Insulating the Combustion Zone
High Temperature insulation is an important component of an efficient wood burning stove or heater. Using insulation that is not rated for the application will result in premature failure. Here we discuss various insulation options and why some are suitable for wood burning stoves or heating appliances and some are not. All materials can be purchased off the shelf and do not require molding or casting.
As you can read in the more about Dragon rocket heaters section, efficiency is directly tied to keeping the combustion zones hot. The primary (burn tunnel) and secondary combustion chamber (the heat riser) must be well insulated to insure maximum combustion temperatures. The heat riser on the dragon burner is made of vermiculite board. The burn tunnel, though it is cast from an moderate insulative refractory, it requires an additional 2” (at a minimum) loose insulation to insure maximum burn temperatures.
Kiln and furnace designs require extremely well insulated chambers to capture the heat and raise the inside temperature to 2,000°F and higher. Any time you want to contain heat rather than release it into the surrounding space, you will need insulation.
Factors to Consider
In order to choose the correct insulating material for your application, you need to consider:
- Working and Melting temperatures
- Thermal conductivity when hot!
- Form factor and strength of material
Working and Melting Temperatures
Whatever insulation you choose must be able to withstand the temperatures at the given location. So it is important to know what the potential temperatures will be 1st. Using this information you can narrow or broaden your choices.
You will notice in the chart above, the working temperature may be different from the melting temperature. A working temperature is the temperature that a material can endure over an extended period without undergoing some other physical or chemical change. A material can loose viability without reaching the melting point. They can change structurally and permanently in some way when kept above their maximum working temperature. So it is important to go by the working temperatures and not the melting temperature.
Low Thermal Conductivity
This is the technical term for “how well does this transmit heat”. Metals have high thermal conductivity; fiberglass insulation is fairly low. Materials with low thermal conductivity prevent heat from being removed. The lower the thermal conductivity, the less insulation material is needed.
A complicating factor is that thermal conductivity in most materials is decreased as the temperature goes higher. In other words, the insulating material becomes less effective at wood-burning stove temperatures versus room temperature.
So when evaluating a material for suitability, check the thermal conductivity of the material at the potential temperatures to which it will be subject. This information is not always available but you can see from the chart below, it can make a big difference. For example, although at 25°C both vermiculite and ceramic blanket have a similar number, at 600°C the ceramic blanket is much more effective.
Many kiln references will suggest 5-8 times more vermiculite, for example than if a ceramic blanket is used.
Form Factor and Strength
Some materials on this chart are loose and must be contained in some way; for example, exfoliated vermiculite or perlite. Some come in a particular shape and can only be cut (vermiculite board, ceramic fiber paper, and calcium silicate board). Others can be molded by the user and dried or cured in place (clay slip with perlite added). Each of these form factors may have a place in your design.
A word about Perlite
Many rocket mass heater designs include a recommendation of clay slip with perlite. Clay can tolerate the temperatures created by an efficient wood-burning stove. However, it has high thermal conductivity (low insulation value). Adding the perlite (which can also tolerate high temperatures) makes the end result insulative. The clay dries and keeps the perlite fixed in a particular shape.
Types of Perlite
There are two types of perlite: masonry and horticultural. What is the difference between the two? Masonry perlite coated with silicone, which keeps water from getting trapped inside the perlite. Horticultural perlite is used precisely for the purpose of storing extra water; it does not have the silicone coating.
Consequently, masonry perlite is recommended for applications that will be exposed to water, such as when mixing with clay slip or it is used outdoors. Water trapped inside the perlite, when heated could theoretically cause steam and ruptures. Having said that, people who have more hands on experience with this issue than myself indicate they have not had issues using the horticultural perlite.
While it possible to treat vermiculite to resist water intake, it is not as commonly found as perlite treated the same way.
While it cheap and available it’s insulating properties derive primarily from the trapped air. But since there is no structure to maintain the trapped air it tends to settle and loose effectiveness. If you want to mix it with clay slip, I think the sawdust burned out of the slip would perform better, but I could not find in thermal conductivity test data to support any ash options. Perlite and Vermiculite are relatively inexpensive and offer better micro structures for insulation, it does not seem worth going with a sub par option.
Fiberglass and Rock wool
Fiberglass and rock wool have binders in them that limit their “working” temperatures. I could find no exact numbers showing this since. Rock wool does serious off -gassing at 400°F (not to be done inside). Check the MDS for the material you are using. Fiberglass should just be eliminated for consideration almost everywhere. I have seen it used around burn tunnels and heat risers, and as you can see from the chart its thermal numbers are way too low.
Rock wool, even though it has a high melting point, is never placed in extreme heat locations, only as a secondary insulator behind fire bricks. I suspect this is due to deterioration from the binders used in its manufacture but I don’t know for sure.
All the numbers shown are from various manufacturer's data sheets. Check out the data sheets for any product you are considering. The numbers can vary significantly from the charts on this blog. Also, be careful to compare apples with apples. Some vendors report thermal conductivity using BTUs and others Watts. It matters, they are not the same!
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