Dome insulation

[BOOMERS WFO]

I am using CSR Bradford Fibertex 650 ceramic blanket for the insulation of my dome. It is 50 mm (2 inch) thick. When I completed the dome, I found that the original bag of insulation was not quite enough to cover the dome, so I had to order and extra one. Since getting the last bag, they have made changes to the manufacturing and the new one is wider and longer with foil backing, so now I have lots of blanket.

I was thinking that I will now have enough to do 2 layers of blanket -meaning an insulating layer of 100mm(4inch). The newer over the older held with wire & chicken mesh

If I go this path, do I still put a thin layer of vermiculite/cement over this before the render of the dome? Or do I just render over the blanket? I probably wont have a lot of room at the base of the dome to the edge of the hearth slab to go too thick on top of the blanket.

The dome will be exposed to weather. I would rather over engineer than go back and redo later.

I have used the waiting time for the blanket to cure my oven. Post to follow.

Cheers

Craig

[Les]

Re: Dome insulation

If you have 4 inches of blanket you do not need the vermiculite layer. If you have the room it obviously wouldn't hurt. As always stated, more is better...

[david s]

Re: Dome insulation

The main problem is that you lose the hemispherical form because of the folds and overlaps of the blanket. The vermicrete layer allows you to correct this so you can add a thinner stucco/render layer over it, achieving a more perfect hemispherical surface.

[cobblerdave]

Re: Dome insulation

The main problem is that you lose the hemispherical form because of the folds and overlaps of the blanket. The vermicrete layer allows you to correct this so you can add a thinner stucco/render layer over it, achieving a more perfect hemispherical surface.

The dreaded"Fred Flintstone" effect....some like it some don't

Regards Dave

[Bacterium]

Re: Dome insulation

I'm going to headed down this route as well Craig.

All, just wondering - if you lose some of your dome shape(due to extra blanket) can you then do a layer of cement or render to even it up, then do your 2coats of render?

The reason for suggesting this route would be to avoid vermicrete due to its water retention which may happen if you got render cracks in an exposed dome.

OR
Is there some kinda of render waterproofing coating that will not bubble if you got moisture under it.
Reason is on my first build this occured.....so did I just use the wrong coating over the top of render.

Hope this helps rather than hijacking this thread....

[BOOMERS WFO]

Re: Dome insulation

Thanks Les, David, Dave, and Damon.

I see the point about the finish of the dome. Its something you look at all the time, so if it doesnt look good !!!

Can one make a paper template as an overlay of the dome, and like a dressmaker, fit the blanket to the dome by cutting sections out to fit?
Or is keeping the blanket intact and folding the bits into creases to make it fit better?

My thoughts are that the fewer the joins the less heat loss throught the gaps.

Damon, I like the idea of using the render as the smoothing tool. I will have a chat to a friend who is a renderer to suggest any products that may suit. As the dome is exposed, I would like to keep water retention to a minimum.

Any suggestions from our learned colleagues would be appreciated.

Cheers

Craig

[david s]

Re: Dome insulation

Waterproofing the outer shell by adding a waterproofing additive is risky IMO because as well as keeping moisture out it keeps moisture in. If there is a siqnificant amount of water left in the insulation layer you risk cracking the outer shell. I think a better way is to eliminate the water from the insulation layer before doing the outer shell, then create the outer shell (keeping it moist for a week) and applying a waterproofing acrylic coating after about 10 decent firings.After all this, even when the oven does get wet again after weeks of rain it does not seem to be necessary to go right back to initial curing fires. Just a couple of long slow burns seems to be enough, or so I've found. Hope this helps.

[cobblerdave]

Re: Dome insulation

[QUOTE=david s;139262]Waterproofing the outer shell by adding a waterproofing additive is risky IMO because as well as keeping moisture out it keeps moisture in.
Gudday
David s is as always ..."right on the money" with this. Consider trying waterproofing till later.
For now yes build up you layers of render remembering to scratch each layer so the next will bond to the one underneath. As you get to the final layer thats smooth and round enought add a little more cement to the mix and trowel off with a smooth steel trowel . This is the same as troweling of a cement slab ....you have to wait till it gets 'firm' before you do this.
Now believe it or not ...if you find the trowel digs in and is hard to handle use a "bundy bottle" its got flat surfaces and round edges and will make your finish easy and smooth.
Finally ....again "David s... after the oven has 10 good firings" and you seal it with paint or whatever make it "Matt" not a gloss coat which will show up any imperfections. Hope something here will help and yes I have the "Flintstone" effect I'll have to fix ......Finish the kitchen first

Regards Dave

[david s]

Re: Dome insulation

If you do the render in layers, then each subsequent layer should contain more lime and sand than the previous one, ie it should be weaker. I actually only do one layer about 15 mm thick but you certainly can do more. You should also keep it moist for a week. I wrap the whole thing in cling wrap.

[cobblerdave]

Re: Dome insulation

Gudday David s
Interested to know about the importance of the builders lime has to the mix
I plan to tidy my dome a little recon it will come up better especially the tiling whitch I did after the render....should have done it first
Regards dave

[david s]

Re: Dome insulation

The lime in the mix gives the render some elasticity and also has the ability to "heal" tiny cracks in the render.

[Johnny the oven man]

Re: Dome insulation

Fibertex 650 is not ceramic fibre, it is rockwool. Loves to suck up moisture.

[cobblerdave]

Re: Dome insulation

Gudday Johnny the oven man
I know you have 30 yrs experience in the refractory industry so I'm interested in knowing more about why Rockwool loves to suck up water?
In my limited experience I have found that firebrick ceramic insulation render in fact all materals in my oven will soak up moisture even from the humidity in the air


Regards Dave

[Johnny the oven man]

Re: Dome insulation

Dave, c/fibre dries out better than rockwool. The batts just dont really recover very well after soaking. Fibre is as good as gold after immersion, dries out to its original configuration, probably becuase it goes through a process called "needling" during its manufacture. Yes I know that all the insulation materials do absorb water, they are all porous.