Here’s a detailed description of the building of a hempcrete studio in my garden, which I started last summer. I have completed the base and the hempcrete walls and will soon be working on the roof.
The base is an unusual foundation, but it will keep the hempcrete building dry, solid and safe. I began by digging a trench around 3 inches (75mm) wide and 7 inches (175mm) deep in the shape of the building footprint. I then poured concrete into the trench and 2 inches (50mm) above it with some formwork. There is one thin rebar in the concrete to aid tensile strength. When the ring beam was set I dug out inside to a depth of around 6 inches (150mm).
A geotextile membrane (Terram) was then laid over the ground and the base was filled with Glapor foamed glass insulation. Glapor is made from recycled glass in Germany. The foamed glass was then consolidated with a vibrating plate and another layer of geotextile was added.
The rationale behind this base is that it enables a garden building to sit very near to the finished ground level, whilst remaining dry and insulated. The Glapor is an amazingly good product as it will not wick water, meaning everything above it will stay dry without the use of a plastic membrane. If in the future the building were to be dismantled the Glapor could be reclaimed, and the concrete ring beam is a minimal amount of concrete. The Terram is necessary as it acts as a filter to stop the ingress of dirt that could be washed in over time or through flooding. If enough dirt did wash in, it could allow water to wick and would reduce the insulation efficiency. The combination of foamed glass and ring beam will give plenty of load-bearing in this local soil with what is effectively a solid, but relatively lightweight, building. Pebbles could be used in the same way but would lack the insulation quality of the Glapor.
A damp-proof membrane isolates the timber frame from the ring beam and the framing was built to follow the ring beam. The frame was built up from the base in 3 x 2 (75 x 50) timber and the walls will be around 8 inches (200 mm) thick. This is a very appropriate size as it protects the timber and allows the hempcrete a reasonable covering over the timbers on both sides. It will also provide excellent comfort levels for the UK climate. The floor joists will be infilled with 3 inches (75mm) of hempcrete and a chestnut timber floor will be laid on top. This will create a very well insulated dry floor.
The Geoplast shuttering is really quick and efficient to use and I would advise any serious hempcrete contractor to get some, although it is very expensive! I used 4mm ply for the curvy walls as I had some left from another job.
I made the timber frame of the base curved largely in the shape that I wanted it and then just banged in some battens into the ground. Then I bent some 4mm ply to the form I wanted and screwed it to the frame and the battens to hold its shape.
This then gave me the form for the outside wall shape. Then I took another sheet of 4mm ply and cut it in half lengthways to make two lengths 2ft high (600mm). I joined them together and fixed them to the floor base 8 inches (200mm) from the outside shutter. As they are curved they largely support themselves, but I added enough support to hold the two shutters in place. For this kind of building I am not trying to get perfect smooth lines so the fact that the shuttering is not perfect just adds to the interest of the walls.
We then started hempcreting and placed the first layer all the way around the building to form the base of the wall.
It took three of us about two and a half hours to hand-cast around the building for one 2 ft (600mm) lift with one person mixing and two hand-placing. The shuttering was removed the next day and the wall scratched back with a nail float where necessary to form the shape of the wall I wanted. When I’m using curved plywood shuttering once I am happy with the shape I give the wall a couple of days to get hard and then I simply screw the next layer of shuttering ply into the hardened hempcrete with 5 inch (125mm) screws. This holds the ply solid enough to hand place the next layer.
This photo shows the first two lifts in place and the framing of the shuttering for the windows goes in at this point. The timbers fixed to the wall plates at roof height cross the building in various directions and so brace the timber frame to stop any movement whilst the hempcrete sets.
The two mixers you see in this photo are the ones I use for small garden jobs. The binder and water are pre-mixed in the small drum mixer. The hemp is placed in the pan mixer on the left and switched on. We take off about a bucket and half of slurry from the drum mixer and pour it into the pan mixer as it turns. This has two enormous advantages. The first is that it makes the whole process much less dusty as it only dusts for a few seconds in the drum before a spray from the hose calms the dust. The pan mixer creates hardly any dust with this method and makes the mixing much more pleasant. The second big advantage is the speed of mixing. A batch of hempcrete will mix in about 90 seconds this way as opposed to 4 minutes when hemp, binder and water are mixed separately. This means that even though we are mixing relatively small batches at a time, we can mix and place up to about 5 cubic metres of hempcrete a day with three of us.
We carried on raising the straight walls using the Geoplast shuttering. Straight walls like this are much easier to shutter, but I really enjoy creating the curves.
Here’s a very simple way to create curved corners. I staple render mesh to two wooden battens and fix the battens to the frame each side of the corner.
As we put the hempcrete in we push a little harder to stretch the render mesh into a curve.
I then use a nail float to scratch the finished corner to the shape I want. You can make a nail float quite easily with a piece of flat wood with a handle and screws sticking out of the face.
Here you can clearly see the three lifts and a rough edge. This is because I would normally have built all the framing for the door and windows and know what sizes I wanted to do. The truth is I haven’t decided what size of windows or door I want to make or salvage so I just left a larger opening. When I decide on the windows and doors I will frame it up and spray them in to make good.
About half-way there! Notice the far end of the wall doesn’t have any timber framing in it. Hempcrete is a remarkably robust material and in these small garden buildings missing a bit of framing won’t make any difference to anything.
If you notice the small level on the top of the shutters it’s because I level the first shutter horizontally so that when I attach more shutters along the wall the line stays parallel to the top of the hempcrete lift. If you put it on at an angle the shutters start to run out of line with the top of the previous lift.
It is at this point that having trestles or a small scaffold tower are really useful.
At the top of the curvy wall now. I have shuttered up to just above the wall plate so I can scratch back to the wall plate to form a straight edge. This will make positioning the roof timbers easier.
The rose bush will eventually grow over the roof, but for now it has been pulled back away from the roof to make the hempcreting easier.
This end wall has a small window in it so I made a simple formwork to give the hempcrete roughly the right shape. I usually make the form a little smaller than the window and scratch back the hempcrete to create a tight fit. The diagonal brace will keep the opening plumb and square. There is no framing or lintel in the hempcrete here as the material is plenty strong enough to cope with this opening.
Using a full sheet of ply like this and simply screwing it to the hardened hempcrete makes the shuttering follow the line of the existing wall exactly. The curve adds solidity to the shuttering meaning no extra support is required
And finally the top lift! We usually do it so the person mixing the hempcrete feeds the person on the scaffold so there is less getting up and down for the one who is hand-placing.
The cross timbers at roof height were removed as the frame is firmly set in the hempcrete now. There is no diagonal bracing on this building as its shape and small scale make this unnecessary.
The curve emerges from the shutters! You can see at the top of the wall two sections of wall plate showing in the hempcrete. This is where the framing is in two short straight sections of studwork. These two sections of studwork are joined at the base and the wall plate, but no attempt to build a curved frame has been made. Two short straight sections of studwork across the two curves gives all the load-bearing strength the roof will ever need and this avoids having to form complicated curving frames.
The rounded corner is revealed and behind that, the wall bulges out around the window. This started out as just some overenthusiastic hand-placing pushing the shutter out, but I liked the shape it left so I carried it on up the wall. When the rosebush grows up over the building the window shape will be more visible.
All the outside shuttering is removed and the walls are largely finished.