I am building a pizza oven into the slope of my garden. I need a DPM under and at the back retaining wall with a drain, not sure how or what to use. Temperature inside are high but after insulation is steam heat max. I have just laid a concrete slab, no DPM yet. The oven usually has a standard double wall construction with insulation sandwich. The inner refectory bricks will be loose laid on the slab onto a layer of insulation so a DPM is possible, temperatures tthere up to 100degC. I need to build a small retaining wall which I imagine would also be the external wall of my oven? The oven is a brick dome wrapped in ceramic blanket insulation. Advice too for the outer stucco layer to keep all snug and warm inside? how do I prevent damp in a short retaining garden wall