Reed in Architecture
Reed is one of the world’s most widely available wetland plant, thanks to its highly-reproductive characteristic. Reed can be found all over the world in moist landscapes, except in Antarctica. Its quick growth and wide availability makes reed a cheap and durable option for a natural construction material.
Looking at reed as a construction material, it has been used historically as a roof covering material. Proof of reed thatching has started as early as the ice age along the coast of the North and Baltic sea, but currently the most famous and well-described roof-thatchers can be found in the Tigris and Euphrates valleys in Iraq.
Reed for thatching is harvested in winter-time and dried for use. Reed contains the highest concentration of silicon of all plants, which is responsible for both its water-resistance and its high flammability. The most common reason (64%) for a thatched roof catching fire is ejected ember, and the second most common reason (11%) is a fire ignited by the chimney. However, all types of roofs have a risk of flammability, for instance asphalt in some shingles is tantamount to highly flammable crude oil. In addition, currently there exist products and techniques to protect the roof from catching instant fire.
Structure of a reed roof: a-d order of roofing, e – completed roof, g-h tools, sketch: Karl Tihase. Source: Stenman, 2008, Reed construction in the Baltic Sea Region
A quality thatched roof is a job requiring expertise, according to the intricate instructions and personal story reporting a brave but disappointed DIY attempt to roof-thatching found in the report by Stenman. A reed-thatched roof with a good quality would last for 50 – 100 years, in comparison to 50 – 70 years for a clay-tile roof and 30 – 50 years for wood tiles.
The characteristics of reed are fundamentally suitable as an insulation material; the heat conductivity (λ) of reed is low 0.055 W/mK as desired for insulation (Köbbing et al. 2013), as compared to clay 0.85 W/mK and 0.11 W/mK for timber shingles. When installing a reed-thatched roof in comparison to a clay-shingled roof, through the level of heat conductivity in most cases no extra insulation is required. The material costs of a reed roof are €32 per m2 in Estonia, €43–73 per m2 in the UK (Rauvola 2007), compared to the cost of clay tiles of around €15 per m2 in the UK, and also adding insulation (for instance 30mm glass wool) this would lead up to around (€15 + €11) €26 per m2.
Reed’s characteristics of heat conductivity in combination with its large air content, allowing for a good conduction of humidity, the material proves to regulate indoor temperatures well throughout all seasons. Reed panels require reed of lower quality than roof thatching-reed, they can be obtained for €6.5–10 / m2 in Austria (in 2013) (Hansmann 2008, Reichel 2013). In comparison to using standard 30 mm XPS foam as insulation which is more toxic and can be found in a range from €0,75 to €27 / m2.
Several modern day projects incorporate the craft of reed-thatching, for instance this project of a reed-insulated beachhouse.
Whether reed is the most sustainable option for roof- or insulation- material, depends of course on the environment climate, application and finish, as well as the other vernacular insulation options.
Reed in a modern architecture design
Information about reed roof thatching from the British Reed Grower’s Association
Köbbing, Thevs, Zerbe, 2013, The utilisation of reed (Phragmites australis): a review
Almusaed, Almssad, 2015, Building materials in eco-energy houses from Iraq and Iran
Stenman, 2008, Reed construction in the Baltic Sea Region