In this post I’m going to describe how the walls were built. By this, I mean the timber frame and the hempcrete infill. The exterior weatherboarding (siding) and interior lime/hemp plaster will be dealt with in separate articles.
Building the timber frame
The building uses a simple stud wall system fixed on top of the previously constructed base with an OSB floor.
Pictured below are the walls in the early stages of construction.
If you study the picture closely, you can get a good idea of how the walls were tackled.
The image below was from a French architect’s site (Amnios) and shows the structure very clearly.
This page on their site no longer exists, so I can’t link to it. Nevertheless, the same architect helpfully mentioned that the studs are normally 120mm x 45mm, spaced 60cm apart. So that’s exactly what I chose for the tiny house. I addition, my walls would have 50mm of lime/hemp plaster on the inside and an exterior weatherboarding thickness of 60mm (Including an air gap). This brought the overall thickness of the walls to around 230mm.
Going back to my tiny house photo above, you can also clearly see the 3 rafter ties on top of the walls. These were added to stop the walls from spreading under the load from the tiled roof. Some leftover joist hangers were perfect for fixing them to the top plates of the walls.
As the framing progressed, horizontal noggins and diagonal braces were added, as can be seen below. Scraps of timber were also fixed to the sides of the studs to act as a mechanical key to lock the cast hempcrete in place.
I later learned that, such is the rigidity of set hempcrete, noggins and diagonal braces are unnecessary. Furthermore, they also make it much more awkward to put the hempcrete in place. If I were to construct using cast hempcrete again, I would omit the noggins entirely and try to devise a system of temporary diagonal braces that could be removed as the work progressed.
It’s probably worth pointing out that the timber used for the walls was untreated douglas fir. All the nailing was done with a Hikoki gas nailer using 65 and 95mm galvanised nails. Having a good nail gun really helped speed the work up and I’d recommend it to anyone undertaking a similar project.
I should also give credit to my ex-colleague and master carpenter Robert (aka magic Rob!) who pretty much took charge of the framing. Having his experience on site was invaluable. I could possibly have muddled through on my own, but I would have been much slower and more error prone.
Adding the hempcrete
You can read in this blog post how I cast the hempcrete around the timber frame. Basically, sheets of OSB are screwed either side of the timber frame and wet hempcrete placed into the void. The next day the shuttering is removed and reattached to form the next ‘lift’. In my case the shuttering on the inside of the wall was spaced from the frame by 5cm to allow the framework to be completely covered by the hempcrete.
It’s not a complicated process but it seemed to take forever. This was largely because I did it on my own: the shuttering, the mixing and the placing of the wet hempcrete. For prospective self-builders I would recommend trying to get a few people along at this stage of the process, otherwise it’s a real slog.
Another thing that didn’t help was the extreme heat. Doing hard work in 35deg C is tough. Especially for someone more used to the Scottish climate. Such were the time pressures of the project, I couldn’t afford to wait until it cooled down. Indeed, I think the only time I took a day off was due to torrential rain!
Anyway, to help cope with the sun and heat I often wore a ‘legionnaires’ cap (protects the back of the neck). They can be had easily on eBay for a few pounds. I also wore an old, light cotton shirt. On the hottest days I had a routine whereby I would regularly soak myself with the garden hose. The water evaporating from the cotton shirt and cap really helped cool me down, especially if there was a breeze.
Another trick I used was setting up a makeshift shade from a plastic tarpaulin. This was used over the concrete mixer to give some respite from the sun while making the hempcrete. 4 wooden ‘poles’ (from roofing battens or similar) were used at the corners and the whole thing braced using camping guy ropes. This arrangement can been seen on the left of the photo below.
Getting back to the building …
As the hempcrete casting progressed, I found the shuttering boards getting progressively smaller as they were cut up to do the fiddly bits around the windows and doors.
Little by little, the gaps in the framework were filled in with grey porridge, producing some very solid walls.
In the picture above you can also see the boards that were used as window liners. These were wide (and unseasoned) so they inevitably cracked a little and were not very flat. In hindsight it would perhaps have been better to have made these from exterior grade plywood.
The picture above shows as far as I got during the first summer of construction. In fact, I ran out of time and so had to leave the triangular area of the gable end wall until the next visit. The exterior face of the wall had been weatherboarded and the black surface which is visible is the fabric rain screen. So no water got into the building, despite the ‘hole in the wall’
On my next building visit, this triangular shape was filled in using a ‘lath and plaster’ type technique. This was done (again!) due to time constraints. I simply screwed some strips of scrap timber horizontally over the studs and pushed wet hempcrete into the gaps.
Admittedly, the overall hempcrete thickness was somewhat reduced by this method but it was quicker and easier than trying to cast hempcrete in this awkward space.
So, that was it as regards the main ‘body’ of the hempcrete walls. The next step was to apply the 5cm thick hemp/lime plaster. You can read about that here.