We use LiteFill Perlite to make a strong lightweight, insulating render material.
Ordinary Perlite is an expanded volcanic glass material, commonly used in hydroponics due to its ability to hold water. This makes it frankly terrible for making lightweight concrete, as you would have to add a significant amount of water to the mix to wet it through completely.
To overcome this, LiteFill Perlite is treated in a solution to give each particle a waterproof coating. This makes it perfect for mixing into lightweight concrete and renders. It has very low thermal conductivity and has a maximum working temperature in excess of 1000 degrees C.
LiteFill comes in 100L bags, and typically a Wood Fired Oven needs 2 to 3 bags to build up a 50mm render shell over the ceramic fibre blanket insulation. We mix the Perlite in the ratio shown below, and build it up in layers 15 to 20mm thick, letting each layer set before the next is applied.
LiteFill Perlite Render Mix Ratio
5 Parts Perlite
1 Parts GP Cement
1 Part Washed Sand
1 Part Lime