Originally Posted by Thermal Mass
V-wiz-I started a build thread with more details on construction process.
Briefly-As the oven is built, there is considerable amounts of time and small fires to cure each step (layer) of the oven. The render is not begun until the brick and insulating layers are COMPLETELY cured and dry.
Yes, if the render was completed before this it would hold in moisture.
The moisture I reference is really minimal. The fire brick on the inside is fairly porous and will hold a little moisture between firings. Similar to a stale saltine cracker.
It takes considerably more time and wood to get it heated to pizza temps after long periods of interactivity. When it is kept dry (inside) I can clear the dome (1000+deg) in about an hour. I also fill the oven with wood when it has cooled to below 300 deg. to both "pre-season" the wood for the next firing and help keep the fire brick "drier".
I do the same with filling the warm oven with wood for the next firing session. I once threw wood in a still hot oven, shut the door, came back a couple of days later and found most had turned into coals. ...lol... I used them in my UDS.
I also went with the igloo design. Being in SoCal with its dry climate I did not worry about trapped moisture during my application of several coats of rendering. But as a precaution I left the very top of the dome unrendered to 'vent' any moisture, if any, trapped in the layers of ceramic blanket. I eventually closed off the top with render after several small fires.
For my final finish render I used surface bonding cement, or SBC (Quikrete Quikwall), applied in several stages. The texture blended well with the house as the oven was built merely inches away from the house.