What thermal mass materials are best for DIY masonry heaters?

Choosing the Right Thermal Mass Material

When selecting a thermal mass material for your DIY masonry heater, consider factors such as heat retention, durability, and ease of use. Firebrick is a popular choice due to its high heat capacity and resistance to thermal shock, but it can be heavy and difficult to work with. A 2-inch thick layer of firebrick can retain heat for several hours, but the weight and installation costs may be prohibitive for smaller heaters.

Mixing and Applying Refractory Concrete

Refractory concrete is a cost-effective alternative to firebrick, offering similar heat retention properties and ease of use. To create a refractory concrete mix, combine 1 part refractory aggregate (such as silica or alumina) with 1 part cement and 1 part sand. Add water gradually to achieve a workable consistency, then apply the mix to the heater’s interior surface using a trowel or brush. A 2-inch layer of refractory concrete can be sufficient for small to medium-sized heaters.

Working with Castable Refractory

Castable refractory is a pre-mixed material that can be poured into place, making it ideal for complex or hard-to-reach areas of the heater. To cast a refractory material, mix the contents of the bag according to the manufacturer’s instructions, then pour the mixture into the desired shape. Use a trowel or brush to smooth the surface and remove any air pockets. A 1-inch layer of castable refractory can provide sufficient thermal mass for small heaters, but may require additional support or reinforcement.