building materilas-1_architecture_Straw as a building material-physical aspects - Basics, Fire, moisture, insects and pests proof. Plastering straw bale walls, straw bale roof.

1. Unit IV
S T R A W B A L E S

2. Under unit IV…….
 Straw as a building material
 Physical aspects
 Fire, moisture, insects and pests proof.
 Plastering with straw
 Straw bale walls
 Straw bale roof.

3. Introduction:
 Straw – the stem material left as a by product of modern grain harvesting.
 Compressed and tied with string/ wire – bales stacked in running bond, plastered with
mud.
 HAY is grown for livestock feed and harvested green, while straw has no nutritional value
and is therefore not as susceptible to rot or insects.
 When properly designed and constructed, straw bale is a viable and sustainable building
system.
 Straw has been used as a building material for thousands of year either as an additive to
clay In the form of adobe or Cob, or as a water resistant layer in the form of thatch.
 Straw – low environmental impact construction for…
- Straw has excellent thermal insulation properties.
- Straw has excellent sound insulation properties.
- The production of straw is a low energy process compared with other building
materials.
- Straw stores carbon within its structure thereby reducing atmospheric CO2.

4.  Straw is inherently structural and maintains its form because of its tubular shape.
 The material’s microscopic waxy coating makes it slightly water resistant.
 A straw bale is a bundle of straw bound with bailing wire or polypropylene twine.
 Sisal has also been used for binding in the past but is not rot resistant.
 Straw varieties include wheat, rice barley, hops and oats.
 There are two types of bales… 2 string and 3 string.
 Bale sizes varies according to the Baler and practices.
 Bale weight may also vary depending on density and moisture content.
 Straw is an organic material that carrier particular risks with it in the context of living
accommodation. These risks included….
- Fire : Straw is highly flammable.
- Rodent and insect infestation : Straw can contain protein and carbohydrates which can
sustain life.
- Decay : Under the right environmental conditions straw is
susceptible to both aerobic and anaerobic decay.
- Structural instability : Straw bales have low compressive and flexural strength and
stiffness.

6. Bale loaded:
 Bale loaded “flat” are loaded perpendicular to their largest face parallel to the
plane of the tie hoops, and generally perpendicular to the straw fibre.
 Bales loaded “on edge” are loaded parallel to their largest face – perpendicular to
the plane of the tie hoops, and generally parallel to the straw fibres.
Different methods of building with straw bale:
 Load bearing
 Light weight frame and Load bearing
 Infill and timber frame
 Hybrid design

7. 1. Load bearing: Nebraska
 Load bearing straw bale, is constructing straw bale walls in such a way that it
supports all roofs.
 This is the original method of building, pioneered by the Nebraskan settlers in the
USA.
 In this method, the bales takes the weight of the roof without any structural
framework.
 They are placed together like giant building blocks, pinned to the foundations with
an wooden roof plate on top.
 The roof plate is fastened to the foundations and the bales with coppiced hazel
and strapped, the roof is constructed in the manner on top of the roof plate.
 Windows and doors are placed inside structural box frames, which are pinned into
the bales as the walls go up.
 This is the simplest method, ease of design, minimal use of timber, and the
opportunity it affords for a modern day wall raising.

8. Advantages of Load bearing:
 A simple, straight forward and accessible building method.
 Easy for non-professional to design, following comprehensible basic principles.
 Designs from one home to two-storey homes can be created using a simple, step
by step approach.
 Curves and circles are easy to achieve for little extra cost.
 Ideal for self-builders for its simplicity, accessibility, ease of design and low cost .
 Great versatility of design shape can be achieved.

9. Disadvantage of Load Bearing :
 The straw must be kept dry throughout the whole building process until it is
plastered.
 This can be very difficult on a large building, or being constructed slowly.
 Openings for windows and doors must not exceed 50% of the wall surface area.
 Maximum unsupported (unbraced) wall length is 6M (20f)

12. Infill style / post and beam / timber frame:
 Infill straw bale involves building some independent structural system - Stud walls, post and
beam, etc, - to support all structural loads, and then straw bales are in-filled between or
around that structural system to create the enclosure.
 Milled lumber is the most frequent material for supporting elements, although concrete block piers
and concrete or steel columns have been used.
 The advantage of this system is greater design flexibility, including the possibility of designing
for more than one storey in height.
 The system has greater long-term stability and can be custom engineered, in contrast with the
prescriptive approach used in the Nebraska style
 It is a more sophisticated system requiring skilled labour, and is, therefore, more costly.
 The infill construction system has posts and beams that extend up to the top of the walls.
 The ceiling and roof are mounted on the upper beams or ring beam. Consequently the weight of
the ceiling and roof is transferred through the posts and beams down to the foundations.
 Traditionally, infill buildings have had a timber or steel structure that is completely self-supporting.
 More often, there are 16 mm steel bracing rods fitted to the structure to provide the lateral stability
to the building.

13. Advantages:
 The roof can be constructed before the straw is placed, giving secure weather
protection.
 Framework and posts can be constructed off site.
 Provides greater stability for window frames than in the load bearing style.
 In conjunction with a steel frame, can create large warehouse space (and gives an
even temperature throughout the year).

14. Disadvantages:
 It is more complicated than the Nebraskan style to construct.
 It requires a high level of carpentry skill (or metalwork experience in the case of a
steel frame) to construct the frames.
 It uses a large amount of timber.

15. Light – weight frame and load bearing:
 It uses a timber framework that is so light-weight that it cannot stand up alone. It requires
temporary bracing to give it stability until the straw is in place.
 The straw is an essential part of the structural integrity of the building, more so than the
timber, and it works together with the timber to carry the load of floors and roof.
 Timber posts are located at corners and either side of window and door openings only, and
are designed such that the timber wall plate at first floor and / or roof level can be slotted
down into them once the straw is in place allowing for compression on the bales.
 Compression of the straw bale infill walls is essential for stability.
 To increase stability, the bales are pined externally, and the pins are secured onto the base
and wall plate of the framework once all the settlement of the walls is complete.
 It is constructed in such a way that the wall plate and roof are kept 100mm above the finished
straw wall height whilst the wall is being built, allowing for compressive settlement of the
straw wall once the bracing and props are removed.

16. Advantages:
 The roof can be constructed before the straw is placed providing secure weather
protection.
 Framework and posts can be constructed off site.
 Provides greater stability for window and door frames than in the load bearing
style.
 Vastly reduces the amount of timber required compared to the more traditional
post and beam method.

17. Disadvantages:
 It is more complicated than the Nebraskan style to construct.
 Greater technical ability is required to make the structure stable whist the straw is
being placed

18. Straw bales as a building materials
Fire Resistance:
 The tight packing keeps the available oxygen needed for combustion very limited. Also the
high silica content in straw (3-14%) is said to resist fire – as it begins burning a layer of char
develops, which insulates the inner straw.
 Straw bale walls are naturally fire resistance. While loose, dray straw is combustible, but
when it is compacted into bales, there’s not enough air from the straw to burn well.
 Combining plaster with the compactness of a bale enhances fire resistance.
 The plaster coating effectively seals the already fire resistance bales inside a non
combustible casing.

19. Insects and other pests:
 Moisture and rot in straw buildings is the concern about vermin.
 There are accounts of major problems with fleas in early hay – bale building in
Nebraska that were not plastered on the interior.
 But with careful plastering, you can easily keep out rodents and other pests. In
areas with significant rodent problems, using a finer woven mesh than chicken wire
for plastering might be a worthwhile precaution.

20. Moisture problems and rot:
 Few organisms are able to decompose straw.
 High moisture level in straw bales – over 70% - can provide a habitat for fungi and lead to decomposition.
 Keeping bulk moisture away from walls – for example, using wide overhangs, sloping the ground away from the
building, and installing a good capillary break between the foundation and bale walls.
 The main source of potential moisture is liquid penetrating into the wall cavity. This can happen in many ways
such as wind blown rain, drifting snow, splash back from dripping roof, plumbing leak, floods, hoes and
breaches of the walls protective layers in leaky roofs and window sills.
 A bale wall is raised on a wooden curb on the foundation or floor will ensure that any spills or floods inside
house will not soak into the walls.
 Plastic or tar paper placed along the top of walls will protect against any water that may come through your
roof.
 Windows and doors also can be installed with proper flashing and drip edges that shed water away from the
walls.
 The other moisture can damage a building is vapour migration through walls.
 During the heating season, the house functions similarly to the balloon, when heat is added to your living space,
your relatively airtight house is filled with warm, moisture-laden air. Extra moisture is added by breathing,
cooking, bathing etc and that air will naturally search for a way out of the house and into cold, dry air outside.

21.  As the warm, moist air tries to travel outdoors, it will begin to cool. As it cools, the water
vapour it carries will condense back to liquid.
 If liquid deposited in our walls and allowed to remain there without drying, it will reduce
the efficiency of your insulation and eventually lead to mold and rot.
 In hot climates, the process can happen in reverse, especially if you use air conditioning.
 The plaster coating on bale walls is an effective barrier against moist air leakage. If properly
tied in with conventional polyethylene vapour barriers installed in the ceiling and under the
floor, a bale house can be made almost airtight.
 Conventional building practice-prevents the moisture through the use of plastic vapour
barriers in the walls.
 Conventional frame construction often relies on a plastic film, vapour retarder paint or
other vapour diffusion retarder to help keep moisture out of walls.
 Climate specific moisture detail-in cold climates use a vapour retarder paint on the interior,
in hot climates use a vapour retarder paint on the exterior, in other climates the walls
probably be left permeable.