Rendering

Table of Contents

Lime Sublime - This article is part of a series.

Part 1: Design

Part 2: Foundations

Part 3: Framing

Part 4: Hempcrete

Part 5: Roofing

Part 6: This Article

Part 7: Floor

Part 8: Door and window

Part 10: Details

A traditional lime & sand mix on both the outside (render) and inside (plaster).

Hempcrete and lime render are natural partners. Hempcrete is an ideal “key” for the render to adhere to; the render gives strength while remaining vapour-permeable. It also adds a significant amount of thermal mass as it is applied to a depth of about 20mm.

Exterior rendering
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My first lime render mix was one part hydrated lime (from Bunnings) to 1.5 parts “plastering sand”, by volume. And enough water for a whipped cream consistency.

Here is a shortlist of things I did wrong with the external base coat:

I should have used a coarse sand
I should have used a higher ratio of sand to lime
I should have added fibre to the mix, or a reinforcing mesh

Despite all that, it worked out.

External render basecoat being applied to hempcrete substrate

What I did right: I roughened the base coat surface with a wood float, and also scratched up to make a key for the topcoat to hold on to.

What else I did right: I kept the lime render shaded, and I sprayed it with a fine mist twice a day for 5 days. If it dries out too fast it will not make a strong consolidated surface.

Top coat
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A few weeks later I applied the top coat. I used the same mix, which is in fact much better suited for a topcoat than a basecoat, except that I sifted the sand though a 5mm mesh to remove any rogue large particles.

I finished it with a wet sponge. This smoothes out trowel marks, and brings sand to the surface. Broader undulations in the surface remain. It is possible to achieve a flawless fine finish using a straight-edge, but I did not do that.

Render was smoothed in to edges of window and door frames

Interior plastering
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This time I had done my homework — reading Weismann & Bryce — so I prepared the basecoat with a coarse sand, a higher ratio of sand to lime (2.5 : 1), and added fibre.

The fibre added to lime plaster is traditionally animal hair, but I used coconut fibre. It works best when cut to 20 - 50 mm lengths. Too long and it can’t be mixed in easily.

Interior basecoat lime plaster mix (fibres too long here)

As with the exterior basecoat, I roughened the surface with a wood float, and scratched it up.

Interior basecoat lime plaster on the wall. Floated but not yet scratched.

The interior walls are divided into panels between the timber framing. That made it easier to do small chunks of work — an hour or two — without worrying about seams between old and new parts. I did have to mask over the timber to avoid excessive staining.

Interior basecoat complete and scratched up. I experimented with a few different mixes.

Top coat
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The interior topcoat was a 1 : 1 mix of lime to sifted sand. I decided on a hard trowel finish. This involves repetitive strokes with a flexible plastic trowel, using a lot of force. The reward for this effort is a surface polished hard and smooth.