns (chemical burns) your skins; wear gloves!

[magicmike]

Thanks all, think ill stay clear of the storage heater bricks, as tempting it may be I don't fancy rebuilding this thing later just to save a few quid now.