Who Builds a House Out of Straw?! Quiz

Answer: Cob

Cob is a traditional building method using a mix of clay, sand, straw and water to form durable load-bearing walls with low-cost materials, often allowing for artistic flair to be showcased. In ancient Egypt straw was used as an additive in bricks. The expression 'bricks without straw' (meaning a task undertaken without the appropriate resources) can be traced to Exodus 5 in the Bible when the pharaoh punished the Israelites by not supplying straw for the brick making the Israelites were required to do.

Modern bricks do not use straw but are typically made from clay, sand and water with mineral additives to enhance properties. Straw may also be used in wattle & daub and adobe construction.

Answer: Roof

Thatching is the craft of roofing buildings with dry vegetation such as straw. It is layered in such a way as to shed water and is packed densely enough to provide insulation. Thatching was common in the UK until Welsh slate became commercially available from the 1820s. Rural depopulation during the 1800s also contributed to its demise. It became of mark of poverty to have a thatch roof. A revival of sorts has occurred in recent decades, partly driven by a desire to preserve heritage buildings. It is now a symbol of wealth for some.

The blackhouse used to be common in Ireland, the Scottish Highlands and in the Hebrides. It consisted of a single storey building generally built of a double wall of dry-stone packed with earth for insulation. It was roofed with rafters covered in turf and topped with cereal straw or reed. The roof needed annual maintenance. There was no chimney so smoke from the central hearth had to find its way through the roof. With modern housing replacing them, most have now fallen into ruin.

Answer: Insulation

A key advantage of strawbales as a building material is its insulation or R-value. Whilst conventional insulation materials can achieve similar R-values, it is typically more expensive to achieve. Local building codes usually require a lower R-value than a standard strawbale offers so standard homes tend to be built to the lesser requirement.

While the strawbales themselves might be cheap, they take up more room due to their size. A strawbale wall might be two or three times thicker than one made of more conventional materials. So for a living space of the same size, if strawbales are being used, you need to factor in the cost of a larger foundation area and a larger roofed area.

Other advantages of strawbales include they are made from a waste product, do not take much energy to make (i.e. low embodied energy) and are biodegradable.

Answer: Correct sizing of roof eaves

The high insulation rating of the strawbales reduces fluctuations of temperature inside. If the eaves are designed to keep the sun out of the house during summer, the house will not normally need to be cooled. If cooling is required, there are unpowered options such as cross-flow ventilation or evaporative cooling.

The eaves should also be sized to allow the winter sun, which follows a lower arc in temperate latitudes, to warm the house in the cold season. This means that a heat-absorbing floor is desirable. Normally this would be a concrete slab floor insulated from the surrounding earth. Carpeting or rugging the floor would reduce the effectiveness of the design, increasing the requirement for heating in winter.

Deciduous vegetation can have a part to play in a passive solar design however pine trees, being typically evergreen, would not normally fit this role. Powered shutters and air conditioners are not 'passive'.

Answer: Wooded valley with good sun

Controlling moisture content in the straw is important. Too much moisture content and the strawbale will compost. Rain on the walls is therefore best avoided. Although wolves may not bother house-building pigs, wind can be an issue most places. Coastal locations and exposed ridges are more likely to see horizontal rain at times. River-side flats are at higher risk of flooding; not a risk worth taking with most housing types if it can be avoided.

The wooded valley provides a more wind-sheltered location. Add an all-round roofed verandah to the house design together with the right kind of weatherproofing paint on the external wall and that should keep the strawbale walls dry.

Answer: It wicks through the wall

Strawbale walls breathe. For correctly-made walls, the moisture inside the house will wick to the outside through the wall with no detrimental effect on the straw itself. Most individual pieces of straw in a bale are aligned pointing to the outside. When the wall is built, choosing the right kind of plaster is important. It needs to be in direct contact with the straw, flexible enough to avoid cracking and help with wicking any moisture away from the straw.

One plastering method is to apply a clay slip onto the straw to help key the plaster layer. An earth plaster is then applied, followed by a lime plaster, with a finished thickness of 50 mm (2 in). Concrete is not ideal as a plaster as it cracks, potentially allowing water in, and has a different coefficient of thermal expansion to straw and so is more likely to fall off.

Answer: Use a timber frame

Timber performs relatively well in earthquakes. It can flex without breaking up to a point and using a timber post and beam construction with strawbale infill works quite well. The strawbale does not have a structural role in the building. Laid like bricks in overlapping layers, the bales are held in place by the timber framing. The framing also helps to keep the strawbale wall reasonably straight and vertical, which helps minimise the amount of plaster required to achieve an even wall surface. On the negative side, the thick plaster adds weight to the walls and so reinforcing is required. This can be achieved in a variety of ways.

In the fable, the brick-building pig fares best, however brick does badly in earthquake zones. One solution for a brick-loving pig is to use a structural timber frame with a brick veneer fixed to it, being more decorative than structural.

So, the strawbale house is really a hybrid, combining straw with sticks and even bricks. If the bricks have a structural role, they will likely be concrete blocks (also known as breeze blocks or cinder blocks) linked together with a lattice of embedded steel reinforcing rods and filled with concrete. Perhaps the moral of the 'Three Little Pigs' fable should be that they ought to work together, combining their preferred building materials for a more durable result.

Answer: No

Strawbales perform pretty well in a fire. First there is the 50 mm (2 in) of plaster to get through then the straw itself is so densely packed that the lack of air is a problem for fires. It may smolder but will not burn well. Other parts of the house will be more of a fire risk. Fire risk mitigation is more than about the house. It is also important to remove burnable materials in the vicinity of the house - pine needles in gutters, firewood stacked against the house, dead vegetation and sticks on the ground, ladder fuels in trees, etc.

Insurance is usually the backstop however the unconventional nature of the build means that finding an insurer can be an issue. This in turn may affect your ability to get a mortgage. Something else to check out before going too far down the path.

Answer: Water

Flooding from a blocked sink or a burst water pipe are risks in all houses and can cause serious damage to strawbale walls. These risks can be mitigated to some extent. The bottom layer of bales should sit at least 50 mm (2 in) above the floor on treated timber. Water pipes should be routed through internal (non-strawbale) walls rather than the external strawbale walls. If going through an external wall cannot be avoid, passing the water pipe through another pipe will lower the risk.

Floor-level drainage to the outside is another idea. Suitable flashing on top of the bales needs to be done to prevent condensation flowing down the underside of the roof into the bales. Window and door flashing is another potential area of weakness, although an all-round roofed verandah would help with that.

Answer: Engineer

While manufacturers provide performance data on their building products, how a particular house design will perform, whether in a standard build or a one-off strawbale design, needs to be proved usually by calculations made by an engineer. With an off-the-shelf house design, the cost of engineering can be split across a number of houses built using the same design. For a unique strawbale design, an engineer's time will be a larger percentage of the overall build cost. Evidence may also be required that, for example, an earth plaster is an acceptable solution. Using professionals with experience in such designs should make navigating the process easier.

An option is to hand over the problem to a building company with its own strawbale designs. You still need to do your due diligence and review their design. For example, they may be tempted to apply a standard weatherproofing solution to make the building consent process easier, however such a solution (such as using a moisture barrier against the straw followed by a vented cavity and external cladding) could compromise the wicking function of the wall, potentially trapping moisture whilst effectively creating a chimney next to the straw in the event of a fire. With no thick plaster layer to protect the straw, it would also increase the risk of rodent and insect infestation.