EIFS or Stucco: How hot does the outside of the oven get?

I am in the final stages of finishing my Cortile Barile oven: four layers of ceramic blanket under chicken wire then stucco. The four layers of blanket are because it gets cold in Upstate NY. The question is: how hot the outside of the oven gets through four layers of blanket then stucco? If the answer is rarely past 150 degrees F, then I can finish with synthetic cement (EIFS) and avoid a lot of cracking and repair. If it gets to 200 degrees or higher, then I will stay with cement. Answers?

Hi Laurence,

I am in southern Connecticut, and my Mattone Barile similarly has four layers of insulation.

In my experience, the stucco shell does not gain beyond the ambient temperature. I wasn’t so sure of this after my first couple of bakes last fall, but it turned out that warmth on the south side of the oven was from solar radiation (external) rather than thermal conduction (internal).

So in my experience, with an exposed stucco surface painted dark brown, the outside rarely gets above 100°F. A lighter color of paint will help, and where you are located will help even more.

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Thank you. Very helpful and reassuring. I have the first layer of stuck done. Not bad. But I could not get the stucco mix to stick on the underside of the oven archway. It doesn’t want to hang upside down. Unless you have some advice, I plan to construct a chicken wire lathe to fit the archway, then support that with plastic-wrap-coated cardboard, layer the stucco cement onto of that and stick up there, supported with some wood posts for a few days. Otherwise the underside of the archway is just exposed ceramic blanket with chicken wire.

You’re welcome, Laurence.

As for this:

Are you saying that the insulation is wrapped in and under the opening of the oven? You definitely do not want that. The ends of the arch bricks should be fully exposed, and the stucco should “reach around” to touch, but not cover, the exposed fire brick.

If I’m understanding this correctly, you need to do a fix, but it is an easy one. Use your utility knife, gloved hands, eye protection and mask. First, remove any stucco you placed on the arch ends. (It should come off easily; if it is enmeshed in chicken wire matrix, use wire cutters to trim the matrix back to the edges of the firebrick arch ends.) Then, use the utility knife to trim the insulation back to that same boundary.

Your matrix ends should be sticking out over the exposed arch ends just a bit. Redo the stucco so that it comes to the brick ends; let it cure. Then do the second (and third if you’re doing three) finishing layer so that your insulation is fully sealed.

Rationale: if you leave it as is, the stucco will quickly disintegrate once you start curing fires. It will flake off and drop into whatever you’re cooking. Worse, the mineral fiber will do the same. Mortar is crunchy, but mineral fiber is both mechanically dangerous and potentially carcinogenic. It is one of the best materials for insulation outside your oven, and that’s where you want to keep it.

Thank you for this explanation, Matt. But, no, you did not understand this correctly. I am not bringing the ceramic blanket nor the stucco into the oven or under the refractory arch (this is the Cortile Barile so there is no fire brick arch.). As per the directions, the ceramic blanket and chicken wire come to about 1-1.5 inches shy of the refectory opening. But I am using three layers of ceramic blanket. So the cut edge of the blanket and the chicken wire lathe on either side and above the oven arch (I’ll call it the “blanket-lathe arch”) is about 2 inches thick. Coating the sides of the blanket-lathe arch with stucco was okay, but coating the underside of it (about an inch above the refractory opening) from the front of the blanket to where is abuts the front oven wall was gravity-defying. If I did not seal this with stucco, the ceramic blanket would be exposed on the underside of the of the blanket-lathe arch.

I did what I described in my last post. I made a plastic wrap-coated triple-thickness corrugated cardboard arch support. I coated the top with stucco/mortar, stuck it into the underside of blanket-lathe arch and supported it with three wooden posts so if would not fall. Will look at it in 48 hours.

Just to be clear, all of this is in front of the oven opening. Not in it.

Make sense? Problem?

Thanks for the clarification, Laurence.

How did the support come out?

(No words are wasted, even if mine didn’t apply here. :slight_smile: )

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FWIW, I ran a pretty hot fire for 5-6 hours this weekend. It was a bright, sunny day with temperatures hitting close to 90. I shot the outside of the dome with my thermometer and read between 140-145 degrees. I have three layer of the glass insulation. I can’t say how much of that was solar load and how much was from the fire but I do believe it’s pretty close to a worse case number for my location.

I’m guessing you’ll see similar results in NY. Arizona might be different…

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