Building outdoors on ground in tropics

[Johnnycantplaytoday]

Hi Dave,

So I built the dome yesterday, learn't somethings, made some mistakes, also some wins (I think). As always took longer than I allowed for!!

Overall, happy with the build, a couple joins in the first layer I wasn't too happy with due to the newspaper slipping all over the place, tearing and getting caught between the layers of cement. Fixed that up by getting rid of the newspaper and leaving the Cling wrap I had put on the night before. Was so much better than newspaper, especially after I did the first run around the bottom and that "stuck" the cling wrap to the dome. But I think there are some places in the first join that have newspaper stuck halfway between the layers.

Also by the time I got to mixing up the 2nd large batch of homebrew and got to laying the 2nd layer the 1st layer had already gone off, (probably an hour or more I guess). So after that I stuck to smaller batches.

By the time I got to half way up I started to make the mix a little bit wetter and that worked really well. After removing the form work today, the inside of the top half of the dome was perfect.

Happy I went with 60mm thickness. I'm sure some places will be a little less and some a little more.

I made up a 60mm mould that I could squash the home brew into and flatten down nice and hard with a wooden trowel and spade. Once flat then a bit of squashing with the fingers and screed off the top followed by some water and a flat trowel to get a nice smooth finish. Then a few good hard bashes on the wall got rid of some of the bubbles and then a good tap to the side and I had a nice thick sticky brick (like fine sticky clay) that I could pick up and lay like tiles over the dome. That worked out very well.

I also substituted the 3 parts brikkies loam sand for 2 parts brikkies loam and 1 part plasterers sand I took from the kids sandpit. Mainly because the loam was very clayey and I already had firelclay. End result was still a sticky thick clayey like material.

Was going to wait a week before I removed the formwork but I was impatient and it was coming out easy so I just did it. The only hiccup was a largish chip (still in place but started to come away from the main build) on the side on the top of the flue were I had to tap the round flue mould out upwards. But nothing that will cause any drama and perhaps one bad join between the layers at the top left of the front opening. But all in all not so bad.

So onto the insulation next weekend, no doubt I will have some questions. when I recover and have some time to think of them.

Thanks for your input as always.

JB

[david s]

Good job John. I suspect the problem you had with the newspaper was that it had dried and wasn’t sticking to itself. It needs to remain wet. Portland cement unlike calcium aluminate cement , requires extended damp curing or it will not aciepve good strength. You should cover the casting with wet hessian or bags and then cover that with a waterproof tarp. This should be left for at least one week before proceeding.

[david s]

I forgot to add that you should fill any voids on the inner surface while the casting is still damp.
It’s looking pretty good.

[Johnnycantplaytoday]

Hi Dave,

Back again, I've had some time to do some more homework and come up with some more questions.

I suspect the problem I had with the newspaper was because it was wet and was just simply sliding down the cling wrap any time I put some homebrew on it (anyways it all done now.)
I've also put some wet towels on it with a tarp and water it daily.

So with the insulation wool I'm doing this weekend, after much searching it appears that there is as usual more than one way to do it. I've seen it where people just cover the dome and leave out the chimney, some wrap the wool up to the side of the chimney and others wrap the wool right up to the edge of the front opening. In my case what would you recommend.

I was thinking of starting on the vermicrete base itself (I've seen others where they start an inch or 2 above the base)
then going over the dome, up to the side of the chimney and just below the opening of the chimney
then wrapping with some fireproof sisalation to help reflect the heat back in,(putting in a few hundred small holes to help steam escape, similar to what UtahBeaver did on his build)
then covering in chickenwire tied down to some stainless steel screws in the bottom sides of the dome
then covering in 4 inches vermicrete in 2 layers.

Just want to ask out there if anyone thinks the sisalation helps with heat retention or is it a waste of time and should I start the wool just above the base or is OK to leave on the base.

Thanks

JB

[david s]

Are you planning on having a separate decorative front arch? One big advantage of doing this is that it can be part of the outer shell leaving the inner dome and gallery free to expand without placing stress and pressure on the decorative arch. The whole dome and gallery should be insulated either in blanket or vermicrete, but it can be thinner over the flue gallery which doesn’t get as hot. Blanket or vermicrete should go right down to the insulating slab so the whole oven is insulated under and over. With the way I have the blanket cut there are some spots at the base that are a couple of inches off the under floor insulation. This space is just filled with the 10:1 vermicertethat covers the blanket. If doing the vermicrete over the blanket any thicker than around 35 mm it is best to do it in a couple of layers because the stuff contains so much water a thick layer can’t dry easily. The vermicrete allows you to get the form and surface back nice and smooth as well as providing a firm surface to work against. If you want more insulation additional layers of blanket are a better solution than adding more wet vermicrete. I’ve been through the foil idea and whilst it seems logical to have it reflect some heat it also gives you a moisture barrier making oven drying slower. I now no longer use it although many other oven builders do. If you are set on it make sure you perforate the foil really well.

[Johnnycantplaytoday]

Hi Dave,

Thanks again for replying.

I wasn't going to have a decorative arch, just the existing concrete on the front of the dome. I think I might ditch the sisalation layer and stick to the 2 layers of 25mm (1") blanket covered by 100mm (4") vermicrete put on in 2-3 layers. Was going to do 2 layers of 50mm each, but I might now do 3 layers of 35mm.

So I guess I will wrap the blanket around the flue gallery as well stopping about 25mm short of the front opening and say flush with the top of the flue gallery. After I add the layers of vermicrete I guess I'll be another 100mm higher on the flue and another 150mm wider around the front opening.

After curing I'm thinking of putting a waterproof membrane (like what you put under tiles in the bathroom) over the oven and eventually covering in some tiles with waterproof grout.

Also think I'll have to end up having to put a roof over it all to "help" with keeping away the rain and moisture uptake.

Dave, you mentioned that for better insulation, it would be better to put another blanket layer over the vermicrete, rather than additional vermicrete layer.

"If you want more insulation additional layers of blanket are a better solution than adding more wet vermicrete"

Does this mean your saying you have a 2" layer of blanket, 1.5" layer of vermicrete, another 1" layer of blanket and then a final 1.5" to 2 " layer of vermicrete. So a layer of vermicrete sandwiched between 2 layers of blanket

Cheers

JB

[david s]

No, I meant perhaps 3” of blanket topped by 1.5” vermicrete. Although I think 2” blanket topped by 1.5” vermicrete is sufficient. More insulation is subject to the law of diminishing returns.

[david s]

When you get to do the vermicrete layer as you obviously want it to be insulative then the leaner the mix the better. I find anything leaner than 10:1 is too difficult to apply. You can use either vermiculite or perlite. I’.ve also found a 50/50 mix of the two works better than either of the two alone for some reason. The finer the grade of the material requires a higher proportion of water in the mix also. My recipe is 5 parts med grade vermiculite, 5 parts medium grade perlite, 1 part Portland cement, 0.25 parts powdered clay, 3 parts water, all by volume not weight, mixed in a barrow by spade, dry materials first, water last. After 24 hrs this mix can easily be carved back if required. It is quite firm to render against for the final outer shell. The drying of the oven is better done after the vermicrete has dried for at least a week. After that do the outer shell. Then to avoid blistering of the waterproofing coat, give the oven several decent firings before applying.

[Johnnycantplaytoday]

Hi Dave,

Thanks for that. Will go with the 2" blanket covered with chicken wire and a couple 40mm layers of 10:1 vermiculite/Perlite mix over the top. I have a fair bit of the poly propylene fibres left, so I imagine that couldn't hurt to add to the mix. Even though they will not melt, I think there original purpose is to provide strength to a concrete mix.

Will let you know how I go.

Thanks

JB

[david s]

Yes, adding those fibres wouldn’t hurt. But as the material is so weak they probably won’t make any noticeable difference. I’ve tried doing the same thing with them but found they don’t disperse at all well and want to remain in a clump. Also excess mixing of vermicrete or perlcrete leads to degradation of the grains so I gave up on the idea of adding the polypropylene fibres to vermicrete.

[Johnnycantplaytoday]

Hi Dave,

So got the fire blanket and the first layer of insulation done. So as mentioned previously, I went for 2 layers of fire blanket, chickenwire, then 40mm coating using your mix of 50% vermiculite and 50% perlite with some fire clay added.
Come up pretty good except that I ran out of mix at the top (only needed one more barrow!!) and had to wait until this morning so I could duck into town and buy a 6 x 5 litre bags of perlite and vermiculite each for a ridiculous price. Anyway, my fault for running out. I "keyed" the remaining top section into the existing bottom section by digging out a 45 degree recess into the existing bottom layer. But all in all came out pretty good.

I did try to add some poly propylene fibres to the initial mix, I added them to the water and got an electric paint stirrer into them but they only fluffed up in a big ball and floated to the top. I did put them in the first mix and they did disperse to some degree but they also stuck together in clumps as well, which I removed when I saw them. Next time I might try to add them to the cement portion of the mix and add water to the cement only to get it into a wet slurry and use the paint stirrer again. That should hopefully keep them separated. Will see how I go.

Dave, you mention that you only really need a single 35-40mm layer of vermicrete insulation outside of the 2 layers of blanket otherwise there are diminishing returns for the extra time and effort..I am tight for room at the back and I still have to allow for some kind of waterproof render once finished. Do you reckon one 40mm layer would be enough. Any idea on what sort of external temp one might get with just 40mm of insulation.

I thought most people tend to have around 100mm ( 4") insulation on the exterior of the dome and with a big fire inside the exterior surface of the oven sits around the 30C mark. Do you think this would be a similar sort of temp I could expect with 40mm insulation layer or would it help to add another 40mm layer. I can just skimp at the rear of the build and the far left and right as I have run out of base, but as soon as I get a bit of height I can then go to 40mm all round.

Thanks again.

JB



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[david s]

Si think you’ll be fine with what you’ve got. Remember that the ambient temp in Cairns is way higher than our friends in the north who frequently deal with life below freezing temps. Once your oven is dry the outside won’t get any hotter than cosy warm. Remember to dry the oven before rendering an outer shell. After a couple of days the vermicrete will go white and appear dry. It won’t be deeper in though. A cheap garden moisture meter inserted into the vermicrete layer will tell you when it’s dry.

[Johnnycantplaytoday]

Hi Dave,

Thanks for that. I intend to have a few big pizza nights before I render the outer shell (after I cure the oven first), so it should all be completely dry by then. Also, I still might add another thin outer insulation layer, like 25mm maybe just because I think I'll sleep better at night.

What sort of waterproof render do you use.

I was thinking of something like what tilers put down in wet areas before they tile over it. Or should I get like an exterior waterproof acrylic render like they put on the outside of houses.
So many choices.

I'm chasing basically a smooth ( or very slight texture is OK ) render finish with a coat of water based paint over it for the colour. I assume you can paint over it.

Thanks again.

JB

[david s]

I use Flexible Pointing which is a product designed for sealing around the mortar on roof ridge caps. It is extremely flexible, outdoor suitable (UV protection) and high build (contains sand). For ease of application I thin it down with 20% water so it can be painted on in a few coats building up to a layer of around 2 mm. Sponge finish and paint over with acrylic if you don’t like the colour.

[Johnnycantplaytoday]

Thanks Dave,

Just confirming that one of the following is the stuff. Looks like they are all similar.

https://www.monier.com.au/-/media/mo...heet-v05fp.pdf
https://www.solutionindustries.com.a...apping-627.htm
https://www.selleys.com.au/assets/488/Pointworks%20A4%20Brochure%20TR930%20web.pdf


Cheers

JB