At the conceptual part of our design process, we decided that we would aim to build with as little cement as possible. This was an environmental choice. Cement is an extraordinarily useful substance in construction but it also carries a heavy environmental price. We decided our floor would not be a monolithic slab of concrete but would be a raised wooden floor.
Putting this idea in practise meant that we had to build and insulate our floor. Our plan was to get as much interior work done until the weather was suitable to plaster the outside walls.
However, Covid19 has resulted in all supply lines to non-essential building ceasing. Currently, we are awaiting a change in the rules so that we can continue with our build. Until then, like everyone else, we wait in hope and observe the constraints that have been put in place for the good of public health.
We did manage to complete our floor and some other work before the lock-down was instigated. Below is the story of building our floor.
Stage 1: Setting out
We worked with our engineer, David, to check the levels on our wooden ring beam (which is the wooden plinth that supports the straw bale walls) in order to mark the correct positioning for the wooden flooring joists. The flooring joists have to span about 10m (north to south) which means that additional supports were required. This span is too great for a single length of wooden joist. We put the supports in place when we were building the car-tyre foundation by placing two sleeper rows of car tyres (running east to west) inside the outer ring of car tyres.
Stage 2: Installing the joists
Our floor used 110 joists which were 3.6m (12ft) long and 225mm x 44mm (9″ x 2″) at 400mm (16″) centres. The joists were secured to the timber ring beam by timber-to-timber joist hangers. Rather than using timber noggins between joists, we used metal herringbone straps, all fastened with twisted shank nails. The advantages of the straps include; they are ready-made; will help to reduce thermal bridging and allow for more insulation in the floor. The main disadvantage is that the insulation will have to be cut around them and that they have to be nailed to the top and the underside of the joists.
There was a considerable amount of nailing in this stage. Before fastening the joists we used folding wedges to help us achieve level and plumb joists. This is an ancient woodworking technique and still really useful when you are trying to achieve good work. We dedicated a lot of time to ensure that we achieved a high level of accuracy with the joists – lots of measuring, levelling and checking.
On each joist we nailed short lengths of 44mm (2″) by 19mm (1″) at a 200mm (8″) chalk line to support the two layers of rigid insulation. We worked out the spacing of these in advance to accommodate the herringbone straps and insulation, as this would reduce wastage.
Stage 3: Insulating the floor
We used PIR insulation from Xtratherm for the floor. Their product – PIR XT/UF – is specifically for raised floors with under floor heating systems. The sheets of insulation are 2.4m (8ft) by 1.2m (4ft) and 100mm (4″) in thickness. We cut each sheet in three to make smaller 800mm (32″) x 1.2 (m) 4 (ft) sheets. This then gave us a spacing for the herringbone straps at 800mm (32″) centres. This simple calculation minimised the wastage on each sheet.
This job was quite time-consuming as we had to install two layers of 100mm (4″) insulation, so as to avoid any thermal bridging. We installed the herringbone straps with the insulation so there was a lot of cutting around the straps, nailing to the underside of joists and dust…lots of dust.
The temperature over the winter inside the house (without the floor being insulated) was usually around 8 to 10°C, even on the frostiest morning with sub-floor natural air vents circulating cool air. After we finished the insulation we noted temperature of 14 to 16°C, which is partially the result of the onset of Spring, but also shows the impact of insulation. We also noted that on really cold mornings, the house feels warmer than outside and on sunny, warmer days it is slightly cooler inside, which is exactly what a well insulated house should be. Obviously when the house is fully finished we will know much more about its thermal capacities.
Stage 4: Sheeting with OSB
The floor was completely covered with a layer of 18mm (3/4″) OSB. This was a fairly simple task. As we had maintained 400mm centres with our joists there was relatively little cutting and waste.
The important thing here is to make sure the first sheet of OSB is affixed perfectly square and plan the sheets so that they are laid like bricks. This gives additional strength and solidity to the flooring joists.
What we noticed was that different manufacturers produce OSB to slightly different measurements. It’s worth checking the measurements of OSB sheets before beginning to lay them as they may not fit exactly if they’re from different manufacturers.
All boards were glued and nailed on to the joist.
The end result was really transformative…at last we had a proper, level floor to work on. Unfortunately soon after this the Covid19 pandemic interrupted our building work as all supply lines were closed.
The next stage for the floor will be to put in the underfloor heating system and screed.
a) Studded partition:
We began work on the internal walls in the house after we completed the floor. The timber used was 100mm x 44 (4″ x 2″). We used a simple Fukuda laser level to set out our walls. There were some challenges here as we were marrying round to square wood in some places. The stud work was more complicated toward the ceiling where we had to accommodate the roundwood rafters and the slight slope of the roof. The laser level was very useful in this situation.
Even with this job being only partially complete, the stud work really changes the house internally. It had been completely open-plan until this point. Now, not only is there a clear indication where the rooms will be but there is a greater sense of how circulation and light will work inside the house.
We took advantage of some of the early Spring weather to clad our fascia with some 18mm (3/4″) larch boards. The underlying fascia boards were horizontal and in a slightly zig-zag arrangement due to the shape of the roof. Affixing vertical fascia boards has ‘softened’ the appearance of the roof and given it extra protection.
Our plumber, Pauric, installed some of the under-floor pipe work before we started to insulate. This included the kitchen and bathroom supplies and some of the pipes for the underfloor heating system.
d) Spring Planting
We planted around 30 native broad leaved trees and some shrubs on the large pile of top soil which was removed at the beginning of our build. Initially we considered using this soil in our garden but we’ve decided to plant trees as a shelter belt protecting us from south-westerly winds (our prevailing wind direction).