Perlite Render - The Fire Brick D130 Instruction Booklet

Perlite Render

7. Perlite Render
To render over the Ceramic Fibre Blanket you'll be using LiteFill Perlite – this is a lightweight aggregate that
you'l use to make an insulating render mixture. The Perlite render layer serves two purposes – it forms a thick,
hard shell over the insulation blanket, and it provides additional insulation to your oven dome.
If you're installing the optional Thermowell and Temperature Gauge, this is the time to install it. Drill a hole
through the wall of the oven, approximately halfway up the dome , then slide the Thermowell rod into the hole.
Use masking tape to cover the internal threaded section of the Thermowell to keep the Perlite Render out.
Leave the Thermowell sitting well off the insulation blanket, then once you have some wet render around it, you
can gently push it into the render, nestling it in until you're happy with its position.
PERLITE RENDER MIXTURE
• 5 PARTS LITEFILL PERLITE
• 1 PART GP CEMENT
• 1 PART HYDRATED LIME
• 1 PART WASHED SAND
Mix up a wheelbarrow (or large tub) of render, combining the Perlite, cement, hydrated lime and sand while dry,
then add water. Mix thoroughly, adding water as you go until you've achieved a thick, stodgy consistency. You
should be able to make a ball with this mixture, throw it up and catch it without it running through your fingers.
The Perlite Render should be put on one layer at a time; for the first layer you'll work the wet mixture into the
chicken wire using your hands. Start around the base, then cover over the top. This layer will only be around 15
to 20mm thick. Leave to set for 12hrs, covered in wet sheets or plastic to minimise drying shrinkage cracking.
Before applying the next layer, wet the surface of the existing render so the new mix will bond to it. You may
need to cover the existing render with old towels and soak them with a hose in order to get enough moisture
into the existing render layer, particularly if it's been drying more than 24 hours. Trowel the next layer on, using
the curved polystyrene float to help shape it. You're aiming to build up around 15 to 20mm in each coat.
Again, let this layer cure under covers for at least 12 hours before applying the final layer. Repeat the step
above, using the float to shape the surface of the Perlite Render and to show you where the high and low spots
are. Stand back and look at the curve of the dome to see how symmetrical it is. We recommend that you bring
the perlite render in behind the vent arch columns, then your render layer remains thick and strong.
*Pro Tip - If you're very familiar with using render, and have a mixing drill available, it's possible to build up the
full 50mm of Perlite Render in one hit. It's critical that the render be mixed very thick and sticky to achieve this.
Please don't try this unless you're 100% confident.
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