A Very Big Mistake

A couple weekends ago, after I loaded my eighth and final load of bread, I noticed smoke coming out of the back of the oven. I quickly got the cordless drill and took the back panel off. I couldn't see anything, and therefore did not take much action for the next 10 minutes. Why was it smoking? I then proceeded to take the fiberglass insulation out of the oven (I hesitated because I knew I wouldn't be able to get it back in without taking the roof off, and I definitely did not want to do that) and was introduced to the flame which was burning up by the chimney. The fire extinguisher was emptied on it. Thankfully the damage was not too great, but a big lesson was learned.

When I built the oven, I seemed to overlook the issue of ventilating the oven, and separating the ventilation from the insulation. By ventilating the oven well, it would have prevented any wood (even the wood close to the chimney) from getting hot enough to combust. This would have been solved by putting in a ridge vent (which we ended up installing after the fire). But even if we did put in the ridge vent from the start, our fatal flaw was worse than not ventilating the oven.

The main thing we did wrong was to put a wooded 2x4 to tie together two rafters which ran right through the insulation close to the chimney and concrete cladding. This 2x4 had constant high heat surrounding it, trapped with the insulation. The wood dried up over the 10 months and eventually smoldered and combusted.

How we solved it: We moved all the wood at least 2 inches away from the chimney. We replaced that 2x4 which caught on fire with a metal tie. We installed a ridge vent and increased air flow from the eaves of the roof. We replaced all the fiberglass insulation with vermiculite so that no insulation would be touching any wood. There will now be enough airflow to prevent the wood from getting much warmer than the ambient air temperature, and the insulation is greater so that the oven actually stays hot longer.

We removed all the fiberglass and pushed aside the vermiculite so we could get in the oven to repair it. I was absolutely delighted, and quite honestly stunned, to see that there are no expansion cracks in the concrete cladding at all! Seems to me that refractory cement would certainly be overkill for any personal purposes, especially since we are firing it very hot for near commercial purposes twice a week and it is still in excellent condition.