Is it too late to add a thermal blanket to my landing area?

[ScottABreaux]

Good Morning,
I built a small perlite oven during covid, (36" interior dome), but at the time I was uncertain about the diameter of the chimney. I used a 3" vent pipe since that was what was readily available at the time, and hoped that if i made it tall enough the draw would compensate for the smaller diameter pipe. Since I wasn't sure if it would work, I didn't put the thermal blanket over the landing area in case I would have to enlarge the hole. I covered the whole dome and landing with Quikrete's One-Coat Fiberglass Reinforced Stucco and painted the whole thing with an elastomeric paint to help keep the water out.

Now 3+ years later the stucco cracked over the landing area, from the chimney to the opening. While I have to repair it anyway, I went ahead and increased the chimney opening to accommodate a 6" chimney. (Btw, the fiberglass stucco is solid as a rock.)

Is it too late to add the insulation blanket over the landing while I have to repair the stucco anyway. If it is not too late, do I need to remove the stucco before adding the blanket?

I'd appreciate any guidance. Thanks, Scott

[david s]

Since you’ve already enlarged the hole you may as well remove the rest of the stucco over the entry and then add blanket or a very lean perlcrete mix to insulate it. A 125 mm angle grinder fitted with a diamond blade works very well for this operation. For fine tuning swap the diamond blade with a coarse flap disc.
often the cause of cracks around the flue pipe is no provision for the pipes expansion. Making the hole slightly larger and wrapping some cardboard around the pipe, stucco up to that, remove cardboard and fill the gap with high temp silicone is a god remedy.
Not sure what mix you used for the inner casting, but perlite drastically reduces both thermal mass and strength, leaving the surface prone to damage and wear as well as lowering the ability to store heat. Too late to change that obviously.