Clay plaster Pt 2

A few weeks have passed and we have finally (almost) finished the clay plastering.

After the application of base (body) coat to straw walls described in the previous blog post it was time to apply the top coats. Top coats consisted of two sub-coats, the initial 4-5mm fine clay coat and 1.5-2mm final coat, which was applied whilst the first coat was still moist. We have initially selected three different colours for the top coat (vanilla yellow, caramel brown and olive green) to differentiate various spaces in the house, but during the application we have opted for additional un-pigmented basic clay for the ‘feature wall’ in the main living space. We’ve also added finely chopped straw in the top coat for a bit of sparkle and a reference to our straw walls.

Normally it is not recommended to apply clay plaster to plasterboard because of poor bonding. However, as we’ve used fibre-based Fermacell boards for internal linings, we felt confident that this would work. We taped all the board joints with glass-fibre mesh and tile adhesive before a clay suck coat and two top coats were applied to internal walls.

Roman really showed off his skills, with beautifully crafted rounded window reveals and all surfaces expertly finished with small japanese trowels. It’s been an emotional process with ups & downs, but it has totally transformed the feel of the house, giving it a sculptural quality rarely seen in new build homes.

Clay plaster Pt 2

Clay plaster Pt 1

We’ve recently re-focussed our efforts on clay plastering. Clay plaster works brilliantly when applied directly to straw, as it allows moisture to permeate back & forth, effectively acting as a moisture buffer. It’s a healthier option compared to cement or gypsum plaster and will add significant thermal mass to the building – we have 7 tonnes of it to put on walls!

Some straw buildings treat the clay plaster as airtightness layer, but this can be problematic, particularly if there are tricky junctions. We use the clay plaster purely as an internal finish, and airtightness is dealt with by a separate membrane, installed to outside face of straw panels. More details on that are in my previous post.

We have sourced our clay plasters from a Czech company Picas, as recommended by our Slovakian plasterer Roman. I met Roman at an Ecococon event in Lithuania in May last year (short video from the event) and it was great when he agreed to plaster our house. The first few days were spent fixing reed mats to exposed timber elements – this will provide a good key for clay plaster. We’ve started with plastering the straw external walls, where two base coats will be applied before the final finish coat. We found our little fans a bit inadequate, so borrowed the neighbour farmer’s big fan to help dry the plaster out – it’s been working a treat.

We’ve installed a couple of DIY moisture probes that will enable us to keep an eye on humidity levels in the straw panels on both north and south facades.

After the first week of plastering Roman has finally finished the first base coat on all straw walls – a bit of a milestone. On to the second coat tomorrow!


Clay plaster Pt 1