Wood fired oven insulation

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A wood fired pizza oven requires an insulation lay on top on the initial internal dome of bricks and under the hearth so as to improve the heat retaining properties of the oven. The insulation can be made out of a number of different materials which are refractory heat resistant and have insulation properties. Generally the insulation is either fire blanket which is a material usually used with industrial ovens, a loose mixture of insulating material which is loosely packed around the oven, or an insulating mortar which is a mixture of cement and vermiculite or pertite. Insulating fire bricks may sometimes be used however these are generally utilised for hobby kilns or industrial ovens.

Heat generated in the oven vault is absorbed by the walls which are generally made of either fire brick or cob. Fire brick in particular is excellent at absorbing the heat and radiating it back into the oven. To prevent the bricks radiating the heat in the opposite direction into the open environment, insulation must be placed on the outside of the oven wall to prevent this dissipation. With the added insulation the oven is able to remain hot for a long period of time. If the oven is built without insulation the time required to heat the vault will be increased and the heat generated by the fire will quickly diminish.

Insulation Type[edit]

  • Perlite or vermiculite can be used for dry and loose form of heat insulation.
  • Ceramic fiber blankets.
  • Perlite and vermiculite cement under the hearth or over the dome.
  • Insulating fire bricks

Under Hearth Insulation[edit]

Insulation is required under the oven hearth in addition to over the oven dome. Although heat generally rises, heat will be drawn down through the hearth particularly if the foundation is a solid material rather than gravel. The hearth insulation is placed upon the oven foundation which is generally a reinforced concrete slab.

Perlite or Vermiculite Cement[edit]

  1. Build a simple temporary wood structure on the foundation to contain the cement insulation until it has dried and cured. The cement insulation should be build to a thickness of at least 4inches (10cm).
  2. Create a mixture of vermiculite or perlite and cement in the ratio of 5 : 1. Mix the materials with a small amount of water in a wheelbarrow until consistently wet. As with dome insulation of cement and perlite or vermiculite, do not use a cement mixer as it may damage the materials.
  3. Place the cement mixture with reinforcing steel on the oven foundation in the wood contained area. Make the cement as level as possible with the use of a builder trowel.
  4. Once the insulation cement has dried remove the temporary wood structure.
  5. Upon this foundation cement the hearth can now be constructed.
Portland Cement Perlite or Vermiculite
1 part 5 parts

Insulating Fire Bricks[edit]

  1. Using fire clay mortar lay insulating fire bricks directly upon the level oven foundation.
  2. Allow the mortar to dry and cure.
  3. The oven hearth can be constructed directly upon the insulating fire bricks.

Glass Bottles[edit]

For cob pizza ovens a cheaper alternative to under hearth insulation is often sought. In this instance many builders will utilise empty glass bottles laid down to create the necessary air space. The glass bottles are then covered with either cob or sand. The hearth of fire bricks or clay is built directly upon the insulation.

Dome Insulation[edit]

Once the internal brick wall is finished construction of the insulation layer may begin. Insulation is required to ensure the oven retains heat so as to rise to a high temperature and maintain the temperature for a long period. Heat up times are important both from the perspective of speed of use and from cost of operations through firewood expenditure. The below table gives an indication of insulation thickness, heat up and cool down times for an oven constructed with fire brick and vermiculite cement as the insulation.

Internal Dome Thickness Insulation Thickness Heat Up Duration Cool Down Duration
4" (10cm) 4" (10cm) 1 hour 12 hours
4" (10cm) 2" (5cm) 2 hour 6 hours
2" (10cm) 4" (5cm) 30 minutes 4 hours
6" (15cm) 6" (15cm) 2 hour 24 hours

Fire Blanket Insulation[edit]

Fire blanket is a heat resistant material commonly used for industrial ovens. The material comes in many different grades depending on the properties required such as the heat tolerance and insulating performance. The lowest grade available is generally sufficient for wood fired ovens as the temperatures realised although high are not extreme. Higher grades are used for industrial application with extreme temperatures which will not be found the the traditional oven. Ceramic insulation due to its effectiveness is generally the more expensive option. Depending on the grade of the insulation the blankets may cost around $50 for a 13mm x 610mm x 1460mm piece. Due to the quality however the thickness of the insulation layer may be smaller than other options. A thickness of 1 to 2 inches (2.5 to 5cm) is recommended. Personal protection is also recommended due to the hazardous danger the material creates when handled. Small sharp dust particles may be created which are damaging to hands, eyes, lungs and irritating to the skin. For this reason it is recommended that when installing the user should ware a long sleeved shirt, a respiratory mask and eye protection.

  1. Cut a piece of fire blanket that may be placed around the circumference of the oven.
  2. Cut a similar sized piece of chicken wire and place that oven the blanket to hold it into place.
  3. Use wire to hold the whole structure stable.
  4. Continue to cover the oven in a minimum 1 inch layer of insulation.
  5. Be careful when placing additional layers of material on top of the ceramic insulation to not compact or squash the blanket. The insulating properties of the blanket are created by the air held within. If the material is compacted the ability for the blanket to function as intended is reduced. The insulation will need to be covered in a final cladding layer however particular care must be taken when applying wet cement or mortar.

Perlite or Vermiculite Cement[edit]

Perlite and vermiculite are products commonly used in horticulture and hydroponics. The material also has good insulation properties. When mixing the materials do not utilise a regular builders cement mixer. The harsh mechanical action of mixer is likely to damage the soft insulating particles.

  1. Cover the oven dome in aluminium foil. The foil is to prevent the existing oven structure from drawing water from the cement mix and hastening the curing process.
  2. Mix perlite or vermiculite with cement and lime at the appropriate rate (13:1:1) with some water to create the insulating material.
  3. Place a layer of about 1 inch (2.5cm) of the cement insulation material over the brick or on top of the fire blanket. If placing on insulating blanket additional care must be taken not to compress the blanket.
  4. Let the insulating layer harden and then apply additional layers.
  5. Build up the insulation to a minimum of 4 inches. This will require multiple applications between drying.
Portland Cement Perlite or Vermiculite Lime
1 part 13 parts 1 part

Loose Materials Such as Perlite, Vermiculite or Ceramic Fiber[edit]

Perlite, vermiculite or ceramic fiber pieces may be used in its loose form rather than as part of a cement mix if desired. Disadvantages of this approach exist in that the material may be messy or escape if small gaps exist in the containing structure. This approach is generally best used only if the oven dome will be enclosed in a rectangular brick walled structure. Benefits of this approach is that the oven dome will be included within a walled brick structure which provides protection from the weather and in particular water. Ideally the loose material will create an insulating layer of 4 inches (10cm) thick or larger. Vermiculite may be purchased from gardening or hydroponics suppliers for around $20 for a 100 liter bag.

  1. Once the oven dome is complete, construct a four walled brick structure around the dome. The walls will need to surround the dome and chimney entirely without any gaps through which the insulating material might escape. Ensure the walls are at least 4inches (10cm) away from the inner dome. Additionally the walls will need to be sufficiently high to allow 4 inches (10cm) of insulating material to protrude above the oven dome.
  2. Pour vermiculite, perlite or other refractory insulating materials into the space created between the outer walls and the oven vault walls. It is recommended that the insulating material is of a middle grade which is easy to handle, neither powder nor extremely large pieces. Ensure that no material is escaping into the oven vault, the chimney, or outside the containing outer walls.
  3. A roof will need to be constructed to contain the insulating material and through which the chimney will protrude. Ideally the roof will comprise of a tiled structure however this will add to the build costs.

Insulating Fire Bricks[edit]

Insulating fire brick is a form of fire brick which is less dense and comprises of many small air bubbles which provide insulating properties. As with fire brick, insulating fire bricks are a purpose built refractory products which may withstand very high heat without suffering structural damage. The insulating bricks will be laid directly on top of the existing fire brick dome.

  1. Starting from the bottom of the dome laying a single layer or bricks directly up against the dome.
  2. Ideally utilise fire clay mortar between the laid insulating bricks and between the insulating bricks and the existing oven dome.
  3. Continue laying bricks to cover the existing oven dome.
  4. Insulating bricks may be cut with a hand saw if a smaller piece is required for the capping stone.
  5. A single layer of insulating fire bricks with a final cladding layer will generally be sufficient. Additional insulation in the form of insulating fire bricks or fire blanket may be added if desired.

Obtaining the Insulation Material[edit]

Insulating materials are available from different suppliers depending upon the material sought and the locality of the construction. In general materials like perlite and vermiculite are available from hardware, gardening or hydroponics suppliers due to the use of the material in plant cultivation. Materials such as insulating fire brick or ceramic blanket are available from hardware or refractory product suppliers. These materials are generally used in the construction of fireplaces or kilns.

Comments[edit]


Anonymous user #1

9 months ago
Score 0+-
Building a cob oven and keep reading conflicting instructions online about using tin foil in the dome....I've read the oven must "breathe" therefore don't use foil...and I've read to USE it for the reason you list above, as well as for heat retention? Help?

Anonymous user #2

9 months ago
Score 0+-

Hi

I'm building a pizza oven and have also read the above but also that you shouldn't cover it. I think I'll leave the foil out but cover the vermiculite in slightly damp clothes to stop the moisture being drawn out too quickly. You can also spray it using a plant sprayer to slow the cure down. I intend to mosaic mine and will be applying 3 coats of exterior insulation. I'm not sure how long this will take to dry. But as with the rest of the build I'll just go with my gut.Good luck with your project 😊

Anonymous user #3

one month ago
Score 0+-

Hello,

I have extra ceramic blankets I can use on the top of the dome, can I use all the extra to make it more insulated?
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