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 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.
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.
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.
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.
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.
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.
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.
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.
hi should i mix Perlite with fire cement and put a layer directly on the outside of the oven bricks
That is correct, but I don't think it has to be fire cement because this layer is not strength-critical.
Hi, we have made a dome using vermiculite mixed with cement. Can I now just mortar over the top of this or will the mortar crack with the heat?