Heat break. To do or not to do

[plokuta]

I'm in the process of closing my dome and have designed the vent. Should I or should I not install a heat break? I still haven't fully grasped the benefits of this.

Any help would be appreciated.

Thanks

[UtahBeehiver]

It depends on what your WFO cooking goals are. If you are just doing pizzas or maybe a roast the next day then calling it good then shutting down the oven then you probably don't need a thermal break. If you want several days worth of cooking, then preserving your heat then thermal breaks are a good option. So it is up to you, but good insulation on the dome and under floor are key and more important than thermal breaks.

[JRPizza]

I too searched to find a definitive answer to the benefits of a thermal break, but that would probably require building two identical ovens (one with and one without), firing them to the same temperature, and taking measurements, or a high powered thermal analysis. A builder just has to decide how important it is to do everything practical to make the oven efficient. Like Russell said if you aren't after maximum baking times, or you have an unlimited supply of free wood, you probably don't need one. That said, breaks can be as simple as a gap in the floor bricks that fills with ash and a bit of gasket around the arch, so in the grand scheme of building my oven the extra few hours of cutting the arch bricks to provide a gap and hold my fiber rope was a drop in the bucket.

[plokuta]

Thanks. I plan on using it on a regular basis but no hard core bread baking. I was thinking that if I go with a break that I would built the inner arch 1/4" away from the dome arch and stuff the gap with rope. I would love to avoid as much cutting as possible.

[Gulf]

Another way to get the heat break without all the "L cuts" would be to set the outer arch back and over over the inner arch. The gap is between the outer radius of the inner arch and the inner radius of the outer arch. Same amount of gap, with a larger reveal, making the oven more accessible much like the flared entry design does without all the intimidation . Doing that would also allow for a slightly shallower entry.

[david s]

Remember that if you plan on using a thermal break in an effort to retain heat it's not all that effective. Iff you only had 1/4" insulation under your floor or over your dome you wouldn't expect it to be adequate in preventing much heat loss. The small gap will slow heat by conduction but plenty of radiant heat will travel that 1/4" easily. Its second function however is to act as an expansion joint allowing the inner parts of the oven to expand without placing stress on the cooler and unexpanded outer decorative arch and igloo shell (if the build uses that style). In ovens without this expansion joint, I've seen enough big cracks in outer arches and dome shells to convince me that it's a good idea to design and build it this way. It's more trouble but very effective. For this expansion joint to work it needs to be filled with material that is somewhat elastic e.g. fibreglass rope, lean vermicrete or perlcrete, CFB etc.Filling the gap with a thick walled stainless pipe may look fancy but it's extremely conductive and quite inflexible.

[plokuta]

I was thinking more aligns the expansion as well. I'm afraid of an outer arch or vent crack A quarter inch should be good and stuffed with rope.

[allegro]

I have two related questions regarding the gap size and insulation material for the thermal break between inner and outer arches, as well as what people do with a floor heat break. In my case I'm working on a 33" oven and would love to give this thing a fighting chance at retaining good heat over multiple days.

Regarding the thermal break between inner and outer arches, what would you all think about a 1" break that would allow me to stuff in a layer of FB blanket (or in my case, 1" superwool) between inner and outer arches, probably sealing on the inside with 1" woodstove gasket rope and caulk to keep nasty fibers away from food?

And what about a thermal break in the floor? I can't find many specifics in the forum regarding floor heat breaks beyond recommendations to avoid designing anything with much of a profile since sliding food in and out of the oven will be a pain if there's a big gap or a rise/fall. Do people tend to just leave a small gap between oven floor and vent floor? Stuff it with rope or other insulation?

[david s]

Most folk want to use a thermal break in an effort to reduce heat loss. Unfortunately a small gap (of around 8 mm in my design) is obviously not particularly adequate in reducing a substantial amount of heat loss. If you only had 8mm of underfloor or over dome insulation you would not have a particularly well insulated oven. So, obviously 1” is far better. By reducing contact you reduce heat transfer by conduction, but radiant heat will still jump a fair gap. The other advantage and a more important one in my opinion is that the gap acts as an expansion joint, provided it is made of a flexible material. Some folk have used a stainless tube in the gap on the floor. It looks nice but is highly conductive and not flexible. CFB is a good choice here and it’s flexibility will protect the cooler outer parts of the oven from the stress of the expanding inner parts. As you rightly point out the CFB will need to be encapsulated, especially if you have used the older style blanket that has not been exonerated as a class 2 carcinogen. (Tumours in lab rats but none detected in humans)
Apologies for the extended reply.

[plokuta]

Thanks for the reply. I was thinking more along the lines of isolating the dome since it will expand. I was going to use ceramic rope to fill the gap which is about a qtr inch didn't think about blanket. Is there a flexible material like a caulk (food safe) that I can seal the gap over the rope/ blanket? I was going to let ash fill the gap on the transition of the floor from the oven to the landing.

[david s]

All the caulks that i’ve tried go hard when they’re heated. The exception is high temp silicon, but the highest rated one I can find is Permatex ultra copper. If used between the oven and gallery though the temperatures it will see there will kill it. It’s ok to use it to seal between the stainless fluepipe and outer render/stucco. If you find a material that is still flexible at high temps let me know please.

[allegro]

I'm leaning toward trying to make a 1" heat break so I can fill it with either insulating blanket or board left over from the build and really isolate the dome from the entry and flue area. Anyone know of any examples of a heat break this large, and possible methods to cover the gap in both the floor and between inner and outer arches?

I wonder if a thin metal threshold could work, assuming it's designed such that it "floats" and is able to expand independently of the brick around it. I'm thinking of a design akin to thresholds that cover expansion gaps in laminate flooring. Greatly appreciate the help, and happy to start a new thread for this plokuta if I'm hijaking / taking this in an unhelpful direction given your original question!

[Toomulla]

I have never understood the thinking of this " heat break" concept, and fail to understand how it will prevent transfer of heat by conduction, convection or radiation. The prime loss of heat for my oven is the door which is steel insulated with four inches of ceramic board. I know of no proven commercial example of such construction can someone post an example that has published proof of the concept

[david s]

Heat travels by conduction through material according to its thermal conductivity. Therefore if a conductive material is separated so that air or a fair proportion of air in the insulating material used in the gap, then it’s travel is reduced substantially. Heat travel by convection will be reduced substantially because of the insulating material and to a lesser extent by radiant heat. Dense brick is actually not particularly conductive compared to most metals (see link). Therefore making steel a poor choice in any area where you are trying to slow heat transfer.

#6


https://www.engineeringtoolbox.com/index.html

[david s]

I'm leaning toward trying to make a 1" heat break so I can fill it with either insulating blanket or board left over from the build and really isolate the dome from the entry and flue area. Anyone know of any examples of a heat break this large, and possible methods to cover the gap in both the floor and between inner and outer arches?

I wonder if a thin metal threshold could work, assuming it's designed such that it "floats" and is able to expand independently of the brick around it. I'm thinking of a design akin to thresholds that cover expansion gaps in laminate flooring. Greatly appreciate the help, and happy to start a new thread for this plokuta if I'm hijaking / taking this in an unhelpful direction given your original question!

Thin metal is a possibility and stainless is quite a bit less conductive that normal mild steel or aluminium. In order to reduce conduction make it pretty thin, say 0.55 mm.