Hemp lime plastering

It’s been a bit quiet here on the blog partly because we’ve been so engrossed for a while in plastering the interior hempcrete walls with hemp lime plaster. We’re far from finished, as you can see, but it’s such gorgeous stuff, I just had to stop and write about it.

With 40 cm thick hempcrete walls, we don’t need any more insulation but decided to use hemp lime for the first coat because, being close to the same raw materials as the walls, it should be very sympathetic in terms of ability to regulate humidity and, with the hemp in there, it’s really sturdy plaster and helps fill in any small cracks and crumbly bits.

It’s also just lovely to work with! I saw it described somewhere else as like plastering with tuna mayonnaise, but since we eat plant-based, I’d say it’s more like hummous. It looks a bit like it too.

We used medium hemp shiv, so a bit finer and shorter pieces than we used in the hempcrete, and the same silky Carmeuse S97 hydrated lime that we used in the walls. We followed Alliance 4’s recipe, mixed it in our Imer mixer and then kept it for at least five days in big tubs covered with a layer of water before using it. However tempting it is to start using it straight away, it’s well worth waiting a few days as it just gets creamier and creamier.

Applying it is like icing a cake, so satisfying! We went for around a centimetre thick or just under, and used fibreglass mesh bedded into the middle of it for extra strength in most places as we found we were getting a bit of cracking in places otherwise in places like the corners of openings and off wall sockets. However much you wet the wall first, the hempcrete just sucks it up so fast, it’s hard to slow down the drying. In places where there had been some shrinkage in the hempcrete, like around the joists, we stuffed in some hemp or jute cord soaked in limewash to fill the gaps before plastering. Thanks to Masons Mortar for that tip, a traditional technique they use in Scotland.

The plan is to finish with a couple of finer coats without hemp in, but it’s looking so nice already, it’s tempting to leave it as it is.