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!
Wednesday 21 September was a big day as we finally started assembling Ecococon straw/ timber panels. One of the main benefits is the speed of construction: it took us only three days to assemble all the external walls, two internal load bearing walls and the main glulam ridge beam. It was helpful that all panels were referenced to colour coded Sketchup renders – this was really easy to follow on site, just like a giant LEGO set. Each panel has a couple of holes at top & bottom, and wooden dowels are used to locate the adjacent panels accurately (like IKEA shelves) before they are screwed together. Unlike ‘traditional’ timber frame, Ecococon system has the airtightness layer on the outside face of the wall, covered with further layer of woodfibre insulation and, in our case, timber rainscreen cladding. Airtightness layer is a vapour open membrane with Sd value of less than 0.2m – this is important in terms of moisture control in the wall assembly. Airtightness membrane is taped to DPM at low level, which in turn is sealed to concrete slab under the sole plates, ensuring no unwanted air leakage. It is critical to keep the straw panels dry – we had a few ‘moments’ during Day 1 when sudden showers forced us to stop assembly and quickly cover the panels up with plastic sheeting.
It is the first time this system has been used in UK and we didn’t quite know what to expect in terms of accuracy of panels. We needn’t have worried – the walls fitted perfectly onto the timber sole plates, even over the longest 17m walls the tolerances were measured in milimeters rather than inches. Very impressive. It was great to have Bjørn Kierulf from the Slovakian architectural studio Createrra available to answer some technical questions during the first day – Bjørn was originally the one who introduced me to the Ecococon system several years ago (thanks!) and I have wanted to find use for it ever since.
Hopefully we will be able to fully wrap the walls up and install the roof over the next week or so, fingers crossed for some dry weather!
Ecococon is a company based in Lithuania, manufacturing prefabricated straw/ timber panels to exacting standards. Combined with internal clay plasters (applied directly to straw), the panels provide a healthy, toxin free internal environment that is comfortable in all seasons. With extra layer of woodfibre insulation fitted to the outside, it is relatively easy to achieve passivhaus levels of insulation. Unlike the traditional straw bale building, Ecococon system is very accurate, predictable and consistent – drawings are worked out in milimeters rather than bale sizes 🙂 Further details about the company can be found here.
After months of detailed design coordination, we have finally taken delivery of the wall panels and materials for roof construction, all fitted neatly on two lorries. The straw fairies were present and we’ve been blessed with the warmest day of September. Six of us and a JCB made a relatively quick job of unloading, stacking and covering up all the materials. Starting with assembling this giant house lego next week – cannot wait!