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Building the Straw Bale Cottage
The Straw Bale Cottage was built along the same principles as the Straw Bale Cabin. Again, materials chosen were local, renewable or recycled where possible.
In any project like this there are many decisions to make; some are made consciously and others are governed by regulation, cost or forces of nature.
[photo courtesy of Graham Crossland]
Below we highlight some of our own thoughts and experiences in the hope that they will help you to plan your own straw bale building:
Foundations for the Straw Bale Cottage
The overriding rule for building with straw is “keep it dry” and that starts with the foundations. All straw bale walls should start 300 – 400mm above ground level so that rain cannot splash up and damage the first course of bales. As Village Farm is only 5m above sea level, we decided to give our straw bale walls a little extra protection from future flooding. The first course of bales on The Straw Bale Cottage is approximately 1m above ground level.
To raise the walls up this far we have used brick piers on concrete pads. Concrete is not the most environmentally friendly building material because of its high embodied energy but it is the most suitable material for use underground. The concrete pads (see below) used 50% less concrete than if the Cottage had strip foundations and 75% less concrete than if it had full raft foundations. If we had had more time we may have been able to negotiate a reduction in the depth of the pads with building control but as it was, we stuck with local convention and dug them down to 1000mm.
Another option for the piers would have been timber but the size, to take the weight of the cottage, would have been phenomenal and they would have had a much shorter life expectancy than that of the building they were carrying. Timber, like straw and all other natural materials, would eventually rot when in close contact with the damp ground.
Recycled tyres could have been used for the foundations but they were discounted because at nearly a metre high they would be unstable and we would have needed an awful lot of them. There were other reasons too; they would have had to be in continuous strips (as opposed to pads), they would be degraded eventually by UV light, ramming them is very hard work and we are not too keen on the look!
The 13 piers (see above) of engineering bricks are spanned by large timber beams. The structural engineer would not accept reclaimed timbers because their strength could not be guaranteed and manufacturers of engineered beams would not guarantee them in this exposed position.
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As wheat straw bales had worked well for the Straw Bale Cabin, we specified the same for the Straw Bale Cottage. The Cabin bales were made by a New Holland baler at 450mm wide and 400mm deep. They were dense, good shaped bales but the length of the straw was quite short and it seemed to pull out of the bale fairly easily. In theory, this could have been a problem for plaster adhesion, but there is no evidence of any problem so far – all plaster still firmly in place!
However, we decided to search for longer straw for the Cottage. A lot seems to depend on the combine harvester used. The large modern machines with drum threshing seem to break up the straw. We found an old Claas Dominator combine which left swaths of long straw in its wake. This time the bales were made with a Welger baler (see below) at 500mm wide and 340mm deep – 50mm extra width of insulation in the walls. Although the straw was long in the field, it still broke up in the bale. Straw still pulled out quite easily. Was this to do with the strength of the baler or the fragility of the straw? The latter we suspect but we have yet to ascertain if this because of the variety of straw, the growing season or the moisture content at which it is baled?
Our first straw bale walls got wet and had to be replaced (see below) so we had to find more bales of the same dimension (all plans were drawn to the Welger bale chamber size). We were lucky and managed to find another neighbour with the same baler and a stack of dry bales in his shed. By chance, the straw in these bales was long. Now having built with long and short, we find the long ones more difficult to handle. Dressing bales and sawing tight fitting notches around door and window posts is much more difficult with long straw!
The Welger bales are more rounded on the edges than the New Holland bales – it proved quite difficult to square them up properly. In addition, the cut side is very uneven and this means more trimming to level up the walls before plastering.
During the summer, we used barley straw bales on another build. Barley straw does not seem as stiff and strong as wheat but this is just a feeling – no scientific evidence! There were a lot of banana shaped bales because the baler tied one string tighter than the other. Many bales had to have the looser string re tied manually to straighten up the bales. Time consuming and very hard work on the hands! There is more to selecting perfect building bales than first thought.
The damage to the barley straw bales was exacerbated by rough handling. Building bales should be carried very carefully to preserve their shape as much as possible.
Another problem encountered with straw bales in 2008 was due to the very wet summer. The harvest was so late, and the farmers so worried that they would not get their crop in, that getting good building straw was the furthest thing from their minds. We needed more bales to in fill the gables and for a while it seemed unlikely that we would get any. It wasn’t until September that there was a brief window of good weather when the wheat could be harvested, and the straw baled.
Previously we had only used bales from the previous year. Some sources say fresh bales have less insects as none have taken up residence in the over winter store. Others say the fresh bales have more insects, straight off the field. One engineer we met said all bales should be individually fumigated to kill any insects in the bales. In our experience with the Straw Bale Cottage, particularly because of the length of time it took to plaster the straw, this would have been a complete waste of time.
During the first year, there were a few tiny insects leaving the walls of the Straw Bale Cabin and running across the windowsills but they were not a problem and soon disappeared.
None of the problems with bales have been insurmountable or caused long term difficulty but to summarise our experience so far, the most important factors for bale selection, in order of importance, are:
Dry – if a wall is to last a very long time, it best to start with the best quality, perfectly dry bales – absolutely no sign of water damage or mould (cut some bales in half to check there too or use a moisture probe).
Dense – solid bales provide better structural and thermal performance.
Straight – straight bales make the bale builder’s life so much easier (unless you’re building a round wall of course!) Straight ends that stack closely together improve the thermal performance of the wall (less air gaps) and if the sides are straight too less trimming is needed before plastering (saving time and depth of plaster).
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Wet bales and temporary covers
When we built the Straw Bale Cabin the weather was kind and the building and timescale were small. Our temporary roof system worked a treat.
Naturally we thought the same would work perfectly for the Cottage. It did not! The larger area of straw bale walls (85m2 as opposed to 40m2) did not take much longer to build (7 days as opposed to 5) but the larger roof was much more of a challenge. In the delay, the temporary tarpaulins took a battering in the wind and rain. They were ripped to shreds and the bales were soaked.
We dismantled all the walls and re built them in 5 days. We paid £164 for the new straw so not a huge material cost. This time we wrapped each wall in its own tarpaulin rather than trying to displace rain over the entire building. We moved the temporary floor in 50mm from the base box all around the building so that rain falling in the middle simply ran off under the building, well away from the straw and the insulated base box.
Both the Cabin and the Cottage are load bearing structures – the bales carry the weight of the roof. The alternative method is timber frame with straw bales stacked in or around the frame purely for insulation. At the low points in the summer, we wished we were building with a timber frame! In this method the frame and roof are permanently in place before starting with the straw so there is no risk of rain damaging the bales. On a large project in a wet climate, a timber frame is definitely the least risky option.
In the Cottage the straw bales in the upper part of the gables are “in filled” against a timber frame.
Because there is no roof compressing these bales they are not as tightly pressed together as those in the lower, load bearing walls. To avoid heat loss through the increased possibility of air gaps, all spaces should be packed out with loose straw or a straw/clay mix. The first coat of clay slip rubbed into the bales tends to highlight any gaps in the bales. A straw wall can look very good until rubbed with clay. The spaces that were highlighted by the clay in the gable bales were initially quite alarming.
However, a few hours and aching arms later the walls were packed out with straw and clay to a very solid finish.
We have since seen ways to compress in fill straw bale walls but the roof pitch make it more difficult in this case.
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A roof is so important on any building. On a straw house the roof should overhang the walls by at least 500mm to give as much protection to the walls as possible.
The Straw Bale Cottage was originally designed with a thatched roof. So that rain is shed as quickly as possible, thatch requires a steep pitch of 50 degrees to prolong its life. An added benefit of this pitch is greater headroom and more space in the bedrooms in the upper half storey.
Rafters 200x50 by 7m long lifted individually into place.
There are significant cost implications of a thatched roof; skilled craftsmen, high insurance premiums and renewal every 20 years or so. Budget constraints meant we could no longer afford this kind of roof. The cedar shingles we used on the Straw Bale Cabin were also cost prohibitive for this size – (140m2 at £23/m2!)
Eventually we decided on a locally made clay tile. Neopantiles from Sandtoft’s factory at Broomfleet are guaranteed for 60 years but should last a lot longer. When the higher embodied energy of these tiles is spread over their longer life, the energy p.a. is not so bad.
The Neopantile is also very light compared to other tiles at 40kg/m3 and we felt comfortable with them sitting on our straw bale walls.
We nailed every tile to make sure they stayed firmly in place at 50°. The top 9 rows were nailed and clipped to make doubly sure.
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By the time the roof was finished and the gable ends plastered and clad so that we could finally get the last remaining scaffold off hire, it was very late in the year. We wrapped big bales of straw around the cottage so that we could continue working in a makeshift tent.
Persevering, the first 3.5 m2 of lime render on the outside was worked into the straw on 29 December 2008. It was covered with a double thickness of hessian and seemed to come to no harm over the very cold winter. However, we decided not to risk any more and moved to inside jobs.
On 9 March we decided that the worst of the winter was over. If there was a slight frost the cottage would be sufficiently protected by the big bale tent. In 5 days Sam and I hand applied the remaining 70+ m2 of lime render, pushing it well into the straw.
A week later, it was dry enough to apply the second coat. This took another 5 days. I worked on the bottom, top, corners and windows where we used hessian reinforcement and Sam manned the mixer and applied the remainder with a float.
For first and second coat we used ready mixed lime render (3 parts sharp sand :1 part lime putty) with chopped hemp added from Womersleys. For the thin top coat we used lime render and fine sharp sand in a 1:1 ratio.
Lime render needs to be protected from the worst of the weather. To cure properly it shouldn’t be too cold but also there shouldn’t be too much wind, rain or direct sunshine. We thought it would be safe inside our big bale tent but it seemed to be drying too slowly. We had problems with the top coat cracking and de laminating so we removed the big bales. We re worked the top coat as necessary and it dried much better now. We applied 4 coats of lime wash that we made ourselves. It took a few attempts to get the colour right!
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Clay is a great material to use; non-toxic, quick to mix, can wait in bucket until needed, very low embodied energy (no baking!) and inexpensive. It is not used on the outside of a building in the UK as it could eventually be washed away by the rain. However, if protected by a porch or rain screen it might work. In the Cottage we have experimented with clay on sections of the external walls (gables and porch), with larch cladding as protection from the elements.
Jim Carfrae, a Phd student from Plymouth University, has inserted moisture probes into both a lime rendered straw wall and a clay plastered/larch clad straw wall on the same elevation so that we can monitor any differences in performance over the years.
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The land at Village Farm is heavy clay. When we dug out the foundations we saved the clay to use as the first internal plaster coat. Not being quite sure what to do with it, we stored it in the corner of an empty building where it dried into some very large lumps. However, it broke up fairly easily into smaller pieces when hit with a hammer and after soaking in a bucket overnight, mixed up to a lovely creamy consistency the next day.
The first coat is rubbed well into the straw on the internal walls. It takes a good grip on the straw and when dry, provides a firm base for the second coat. Any small gaps between bales are stuffed with a straw/clay mix at this stage. This ensures there are no air gaps in the walls, which could lead to heat loss. It also levels out the wall reducing the amount of second coat clay plaster needed. It is better to fill with free straw/clay than bought-in second coat.
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The second coat of clay is a mixture of clay, sand and a material that will form a “mesh”. This added material is chopped hemp in the plaster that we buy from Mark Womersley but it could be chopped straw or, as in the old days, animal hair.
We could have made this second coat from our own clay on site but decided we had enough to do. Clays vary considerably and tests are needed to establish the correct ratio of clay to sand to minimise cracking. It is much easier to buy the correct blend, ready mixed in tonne sacks! On site, the only thing left to do is to add water to achieve the right consistency for plastering. A wetter plaster is easier for application by trowel but a drier consistency is better for hand application in trickier places.
The second coat is thicker (8-12mm) and hides the straw completely. Hessian is worked into the second coat (with the help of some clay slip) to strengthen corners and to avoid cracking where there is timber or strapping in the wall.
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The top coat of clay is a mixture of clay and a finer sand to provide a smooth finish to the internal walls. We bought the top coat ready mixed, again from Mark Womersley in Heckmondwike.
The top coat is applied thinly (2-3mm) and is sponged to a smooth finish before completely dry.
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A natural, breathable wall needs a natural, breathable finish and there is nothing better than clay paint. The clay paint we used in the Cabin, however, was quite smelly. For the Straw Bale Cottage we found the Earthborn range to be odourless and easy to use – simply a first coat thinned with water followed by one further coat at full strength. Although there are many lovely shades to choose from, we preferred the simplicity of “vanilla” throughout.
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Industrial hemp is grown on a handful of Yorkshire farms. The fibre has a number of applications and the shiv (the non fibrous inner core) is usually sold as animal bedding.
However, hemp shiv can be mixed with lime to form a bio-composite material that can be cast in situ to make walls. As an experiment, we decided to use hempcrete to cast the porch wall. It is only 3 metres wide and links the other 2 straw walls of the porch. There was a door, a window and a gable to contend with in this small wall.
We mixed hemp from 2 sources; one was a very clean sample of shiv only and the other contained a good proportion of fibre. One bucket of the fibrous hemp was mixed with a half bucket of the clean hemp, half a bucket of lime and 7 litres of water in a cement mixer for a few minutes.
The mixture was then tipped between temporary shuttering around a timber frame and loosely tamped (more firmly at the edges). The boards used for shuttering were removed the next day.
The timber frame can be on the edge or in the centre of the wall. In the picture above, the frame is external because this wall was clad with larch boards to finish.
There is ongoing research into the type of lime to use when making hempcrete. The lime must dry at the correct rate to gain its full strength. For this 350mm external porch wall we used the recommended French “Batichanvre” lime. For a thinner, internal wall we experimented with a local lime (see below).
The resultant porch wall has a very solid feel. It is also very straight and we created lovely window reveals through angling the shuttering. Hemp, like straw bales, stores carbon for the life of the building. However, as lime is used throughout the wall (rather than just on the outside as for lime render on a straw bale wall) there must be a higher embodied energy in a hempcrete wall. Insulation must also be less in a hemp wall compared to a straw wall.
There is a lot of preparation involved in framing and shuttering. Throwing hemp and lime into the mixer is a dusty job, using a conventional mixer. There is also noise, petrol fumes and breakdowns to contend with.
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Most of our internal walls are made of 4 x 2 FSC studwork, insulated between with sheep’s wool, boarded with Celenit wood fibre boards and clay plastered. We decided to experiment with hemp and lime again. We used the same recipe as described above but this time we used a weak Lincolnshire lime (NHL 3.5). The wall is 2.5m high, over 4m long and 160mm thick.
We used 4 x 2 studwork again, left exposed on the kitchen side for fixing the units to. We cast the whole wall in a day – Sam on the cement mixer, Julie and Carol tamping the hemp and lime.
The kitchen side was covered in either cupboards or tiles fixed directly on to the hemp and lime.
The study side of the wall was simply rubbed with top coat lime (lime putty and fine sand 1:1) then clay painted like the other walls. It could have been smooth plastered but we are happy with this rustic effect.
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We prefer to fix windows on the outer plane of the building. That way rain is kept out of the wall as much as possible. We also find it much easier and stronger to attach the windows to the top box. Sam made all the windows and doors.
All windows double glazed with low E coating, 16mm argon filled and plastic spacing. All have 3 point locks and draught proofing strip all around the openings. Many of the windows have small top openings for high level ventilation.
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Having the first floor raised off the ground has several advantages; the straw is well away from rain splash back and flooding, the impact on ground is minimised, the use of concrete is greatly reduced (see above), the final position of plumbing and drains can be decided on later and the use of temporary floor boards helps to naturally dry the internal plasters as the boards can be moved aside to increase air flow through the building. The disadvantages are a need for more insulation and the final sealing against draughts.
Finishing the first floor was one of the last jobs. The budget was seriously stretched at this point so Warmcell was chosen for insulation as it seemed to be the cheapest green insulation product available. It was cheaper than sheep’s wool but we did use quite a bit more than recommended for the area we were filling. We probably didn’t fluff it up as much as we should have.
My only reservation about using Warmcell is that it cannot tolerate water damage. There is no washing machine or dishwasher in the Cottage but the sinks could overflow or the shower leak. However, if this were to happen, it would be possible to repair the damage from underneath the building.
After 275mm of warmcell, the floor joists were covered with 3mm of sheep’s wool (Silent Joist from www.environomix.com ), the OSB boards were re laid and then 3mm cork/rubber (from www.environomix.com) followed by 22mm character oak floorboards in the lounge and study. In the kitchen, lobby and cloakroom 22mm plywood was used and Forbo Marmoleum tiles (www.brighouseflooring.co.uk )
The second floor make up was 15mm Celenit wood fibre boards below 200mm softwood floor joists (clay plastered and clay painted as the internal walls) to make the ceilings. Then 100mm sheep’s wool was fitted between the joists for sound insulation. As for downstairs, 3mm Silent Joist was run along the joists, the OSB boards were re laid followed by 3mm cork/rubber mat then softwood floorboards.
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Pipes are best kept away from straw walls – just in cast they leak and the most efficient design, in any building is the one that minimises pipe work. In the Straw Bale Cottage we didn’t quite manage to optimise either of these strategies.
We did, however, remember to insulate all the pipes – to keep the hot water hot and the cold water cold!
And we have spray taps and an ES4 low flush toilet from The Green Building Store. The recycled toilet downstairs is fitted with a varyflush device designed and sold by www.interflush.co.uk
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Our Highlander 8 multi fuel stove is made in Yorkshire by Dunsley Heat. It can burn coal or wood but we hope to use only wood as this is a renewable fuel source. We should have enough timber off cuts from the build to keep it fuelled for the first year or two!
As the East/West roof of the Cottage is not the best orientation for solar hot water, the Highlander stove is fitted with a back boiler to provide hot water. In summer, the immersion heater can be used. 100% renewable electricity is provided by Good Energy. The immersion has a boost button so that it can only be used for an hour or two. It cannot be inadvertently left on for days on end.
As well as heating the water, the log stove also warms 2 towel radiators – one in the upstairs bathroom and one in the downstairs cloakroom. The whole system is gravity fed so that it will not be affected by a power cut!
The log stove is located in the centre of the Straw Bale Cottage. The 10-20mm of clay on every wall and ceiling should be sufficient thermal mass to moderate the diurnal temperature swings of the UK climate. The flue from the stove provides a considerable heat source upstairs.
Heat flow around the cottage can be managed to some extent by opening and closing doors.
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Due to budget constraints there is no electricity generation on site. Instead we decided to buy renewable electricity from Good Energy and to minimise its use. Electricity is not needed for heating or cooking. All lights are low energy and the fridge is A+ rated.
The hot water tank is fitted with an additional element so that renewable power can be fitted at a later date.
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As the water table at Brind is quite high we decided to minimise waste water and to disperse it around the site. Both toilets are low flush (see above) and basin taps low flow. These drain to the large septic tank with a high level covered outlet that is planted around with willows which can be coppiced for firewood. The grey water from the shower and bath drain to the opposite side of the garden where it will soak away under the hedgerow and trees.
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