What is straw?
Straw is the dry stalks of cereal crops such as wheat, left over after the grain has been threshed. As a building material it is cheap, renewable and very old. It appears in three main forms: compressed into bales for walls and insulation, as fibre reinforcement in clay and daub, and mixed with clay as light straw-clay. Pressed straw panels are a separate product (see straw board).
Straw-bale construction began in the Nebraska Sandhills in the late 19th century, after mechanical balers became widespread; the soil there made poor building sod. The Pilgrim Holiness Church in Arthur, Nebraska, built in 1928, is listed on the National Register of Historic Places. Bales can be load-bearing (“Nebraska style”), used as infill in a timber frame, or combined in hybrid systems. Walls are finished with a thick lime or clay render. In the USA, the 2015 International Residential Code added appendices for straw-bale and light straw-clay construction. Measured thermal conductivity of bales mostly falls between about 0.038 and 0.067 W/m·K.
Straw in clay is far older. In wattle and daub, straw is one of the fibres that hold the daub together, control shrinkage and add some flexibility. Light straw-clay is clay mixed with chopped straw, used to fill timber-frame walls.
In practice
- Keep straw dry from delivery onwards. Wet straw loses its insulating value and rots. Studies of existing walls found straw at under about 20% moisture content in good condition.
- Raise the base and protect the top. Use a capillary break under the bales and a generous roof overhang.
- Use a vapour-permeable finish. Lime or clay render lets the wall dry out; dense cement render traps moisture.
- Render with no gap. Many fire rules require no void between straw and its finish.
- Check local acceptance. Building control approval varies by country, so ask early.
Common mistake: confusing straw with thatching reed. Reed is a wetland plant; straw is a by-product of cereal growing, although straw is also used for thatch.
Related terms
- Straw board
- Clay plaster
- Clay
- Thatching
- Bio-based construction
- Timber frame construction
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