Perlite insulation at vent

[Randymoses]

I have seen some ovens built with a metal vent hood leading to a single wall chimney. That assembly was then surrounded by a 5:1 perlite/pc mix in a form so that the vent and part of the chimney was encased in it. Are there any concerns with that mix being in direct contact with the heat of a metal vent/chimney?

[Pizzarotic]

Expansion of the metal flue & hood will greatly exceed that of the render and it will fail.

[david s]

I encase the base of the flue pipe in about 3cm thick 5:1 vermicrete. It seems to work fine, as there is some give in vermicrete and my tall flue gallery means that area would be cooler than most other designs. What you can’t do is to render around the pipe with a stucco mix. The layer is too brittle and thin and will crack. But if you wrap some cardboard around the pipe, render up to it and remove the cardboard leaving a gap of around 4-5mm, it can be filled with high temperature silicone, which then prevents the render from cracking and allowing water entry.