What Material Should You Use
Beneath Your Kamado-Style Cooker
On A Wood Table?


Introduction
When looking for an answer to the question "What should I place beneath my kamado cooker to protect the wood table it sits on?" you might be tempted to look to your manufacturer for an answer. However, only Big Green Egg seems to address the issue and has this on their website:

DO NOT PLACE AN EGG DIRECTLY ON A WOOD SURFACE OR ON OR NEAR ANY COMBUSTIBLE SURFACE! The bottom of the ceramic does generate significant heat when in use, and may cause combustion if placed directly on or in proximity to any combustible material. The EGG is designed to be used in a metal Nest or with the metal table Nest, allowing for an air gap below the EGG. Alternately, a concrete paver block may be used to support the EGG when used in a table. For maximum heat protection, a paver block and a metal table Nest can be used in combination.
All good advice, except possibly for the sentence highlited in red. We have seen numerous photos of damage to wood tables caused by the use of a concrete paver beneath a Big Green Egg cooker. Photos like these:

Damage to a wooden table when using a
2-inch thick octagonal concrete paver.

A wooden table that got charred despite
using a concrete paver.

So, what's the answer? One answer often repeated is to use firebrick instead of concrete pavers. But is this actually the best answer, or even a better answer? What other materials might be used besides a concrete paver? We decided to look into this in greater depth and were surprised at what we found.


What Materials Might We Use?
We decided to look into four different materials besides concrete pavers: refractory firebrick, kiln firebrick, vermiculite and calcium silicate insulation.

Firebricks are specially made high temperature bricks for use in fireplaces, circulators, stoves, barbecue grills, furnaces and inserts. They are made from ceramic material and, depending on the type, can take up to 3000 degrees. There are two types of fire bricks, heavy (refractory) and insulating (kiln). Here is what Wikipedia has to say about refractory fire bricks:
"A fire brick, firebrick, or refractory brick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperature, but will also usually have a low thermal conductivity for greater energy efficiency."
And here is what Wikipedia has to say about insulating fire bricks, also known as kiln bricks:
"[Kiln bricks] are weaker.... [and] much lighter, easier to form, and insulate far better than dense bricks. "
Another type of insulating brick that we will be looking at is vermiculite. Vermiculite bricks are often used in quality woodstoves. Here is what one manufacturer says about them:
"Vermiculite is a naturally occurring mineral formed into a pressed piece of board and then cut into shapes. Vermiculite fire bricks are non-combustible. A fire resistant product which is light and easy to cut with a wood saw, unlike the old heavy clay fire bricks which were in a lot of stoves. Vermiculite fire bricks have excellent thermal insulation properties and have minimal shrinkage at high temperatures. They are fire resistant, which make them an ideal product to use in your wood burning or multi-fuel stove, providing fire protection exactly where it is needed."
And finally, there is one last material we decided to test, calcium silicate insulation, suggested to us by the very helpful gentleman at Neuex Hearth Products who provided us with data sheets for some of the materials we were using. Again paraphrasing from Wikipedia:
"Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite. Calcium silicate is commonly used as a safe alternative to asbestos for high-temperature insulation materials. Industrial-grade piping and equipment insulation is often fabricated from calcium silicate."


Material Properties
Let's take a quick look at the various materials involved in this discussion.

Here are the weight and dimensions of the various materials we are testing. By the way, all types of fire bricks come in primarily two sizes—full bricks and splits that are half as thick— so we include the weights and dimensions of both sizes. Also, cement pavers generally come in one thickness, while calcium silicate boards come in a variety of thicknesses.


Split WeightFull Brick WeightSplit DimensionsFull Brick Dimensions
Concrete Paver15.6 pounds
12" x 12" x 1½"
Heavy Fire Brick3.8 pounds7.6 pounds9" x 4½" x 1¼"9" x 4½" x 2½"
Insulating Fire Brick1.2 pounds2.4 pounds9" x 4½" x 1¼"9" x 4½" x 2½"
Vermiculite Brick1.4 pounds2.8 pounds9" x 4½" x 1¼"9" x 4½" x 2½"
Calcium Silicate3.8 pounds
12" x 12" x 1"

Table 1. Weights and dimensions of various materials used in this project.


The next property we'll look at is the important one, thermal conductivity, i.e., how fast is heat conducted through the material. In the International System of Units (SI), thermal conductivity is measured in watts per meter-kelvin (W/(m·K)). We won't bore you with the details, but suffice it to say that the bigger the number when looking at thermal conductivity, the faster heat is conducted through the material. Conversely, the smaller the number, the better the material insulates.

Material  Thermal Conductivity  
  Refractory (Heavy) Fire Brick  
1.40 ²
  Concrete (Paver)
1.13 ¹
  Kiln (Insulating) Fire Brick
0.24 ³
  Vermiculite Brick
0.18
  Calcium Silicate
0.07

Table 2. Thermal conductivity of various materials used in this project.


So based on these values for thermal conductivity, we should expect to see the insulating property of these materials to increase as the value for thermal conductivity decreases. We'll see if this holds true when we present the data in a subsequent section.

Finally, here are some photos showing each of the materials we'll be testing.

Photo 1. From left to right, kiln (insulating), refractory (heavy) and vermiculite bricks.


Photo 2. From top to bottom, calcium silicate insulation board and a concrete paver.


Methodology
The idea here is not to actually measure thermal conductivity. That is a highly technical task which we are clearly not equipped to perform. However, we can get a feel for the relative thermal conductivities of these materials in order to pick the best one for placing underneath a ceramic cooker when sitting on a wooden surface.

To do this, we are simply going to place a thermocouple on one side of the test sample and subject the other side to heat. We can then see which material keeps the temperature the lowest. More specifically:

  1. We placed a concrete paver on the ground to serve as our base.
  2. Next we placed a wire thermocouple on top of the base.
  3. Then we placed the test material on top of the thermocouple and base.
  4. Finally, we placed a Weber chimney starter on the top of the test material and burned charcoal in it until the temperature measured by the thermocouple stopped rising.

Here are some photos of our test set up:

Now it is important to understand that we don't claim that the bottom of your ceramic cooker will get anywhere near as hot as the underside of our chimney starter. (Indeed, it won't. The temperature underneath the chimney starter was approaching 1250°F. The temperature of the bottom surface of your ceramic cooker will normally stay below 300°F) We aren't trying to measure the temperature that a wooden table would be exposed to. We are simply trying to determine which material keeps the underside coolest when the top side is exposed to heat. Using extreme input temperatures (the chimney starter) yields extreme output temperatures (the thermocouple under the test sample) which in turn exaggerates the differences between the different materials.


Observations
So, after several weeks of testing and burning several bags of charcoal in our chimney starter, it all comes down to this table. It shows the maximum temperature measured for each material and configuration that we tested.

MaterialThermal
Conductivity
Thickness
of Material
Maximum
Temperature (°F)
Refractory (Heavy) Fire Brick Split
1.40
1.25"
500.4°
Concrete Paver (1)
1.13
1.5"
403.5°
Kiln (Insulating) Fire Brick Split
0.24
1.25"
390.1°
Vermiculite Split (1)
0.18
1.25"
370.4°
Refractory (Heavy) Fire Brick
1.40
2.5"
324.9°
Calcium Silicate 1"
0.07
1.0"
323.9°
Kiln (Insulating) Fire Brick
0.24
2.5"
274.8°
Concrete Paver (2)
1.13
3.0"
253.5°
Vermiculite Split (2)
0.18
2.5"
224.4°
Calcium Silicate 2"
0.07
2.0"
210.4°

Table 3. Maximum temperature achieved for various materials.


And just for fun, we plotted the temperature data for all 10 tests in one magnificent spaghetti graph so you could also see how fast each material allowed heat to pass through it.

Graph 1. Graph of temperatures that would be experienced by wood protected by different materials.


Conclusions

  1. Clearly, refractory firebrick is not the panacea that everyone (us included) had thought it to be. A refractory firebrick split provides the least protection of everything we tested, and a whole refractory brick doesn't provide as much protection as two concrete pavers.
  2. Insulating kiln bricks also aren't as wonderful as we might have hoped. While clearly better than the heavier refractory firebrick, a kiln split is only slightly better than a concrete paver and a kiln brick is slightly worse than two concrete pavers.
  3. Vermiculite splits when used in a double layer provide excellent protection, better than anything else except for the calcium silicate boards.
  4. Two inches of calcium silicate board provides the best protection of all the materials and configurations we tested.


Recommendations
If you want to go for the gold, clearly 2" calcium silicate board provides the best protection. However, it isn't that much better than vermiculite bricks or even doubled up concrete pavers if all you look at is the ultimate final temperature. On the other hand, if you look at the graph showing the length of time it took for the calcium silicate board to get to its maximum temperature, it's clear than in the long run, calcium silicate will provide the best protection.

Vermiculite bricks were the next best option as far as protection is concerned. They are less expensive than calcium silicate board and can be sawn with a wood saw to allow you to cut them to size if needed.

As far as fire bricks are concerned, neither the refractory bricks or the kiln bricks provided a level of protection as good as that provided by two cement pavers, so they probably aren't worth the cost or trouble in obtaining them.

So, that brings us to those lowly concrete pavers. We know a single concrete paver isn't sufficient protection for a wood table as evidenced by the stories and photos we have seen. However, our testing shows that two concrete pavers gives almost the same protection as vermiculite bricks and calcium silicate board. And considering they are dirt cheap compared to anything else we tested, they would appear to be the best budget option.

Whatever material you choose to use beneath your cooker, you can increase your safety factor by also using a BGE Table Nest or ceramic feet on top of the material you choose. That layer of air between your cooker and your insulating material will help to keep the temperature down and reduce the possibility of damaging your table or even starting a fire.


What About Other Materials?
Owners of kamado-style cookers have at times used other materials such as slate pavers and ceramic tiles. How do these types of materials work for insulating a wood table? The following photos should provide the answer:

Ceramic tile that has shattered from the heat of the cooker.

Another ceramic tile that has shattered from the heat of the cooker.

A sheet of slate that has shattered from the heat of the cooker.


As you can see, these materials are totally unsuited for exposure to heat. We know from personal experience that slate is a poor choice. We had 3 sheets of slate crack just like is shown in the last photo. It is best to stick to materials made for exposure to heat such as the ones we tested in this article.


Sources For Materials
Regular fire bricks can usually be found at your local brickyard, Home Depot, Lowe's etc. Here is where we obtained some of the materials we used in this article:

Vermiculite splits – 9 for $44.00 from Neuex Hearth Products
Kiln (Insulating) fire brick splits – 6 for $39.95 from Amazon, Lynn Manufacturing
Kiln (Insulating) fire bricks – 4 for $32.90 from Amazon, All About Steel
Refractory (Heavy) fire brick splits – 6 for $36.99 from Ace Hardware, Rutland Products
Cement Pavers – $1.78 Lowes
Calcium Silicate Insulation – 12x12x1, $25.23 from Grainger. Also available from
     from Neuex Hearth Products who can provide custom sizes.


Citations

1. Field and Laboratory Assessment of Different Concrete Paving Materials Thermal Behavior,     Ivana Bariši´c, Ivanka Netinger Grubeša, Hrvoje Krsti´c and Dalibor Kubica, Sustainability 2022.

2. Heat Retention Qualities Of Fire Brick, Steven Colbath, HomeSteady web site

3. Ibid, note 2.

4. Thermax Boards SF (1100°C) Technical Specification, SF750 vermiculite technical data.

5. Thermal conductivity of calcium silicate insulation..., The Engineering ToolBox web site.


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