Protect the rendering around the flue from heat and rain!

[BittenOffMoreThanIcanChew]

Hi All

I’d really like some advice on how to proceed with the following.

I’ve created flanges and screwed a 6 inch, single wall stainless steel flue to my brick arch. I stupidlyfilled the gaps and remaining space with fire mortar rather than a vermiculite cement mix. However, I did insulate over the area with a vermiculite cement mix (12-1 ratio). The insulation blanket comes up to the flue area, but not beyond it.

The plan now is to render the dome and arch area using a perlite (water repellent grade) render mix using the following ratio: perlite, sand, cement, hydrated lime: 5,1,1,1.
However, I’m doubting whether this mix will stand up the ferocious heat of the flue. Would I be best to ‘join’ the perlite render to the flue, using a vermiculite cement mix? I had thought about using a high heat sealant as a final ‘ring’ of protection and painting the vermiculite cement (and perlite render) with elastomeric paint.


Do you think this would work? Does anyone have any alternative suggestions, please?


A secondary concern is should I be sealing/ waterproofing my decorative arch against the elements?
My concern stems from anticipating that moisture may seep into the second arch firebrick vent area, which is obviously an area that I want to keep dry. There has been some slight cracking that extends from the flue to the decorative arch, due to lack of containment of heat***. Yes, I’ve cured the oven over 5 days’ worth of firing, reaching approx. 450 C.

***This is something that I will address properly next year, during the limited 2 week Scottish Summer. Unfortunately, it’s going to be a major job… but it will allow me to strategically address the smoke coming out of the mouth of the oven.

In the meantime, I’ve added another couple of inches of vermiculite cement. I’ve also read a little bit about freeze-thaw, which has me slightly worried.
Pics to follow!

Thanks in advance.
Tom

[fox]

Hi Tom, I am a bit confused, did you build a brick oven?
Did you cover the oven with insulation ie ceramic fibre?
You want to cover the insulation with a waterproof perlite mix?
if you used a single wall chimney then you need to allow for expansion.

[BittenOffMoreThanIcanChew]

Hi Tom, I am a bit confused, did you build a brick oven?
Did you cover the oven with insulation ie ceramic fibre?
You want to cover the insulation with a waterproof perlite mix?
if you used a single wall chimney then you need to allow for expansion.

Hi Fox

Thanks for your reply. The answer is yes to all three questions. How would you allow for expansion whilst protecting against the weather?

Tom

[fox]

Well I use twin wall chimney flues on the ones I built now and then cement the top coat right up tight.
On my first build I placed ceramic fibre around the pipe and slid two clay flower pot over the top.
For the top coat over the main dome insulation I use 4-1 cement mix over a 3-1 cement mix with poly fibres in the mix.
Then I coat with weather shield house paint.

[BittenOffMoreThanIcanChew]

Well I use twin wall chimney flues on the ones I built now and then cement the top coat right up tight.
On my first build I placed ceramic fibre around the pipe and slid two clay flower pot over the top.
For the top coat over the main dome insulation I use 4-1 cement mix over a 3-1 cement mix with poly fibres in the mix.
Then I coat with weather shield house paint.

Nice. What did you use to seal between the chimney pots and the single wall flue?

if I ever build another oven... I’ll look into using a double walled flue.

[fox]

I wrapped the outside of the pipe with ceramic fibre and slid over the pots,I sealed the top and bottom with heat resistant silicone,
i have since added tiles around the jointed areas.

[david s]

Here is a typical problem when using a single wall and not a llowing for expansion that I've seen more times than i can count. If you wrap some cardboard around the pipe, render up to it, the remove the cardboard after the render has set, leaving a 1/4" gap, it can ten be filled with high temp silicon. It is advisable to shape some more 10:1 vermicrete around the pipe so the silicon joint will be higher and the pipe a little cooler if higher up. it also provides better support for the flue pipe (see pic on my oven). The high temp silicon will not stand the temp if you have flames leaping up the flue pipe.

[david s]

Here's a pic of what I meant.

[BittenOffMoreThanIcanChew]

Here's a pic of what I meant.

This is exactly what has happened, again. Despite adding more insulation before I rendered.