Applying insulating render

[itiejim]

The build instructions for my oven suggest an insulating render of about 7cm of insulating refractory cement. Can anyone suggest how many layers it is best to use to get that sort of thickness? Presumably I need to let each layer set properly before applying another?

[moonhead]

Hi and welcome.
If you want to insulate your oven then it's not refactor cement your after. Refractory will withstand the heat from the oven but won't insulate.
Ideally you need a 25mm or 50mm thick layer of ceramic wool blanket then a layer of vermicrete over the top of that. Depending on how much space you have I'd go for 75mm thick. This is a mix of vermiculite which you can get from builders merchants and cement at a ratio of 6:1. Finally a couple of layers of waterproof render to keep it all dry. A lot of builds are using K- Rend for this.!
Sound like a lot of work but it will keep the heat where you want and increase the efficiency of the oven too.

Check back in the builds section on the forum and and you'll find a huge resource of information and tips from the builds in there.

Rick

[itiejim]

Hi, thanks for the reply. Some more info might help. Firstly, I won't be relying on this for insulation. The whole thing will be built in to a housed brick structure and then the air gap filled with loads of vermiculite. Second, all this kit comes from Italy, so it may be that due to my lack of building knowledge I haven't translated it well. The render is called granulato isolante rifrattario, which I think would literally translate as isolating granular refractory (cement) it needs to be mixed at a ratio of 100kg of the refractory cement to 25kg of normal cement.
I'm building as per the Italian building instructions but could just do with knowing whether the 7cm thick render they specify needs to go on as one layer or several.
Apologies if I've put this in the wrong forum. Perhaps a mod could move it?

[downunderdave]

Feb 27, 2016 12:28:25 GMT itiejim said:

Hi, thanks for the reply. Some more info might help. Firstly, I won't be relying on this for insulation. The whole thing will be built in to a housed brick structure and then the air gap filled with loads of vermiculite. Second, all this kit comes from Italy, so it may be that due to my lack of building knowledge I haven't translated it well. The render is called granulato isolante rifrattario, which I think would literally translate as isolating granular refractory (cement) it needs to be mixed at a ratio of 100kg of the refractory cement to 25kg of normal cement.
I'm building as per the Italian building instructions but could just do with knowing whether the 7cm thick render they specify needs to go on as one layer or several.
Apologies if I've put this in the wrong forum. Perhaps a mod could move it?

The problem with adding a wet insulating mix is that it uses a large amount of water (around a third of the volume of the dry mix) This needs to be eliminated. For this reason I do not make any layer of it thicker than around an inch and a half. I then allow the weather ( assuming you have sun and wind) to dry it for a week before adding a subsequent layer. This is preferable to using fire to dry a wet layer because it makes it dry more evenly. If you are not using it for insulation then adding dry vermiculite is better. Unless the mix is also designed to provide structural support for the inner dome sections (which it may be, particularly as it sounds like a very strong mix if they are asking you to add so much cement) there is little point in using it. You should contact the manufacturer and discuss your plan with them.

[itiejim]

Spoke to the manufacturer about this. They insist that the render is necessary to increase the thermal mass of the oven. The mix is quite odd, looks like a lime mix with lots of tiny bits of broken ceramic tiles in.

[downunderdave]

Mar 7, 2016 21:11:39 GMT itiejim said:

Spoke to the manufacturer about this. They insist that the render is necessary to increase the thermal mass of the oven. The mix is quite odd, looks like a lime mix with lots of tiny bits of broken ceramic tiles in.

If this render is necessary to increase thermal mass, contains a large proportion of cement and also broken ceramic tiles, then it won't be much use as an insulator. You will need to insulate over it, as your plan with the loose vermiculite, if you want to retain heat.

Regarding the addition of the 70 mm thick thermal mass layer, because you will be adding a large quantity of water to this mix it needs to be eliminated and that will take many firings. You can help this a lot by doing it in layers of about an inch and a half with a weeks drying between each layer. hopefully you can get some good sun and drying wind on it during these times.

[itiejim]

Dave, thanks for that, the plan was always to insulate well with vermiculite - the render is all on now anyway, two layers went on yesterday on a suitably dry, sunny windy day. Making progress now!