80% clay. You will have a hard time to work the sand in, and the more sand, the harder it gets to work it in. Too much sand and you lose all pliability. The 80/20 ratio is just a rough guide. I might have put more sand in myself.
I have a small confession to make: the oven started to sag after 2 years. A good solution would be to coat it in something waterproof like small swimming-pool / mosaic tiles and grout.
It was 80% clay, 20% sharp sand. In hindsight, I should have made the sand proportion higher since the clay cracked so much as it dried out. But getting sand worked into clay is hard physical labour!
The door stays open while in operation, as this is where the fresh air is drawn in from. (Sorry for the late reply!)
The vermiculite functions best as an insulator if it is not compressed. It is to be expected that the top of the oven feels warm, if it is scalding hot, however, then you must have lost some of the insulation of the vermiculite, i.e. it got compressed, or the layer was not thick enough. Still, I am sure the pizzas will be fabulous either way.
Internal diameter: 70cmExternal diameter: 110cmI used 6 x 25kg bags of cheap potter's clayThe vermiculite insulation was about 5cm thick. HTH & have fun!
Next time I am going to use refractory (heat-proof) bricks and refractory cement for the inner dome and then make an insulation later from someting new and interesting like rock wool as used for house insulation. I also found that the terra-cotta tiles on the baking surface shaled a little so I might use refractory bricks for that too.
You absolutely have to put a waterproofing layer like roof tiles over the whole thing or it will start to disintegrate after a few rain seasons (Britain has one continuous rain season, come to think of it it...). It lasted 3 years without a waterproof cover and was occasionally patched. And then one day after a heavy rain.... well, you can the guess the story's ending! Suffice to say, it is not more.