Heat break at arch

[Brads Build]

after reading the forums, we started our oven a Pompeii 39 inch. We closed in the dome yesterday and the floor is set. While checking the forums for the outer arch and chimney vent we saw that some people put a fire/heat break between the outer arch and the inner arch. How is it done, and is it necessary?

[Brads Build]

Re: Heat break at arch

To add a second question; cladding. I have a bucketfull of fireclay. Can I mix it with portland cement and sand to make a cladding to go on under the insulation blanket? If so, what quantities are recommended?

Thanks

[Greenman]

Re: Heat break at arch

The heat break may not be completely necessary but it does significantly help curtail the loss of heat from the dome via the outer arch and chimney vent when you have the oven sealed up.

There are a number of methods of filling the heat break, I used light walled stainless steel and vermicrete. Others have used a variety of insulation material, ceramic fibre rope and the like but whatever you use it should not be capable of dropping fibres or other material once you seal it. That is why I opted for stainless.

A perusal of the threads will give you a good overview of the variety of methods used.

In short, I think if you are looking to retain heat over a period of time then a thermal break is a very good thing to have. The differences in temperature of the hearth and the entry floor with a break is quite significant.

[NCMan]

Re: Heat break at arch

I agree w/Steve. Are heat breaks necessary? Not at all. They are, however, a good idea. One thing to remember when considering using them, is that they can and do also serve as a construction and expansion joint, placed in locations that are exactly where they should be to do the most good. As Steve mentioned, just be sure you have a plan to seal/caulk them (except when using stainless, etc.) after you're done and before using your oven. The joint in the hearth/floor does not have to be sealed. All you need there is a small gap, which can be left alone and fill up w/ash, if you like. Studying the Builds of others can help explain it further, if you go that route. Either way, best of luck w/your Build and enjoy!!

[Brads Build]

Re: Heat break at arch

Okay, thanks. So it does make sense then to try to incorporate a heat break. We are having a bit of trouble sourcing some materials here. For example, can't find hydrated lime anywhere to make ahome brew for the dome cladding, and at $60 a bag, heat stop is too expensive.

Have not found ceramic fiber rope nearby, so i am wondering if it would work to attach the vent area to the inner arch with vermicrete itself, or is there no structural strength to vermicrete? I will see if I can figure out how to post a picture ..

Thanks for your help, really appreciated.

[Brads Build]

Re: Heat break at arch

Okay, thanks. So it does make sense then to try to incorporate a heat break. We are having a bit of trouble sourcing some materials here. For example, can't find hydrated lime anywhere to make ahome brew for the dome cladding, and at $60 a bag, heat stop is too expensive.

Have not found ceramic fiber rope nearby, so i am wondering if it would work to attach the vent area to the inner arch with vermicrete itself, or is there no structural strength to vermicrete? I will see if I can figure out how to post a picture ..

Thanks for your help, really appreciated.

[Greenman]

Re: Heat break at arch

Vermicrete would be fine as a joint there. It doesn't have structural strength but there is no stress on this joint anyway.
The only issue would be holding it there while it hardened and loose bits falling out as the temperature cycles. I used vermicrete but had the s/s to hold back.

[Les]

Re: Heat break at arch

This has been discussed many times - I am one of the nay sayer's but it really does no harm if it makes you feel good. I posted this some time ago - I had a thermal list of this as well but would have to dig too deep to find it.

These bricks can absorb and retain heat pretty well, in regard to conduction, they are pretty poor. I have no doubt that the break will cause no harm (if constructed properly) but no one has posted empirical evidence that it helps. Any one with the break, please document the delta across the divide, with pictures so we can compare ( without the door on, that's just another variable) If the data supports the benefit, we should get it incorporated into the plans. Here is a test I did last week? https://www.youtube.com/watch?v=03Pwaf8NlOU

[david s]

Re: Heat break at arch

We go on and on about making sure the oven is insulated well yet fail to insulate around the mouth of the oven. The heat break is an attempt to reduce the heat loss there. By moving refractory bricks apart so they are no longer in contact goes a long way to reducing heat transfer by conduction, but a 10 mm insulation space would be considered totally inadequate if used under the floor in an attempt to retain heat. The same applies for a heat break to reduce heat loss from the oven. It works to a certain extent, but who would make it 4" wide ?

The big advantage is where it can act as an expansion joint, providing an air gap or flexible material is used in the space. Used in this way it will protect the outer arch or outer shell (if you're building an igloo style) from cracking due to expansion, allowing the inner parts of the oven to expand independently from the outer shell and decorative arch. If solid material is used to fill the gap then this advantage is lost.

[NCMan]

Re: Heat break at arch

Personally, I find that the majority of builders who are not big fans of heat breaks are....surprise......ones who did not use them. As David just said, we go to great lengths to insulate under the floor and around the domes, but yet we have people who refuse to acknowledge the (multiple)advantages of heat breaks to slow heat loss. They need "proof". Go figure......

[NCMan]

Re: Heat break at arch

Okay, thanks. So it does make sense then to try to incorporate a heat break.

Have not found ceramic fiber rope nearby

I found and ordered mine online.

[Brads Build]

Re: Heat break at arch

Thanks everyone. We have decided to compromise and have found a piece of stainless steel to place at the top of the joint, and will lay it on perlite or perlcrete. While one of us is working on that, the other is doing the cladding, using the homebrew 3:1:1:1 mix. As it is drying, it is cracking - any ideas? It is only about 1/2 inch to 1" thick, but it is a bit worrying as it is supposed to be extra structural layer between firebrick and insulation blanket.

[Volongo]

Re: Heat break at arch

IMO, I do not believe it is logical to compare the need to insulate the dome and hearth versus the mouth of the oven. The surface area of the mouth is insignificant compared to the dome and hearth.

I believe having a well insulated door is more important than a heat break.

The fire bricks have relatively poor thermal conductivity as Les pointed out above. So I do not believe it significantly effect the performance of the oven, especially if design/use to primarily make pizza.

But if you have the extra money for it then go for it

[david s]

Re: Heat break at arch

IMO, I do not believe it is logical to compare the need to insulate the dome and hearth versus the mouth of the oven. The surface area of the mouth is insignificant compared to the dome and hearth.

I believe having a well insulated door is more important than a heat break.

The fire bricks have relatively poor thermal conductivity as Les pointed out above. So I do not believe it significantly effect the performance of the oven, especially if design/use to primarily make pizza.

But if you have the extra money for it then go for it

It is not only the surface area of the mouth that would be in question. It also includes the whole of the surface area of the interior of the flue gallery plus the area of the floor in the entry as well (totalling a significant area), which is subject to ambient air temperature being cooled by cool air being sucked past it through the action of the still hot flue. The entry with its heavy mass can act as a heat sink if directly connected, meaning more energy required to heat it and more mass to suck heat out of the dome and floor via conduction.
But I do agree with you that a well insulated door is more important than a heat break in retaining oven heat for retained heat cooking. I don't understand why oven manufacturers offer heavy, highly conductive steel doors that are uninsulated.

In the case of my oven design my flue gallery is cast and only weighs 10kgs, so is less of a factor. That is part of the reason my heat break is between the flue gallery and the outer decorative arch.

[Greenman]

Re: Heat break at arch

Thanks everyone. While one of us is working on that, the other is doing the cladding, using the homebrew 3:1:1:1 mix. As it is drying, it is cracking - any ideas? It is only about 1/2 inch to 1" thick, but it is a bit worrying as it is supposed to be extra structural layer between firebrick and insulation blanket.

I wouldn't be too concerned about some cracks in the homebrew mortar over the dome. The dome is the structural strength and a layer of homebrew mortar over the top will only provide some additional thermal mass. Once the blanket goes over the top the only person who will know about them is you.

Crack on, you are doing fine. For what it is worth I agree totally with David s about the heat break and the door material. I have been using thin aluminium sheet stuffed with c/f blanket for doors and that works fine. It may be improved if you used the insulating board in the door but I am not sure. Others may have experience of that or have some technical knowledge to offer.