i have just build my first brick oven in my garden using fire bricks and refractory cement i now have to decide what to cover it with. is a vermiculite-cement mix all it needs? if yes at what mix ratio? or should i cover it first with a ceramic blanket and then plaster it with the vermiculite? and if i do this should the ratio be different?
Congratulations on your build; do let us see some pictures! By and large, the more insulation the better, but you usually need a rigid outer layer so that you can put your weatherproof outer render on top of it. Most oven builders start with a layer of ceramic fibre blanket, since it is an excellent insulator. I think its insulating value is about double the vermicrete. Then you need a layer of chicken wire, followed by the vermicrete. Probably about 6 vermiculite to 1 cement. Then you normally need an outer weatherproof coat of render, since the vermicrete is not waterproof. Have a look at my modular oven build Here for one example of how to finish your dome. There are other ways to do it - some have used K-rend as a more flexible outer layer, less prone to cracking, and others build a roof structure over the top of the oven.
thanks for the response the vermiculite-cement ratio is it volume or weight? and can you tell me more about the top coat the k-rend what is it where can i find it?
Hi, vermicrete is mixed by volume - whatever ratio makes a workable mix for you, the more vermi, the more insulative it is, but as Albacore says, about 6 to 1.
K-rend comes as a bag of base coat (with or without reinforcing fibre) and a bag of smooth top coat. Both mix easily (I've forgotten the ratio, but it's just a measure of powder into a measure of water, details on the bag) and it's easy to apply. I used less than a bag of each, and got it from MKM builders merchants cos there was one nearby. It is available in a range of colours, doesn't have to be white but it's most commonly stocked.www.k-rend.co.uk
One thing - K-rend is pretty waterproof. Make sure your dome and vermicrete has fully cured and dried before you render over or you'll trap moisture inside the dome.