Are green houses more expensive than traditional houses

Green building materials: straw instead of concrete

Natural materials such as straw, clay or wood are in no way inferior to bricks in terms of quality. However, the costs are sometimes higher and processing is often time-consuming.

Straw house in the shell. The outside is then plastered with lime and the inside with clay (Photo: anandoart - Thinkstock.com)

Despite his 66 years of age, he is considered a shrill bird in the village. “The neighbors clapped their hands over their heads,” says Siegfried Schönbauer, remembering the time when he and his partner started building a house in Sankt Andrä-WIERT, Lower Austria. White facade, red tile roof and a biotope in the garden. At first glance, one cannot guess why Schönbauer's house caused a sensation in the neighborhood. When construction began in spring 2013, the trucks did not bring bricks but 200 bales of straw, as we know them from agriculture. It is the building material of the 90 centimeter thick walls. Outside they are wrapped with lime plaster as protection against the weather and inside with a layer of clay.

“We always wanted to build a passive house and by chance ended up with straw as a building material,” explains Schönbauer. The ecological aspect immediately convinced him, because the natural product does not consume any energy during production and, in contrast to synthetic materials such as styrofoam, does not contain any petroleum. There is also no need to worry about recycling later.

During the almost four-week straw construction work, a bizarre picture emerged: “Little by little, the workers stacked bales of straw on top of one another, lashed them with straps and then cut the walls straight with the chainsaw,” says Schönberger. No wonder they labeled him a weirdo.

Today, four years later, Schönbauer draws a positive interim balance: "Living in a straw house is even better than we imagined, because the indoor climate is fantastic." The humidity is constant at around 50 percent, although it took a few months until the heat pump and air circulation in the house were optimally set. The only downer: In the interior, small expansion cracks can be seen between the ceiling and the wall. Schönbauer names the costs at around 2,400 euros per square meter. That is a little less than he had calculated for the same house in the brick version.

In the end, the neighbors were surprised that a house was made out of the bales of straw. They gave Schönbauer a photo album about the building of the house, because a lot of photos were taken, driven by curiosity.

No risk of fire in the straw house

Straw houses like that of Siegfried Schönbauer are exotic in this country. There are an estimated 1,000 objects. “The trend is rising, because awareness of ecological houses is increasing. Not so much because of the environment, but because of the healthy and comfortable indoor climate, ”says Herbert Gruber, founder of the straw bale cooperative“ Strohnatur ”and chairman of the“ Austrian Straw Bale Network ”.

For the low-energy standard, wall thicknesses of 36 centimeters of straw are sufficient, which corresponds to around 30 centimeters of styrofoam insulation. The most common construction is the wooden stand construction. The load of the house is carried by wooden elements, the spaces in between are filled with straw. The rare load-bearing variant does not use wood. For this, the straw bale walls must be sufficiently thick so that they can brace the roof structure. “The durability of straw houses is over 100 years, provided that everything is done correctly during construction,” explains Gruber. Because moisture and vermin are the straw's greatest enemies. The concrete foundation must be at least 30 centimeters high so that the house does not get “wet feet”. Sealing tapes and plastering strips are also mandatory to prevent cracks and moisture ingress. The bales of straw must be very tightly pressed (density between 85 and 120 kilos per cubic meter) and dry (moisture up to 14 percent) to protect against vermin infestation. Incidentally, the pressing is also the reason why Schönbauer from Sankt Andrä-Wölker is not worried about the susceptibility of his house to fire. Straw burns brightly when loose, but only as good as a telephone book when compressed.

When it comes to costs, straw houses can compete with conventional houses. If you let the builder do everything, the square meter costs start at around 2,500 euros for single-story houses and from 2,000 euros for multi-story houses. Straw expert Gruber organizes workshops in which you can familiarize yourself with the building material straw and learn straight away how to help yourself on your future construction site. That pushes the costs down to 1,500 per square meter for a low-energy straw house. With around 8,000 to 10,000 euros in material costs per house, straw as a building material is cheaper than brick or concrete, but the bottom line is that a straw house costs roughly the same as a solid construction. The reason: clay plaster for the interior is more expensive than plasterboard or conventional interior plaster. But without him, the straw house is only half the fun, as he is responsible for the indoor climate. "Clay keeps the moisture away from the straw and releases it evenly into the room air," says Gruber. In practice this means: Do not ventilate immediately after showering or cooking so that the plaster is enriched with steam.

Mud house à la Grand Canyon

If you like that, you can build your whole house mostly out of clay. Like earth building project developer Martin Rauch from Vorarlberg: “I wanted an African hut with European standards.” Rauch's house walls consist of clay through and through, also known as rammed earth. For this purpose, clay is poured into a formwork and tamped in layers. The density of the material roughly corresponds to that of concrete. Ecological advantage: a large part of the excavation can be used for building a house.

Another special feature of earth buildings is the controlled erosion of the facade. Purists do not plaster them, but let wind and weather work on them, so that a rough, grainy surface is created. “This is not an ongoing process, but rather comes to an end when it looks like it is in the Grand Canyon,” says Rauch. So rain does not continuously carry away the house, but predominantly only washes out the fine elements. The more solid components such as pebbles defy wind and weather. Inside, reeds are suitable as insulation and - how could it be otherwise - fine clay plaster as a finish.

However, earth building has an absolute niche existence in Austria. This could also be due to the costs, because building with clay is very labor-intensive. The cost of a house is roughly 30 to 40 percent higher than that of a conventional house.

Spruce wood as a substitute for concrete

If you like it less exotic, but are still looking for a natural, local building material, you can of course also use solid wood. Like Günter Pichler, architect and professor at the Technical University in Vienna. In the allotment garden idyll on the Old Danube, between garden gnomes and flower beds, his completely black house in the shape of a rectangle clearly stands out. The walls of the 17 by four meter house are made of cross-laminated timber, or CLT for short, a high-tech version of the centuries-old solid wood construction. CLT, in German: cross laminated timber, consists of several layers of cross-glued spruce boards. As a result, the wood achieves static properties that are comparable to concrete. "In practice, CLT is even the better concrete," says Pichler, aiming at easy processing. Instead of concrete mixers and formwork, the components can be prepared in the carpentry and only need to be screwed together on the construction site. In terms of thermal insulation properties, 16 centimeters of CLT correspond to around 160 centimeters of concrete. For a low-energy house without additional insulation, you have to calculate with 28 centimeters. However, the material is around ten percent more expensive than concrete or brick. For a finished house you can calculate with square meter prices of around 2,700 euros.

One disadvantage compared to concrete or brick: CLT has a lower storage mass and therefore stores
z. B. the heating energy in winter less long. Pichler counteracted this in his house with an exterior layer of rock wool. “The mineral insulation material has a high storage mass and at the same time acts as fire protection,” is his strategy. The black outer skin, on the other hand, is a material experiment. The sun warms it up very quickly - a blessing in winter and a curse in summer.

Links & contacts for green building