In general, Heat Stop premix products - which includes Domestic Fireplace Mortar, a similar product to Heat Stop premix, only beige in color - should be applied with a maximum joint thickness of 1/16 - 1/8". Heat Stop II and Heat Stop 50, which are dry mix products, should also be applied with a thickness of 1/16 - 1/8" but with these dry mixes a thicker joint can be used if desired. For each of these refractory mortar products, installers will need approximately 50 lbs. of product to lay up 100 firebricks (assuming a joint thickness of 1/16 - 1/8").
Requirements for refractory castable Heat Stop products are different. For example, Heat Cast 40, a product used to cast shapes or used as a replacement for firebrick, should be applied 2" thick. Installers will need approximately 20 lbs per square foot when applied 2" thick. Insul Stick, a castable product used to parge over firebrick in a smoke chamber or for exterior applications, should be applied 1/4 to 1/2" thick. You will need approximately 2.5 lbs. per square foot when applied 1/2" thick. when applied 2" thick. Finally, Insul Cast, which is a backup insulating product that can be applied under the hearth or over the dome in a fire pit or bake oven, should be applied with a minimum thickness of 2". Installers will need approximately 6 lbs. of product per square foot when applied 2" thick.
The best advice we can give is to follow these guidelines. Remember, when using the premix products, you may want to give them a through mixing and if they appear dry, add a little water and mix them again. Adding water can give these products a thinner consistency, if that is desired.