Technical principles of straw bale construction - Architekt Dirk Scharmer

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Technical principles of straw bale construction

Straw as building material
Using straw as insulation is a relatively recent practice. By means of tests and analyses plenty of its features and characteristics are known and defined by now. Neither vermin nor moisture and fire can harm professionally installed bales of straw. However, certain rules must be followed so that the approved quality characteristics are valid on a German construction site. The straw bales from nearby fields have to turn into an approved construction material proven to be applicable for its purpose.

Such a proof of applicability can be the general building supervision (Allgemeine Bauaufsichtliche Zulassung) Z-23.11-1595 for the thermal insulation material "bales of straw for building" issued by the company "Baustroh" in Verden/Lower Saxony.

In order to be permitted to apply straw bales as thermal insulation material in accordance with German building law, the bales of straw must come from or be certified by "Baustroh". The latter occurs, for example, if straw bales are to be applied as regionally as possible. For more information, please see www.baustroh.de and www.fasba.de.

Building license
Depending on the intended type of construction it can be necessary to not only make use of approved bales of straw for building, but additionally to request an individual approval from the supreme construction supervision of the respective state.

How well do straw bales insulate?
Straw bale insulated construction components outperform the thermal insulation requirements. The approved thermal insulation material "bales of straw for building" is characterized by a proven and regularly officially controlled thermal conductivity. The rated value for calculating the U-factor is
λ=0,052 W/mK. Therefor the bales of straw must be installed in such a way that the heat flow through the construction component runs at right angles to the predominant alignment of the straw. A straw bale wall 35 cm in width equates to passive house standard. U-factors from 0,17 to 0,12 W/m²K are possible.

How high is the fire risk of straw bale insulated construction components?
Bales of straw as construction material feature excellent fire protection. Thorough production of the bales of straw for building as well as the inspection of an approved supervisory board ensure high grade straw bales. Thereby these straw bales can be classified into the building material class ‘normally flammable’ (DIN 4102 - B2 / EN 13501) – which meets the minimum requirements for the certification as construction material.

Yet another cladding of the straw not only completes the construction component, but enhances the fire protection. For this purpose, it is advisable to apply a mineral facing of at least 10 – 20 mm. On the inner surface this might be clay plastering directly applied to the straw. Flammability tests could demonstrate that already a 8 – 10 mm thick clay plastering can result in a fire-resistant component (DIN 4102 F30) and a flame-retardant surface (B, s1, d0 according to EN 13501-1: 2007). In general, the clay plastering of straw bale walls is 20 – 40 mm thick and thereby exceeds the certified requirements considerably. A straw bale wall can be officially used as a fire-retardant wall if it complies with the general test certificate from FASBA. Thereby even large buildings with multiple units can build, if no further fire protection requirements have to be be fulfilled. An ongoing research and development project which will end mid 2013 shall verify that construction components insulated with straw and plastered with clay or lime can be used for up to five-story buildings from a technical and from a building regulations’ point of view.

Moisture proofing
Moisture in straw bales on exterior components will not raise a problem if a construction pervious to vapor plus an adequate exterior cladding are being used. They also need to be effectively protected against driving rain. The indoors plastering has to be free of cracks and as airtight as possible. While installing the bales of straw and applying the plastering the seasonal climate has to be considered. In general, the facing has to be finished before winter. The current general building supervision specifies a detailed framework which complicates the direct plastering. Further studies and verification shall be done until 2014 to achieve a general proof of usability for the direct plastering of exterior walls insulated with straw. At present individual case surveys already exist demonstrating that under certain circumstances a straw bale wall plastered directly with lime is complying to the general building supervision.

 
 
 
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