Lime plastering is one of the most rewarding and hardest jobs we’ve undertaken so far on our build. The reward is the smooth organic finish which can be achieved by weatherproofing the exterior of the walls. However, this is only achieved with a lot of effort which arises from the very tough physical demands of plastering.
Our schedule for 2020 had already been affected by the time lost during the lockdown, imposed in the early part of the year. Unfortunately, it was further impacted by Maria sustaining a serious injury in early July; a broken and dislocated ankle. This meant that our regular workforce was reduced to one for plastering the entire house. Maria was unable to take part in building works for at least three months! Needless to say, this was one of the things we feared most when we started to build, and, unfortunately, it happened. This meant that our potential to be completed by the end of 2020 had vanished. As always with self building, you have to remain positive and embrace the challenges.
Sadly, our wonderful Springer Spaniel companion, Jal, passed away in September. She was with us on every day of our build, but sadly her part in the story has ended.
Early summer was all about preparation for plastering. During this time, we met fellow straw bale builders Pat and Heidi. Luckily they had arranged for Miki, a plasterer, who was trained by Straw Works, to work on their house. I spent a day plastering their house and Miki spent a week on our site, getting us started. I would recommend to any straw bale builder starting plastering to get some experience beforehand. The job is quite daunting and not one you want to repeat.
Lime plaster is the proven protector of straw bale walls. Straw has a natural ability to allow moisture and air through it and lime is a breathable material. If cement was used, it would cause the straw to rot, as it is not breathable, which would lead to a build-up of moisture in the straw walls. Lime plaster also has greater flexibility and is more suitable for natural building methods. In its lifetime lime plaster will continually absorb C02 from the atmosphere as it returns to its original chemical composition (CaCO3/limestone) and is an excellent, sustainable building material.
Mixing the plaster
Due to the moderate exposure of our site, especially to prevailing south-westerly winds and rain, I used NHL3.5 with a 5mm down coarse aggregate for the exterior walls.
The quantity of silt in the aggregate is an important factor to consider. Too high a content and it will cause cracking in the plaster. Our aggregates came from Shiel Sand and Gravel and have a silt content of around 2%. For a very detailed understanding of plastering with natural materials, I found Weismann and Bryce’s book, Using Natural Finishes, a really useful reference. Our lime was supplied by the Traditional Lime Company who also offer advice and training.
a) First Coat (Scratch Coat)
The plaster was mixed 2:1 (aggregate to lime) by volume. The aim of the first coat is to get the edges of the bales well covered and provide a suitable substrate for the second coat. The plaster is worked deeply into the bales by hand. The straw on the edge of a bale is alternately chopped or folded. The chopped edge is much easier to work the plaster into (photo below shows the first coat).
Plastering is a very physical task, involving a considerable amount of heavy lifting. Each tonne is lifted three times; firstly into the mixer, then into buckets and finally onto the wall. Plastering by hand is quite tough, as it uses different muscles than usual. It takes a while for the strength to build up in your hands and fingers. It also takes a while for your hands to get used to the feel of the plaster.
For plaster to dry (cure) properly, it is important to control the atmospheric conditions of the wall. I protected the walls from the elements (mainly wind and sun) by hanging tarpaulin from behind the fascia. In addition, I hung hessian between the wall and the tarpaulin, which was dampened periodically to aid the curing process. Luckily, the very humid, calm summer of 2020 provided the ideal weather conditions for plastering.
The plaster changes to an off-white colour as it cures (see photo above). When it becomes solid to the touch, it’s time to add a second coat.
b) Second Coat (Straightening Coat)
The mix is also a 2:1 ratio, but chopped straw is added to give additional strength. The purpose of this coat is to fill in the grooves and any other holes in the first coat. This coat is then ‘keyed’ using fingertips to provide a suitable substrate for the top or finish coat (see photo below).
As NHL3.5 cures quite quickly, I aimed to apply the first coat for about a week and then add a second coat. The second coat is easier to apply as the more difficult task of getting the plaster to stick to the straw isn’t a factor, as it is in the first coat. The plaster sticks better with this coat but still has to be worked into the bales, albeit with slightly less force.
c) Top Coat (Finish Coat)
Due to Maria’s injury, it wasn’t possible to complete all three coats in the time available. I was lucky to have help on several days, but as I was mainly working on my own, it was only realistic to complete the first and second coat by mid-September. I set this as the cut-off date as the colder, frosty winter weather from mid-November onwards can arrest the curing process. We will complete the exterior top coat in late April/early May next year.
By mid-September, practically all of the first and second coat had been completed. It was a considerable task as the total exterior wall space is around 200 square metres and each week I added between 1 and 1.5 tonnes of plaster.
When I worked with Miki, we completed a small portion of top coat. Some of this coat was worked in using a float and, where this was not possible, we used our hands. The mix was slightly weaker 2.5:1 with fibres added (see photo below).
a) Underfloor Heating System
In early July, our plumber, Pauric, completed the installation of the pipes for the underfloor heating system. We will be installing an air-to-water heat pump for hot water and heating. This type of heat pump is practically standard on new builds and will satisfy some of the requirements of Part L of the Building Regulations.
Prior to the pipes being laid (see photos above) we covered the OSB floor with polythene and also added an expansion gap around the exterior walls, and the posts to accommodate any expansion in the floor.
b) Floor Screed
The floor screed is a vital part of our heating system as it forms our main thermal mass. It needs to cover the pipes and set properly without cracking (which would obviously damage the pipes). We were recommended Cemfloor and got in touch with a local contractor – Bracken and Mulligan – who poured the floor in about half a day. It has a relatively quick set, however as we didn’t need to work indoors, the screed was left for over a week before we walked on it.