Bjørn Kierulf from Createrra explains how you can have the best of both worlds, using almost entirely natural materials to create a house that reaches the Passivhaus Standard.
Interview with Bjørn Kierulf
Bjørn is an industrial designer and his wife is an architect, and together they run Createrra.
They have always been into building with natural materials, but now have adapted their approach to achieve Passivhaus Standard as well.
Natural Materials are Often Good Insulators
Most natural materials are fibrous and as such tend to be good insulators. They also have protection against summer overheating, though do need to be protected from humidity and moisture.
In order to reach high levels of insulation, however, a greater thickness of the material will be required (compared to artificial insulations such as aerogel).
Straw has the Ability to Take Structural Loads
When compressed, straw has very good insulation values and unlike most other materials also has the ability to take structural loads. While there are a number of load-bearing straw houses the structural calculations can be quite difficult which is why timber is usually used as well.
A further benefit is that it can be added to the compost heap, rather than landfill, when the building is demolished!
A Straw Bale House is Not a Flammable One!
Fire needs air, but straw bales are so densely pressed that there is no air to burn. When combined with a typical clay plaster it can have a 2-hour fire protection, meaning that even with a 1000 degree fire burning for 2 hours on the inside, there is no noticeable temperature difference on the outside.
A Danger of Lime Render is High Water Absorption
Lime render is typically used by self-builders who are building with straw. Bjørn references the recent International Passivhaus Conference where the dangers of high water absorption in lime render were discussed. At Createrra they are looking into other options, such as industrial certified render plastered on wood fibre board which creates a thinner layer.
There is Often a Trade-Off When Using Natural Materials
Bjørn explains the difficulty of trying to get the right balance between using natural materials and materials that will last well under humid conditions. He sees this situation persisting for the foreseeable future, though predicting huge development and innovation in this area.
Natural Materials are the Basis of Innovative Products
Bjørn uses the example of a Swiss company who have experimented with using enzymes to treat the straw, thus giving it very different properties once it has dried out.
He also explains how a Danish company are using crushed mussel shells to make a foundation. The shells don’t absorb water and with air gaps they do have some insulating properties.
Much More Experimentation is Required for Natural Foundations
Bjørn feels they are still some way off an ideal natural foundation that he would recommend for clients. Hempcrete is another example he gives which, while appropriate for some areas, doesn’t have particularly good insulation values so wouldn’t create a thermally efficient building.
Airtightness is Crucial and Has Often Been Overlooked by Self-Builders
Problems are caused by humid air going through gaps in the construction and condensing in the structure.
One solution is to use a clay plaster, which is airtight, however it is difficult to achieve full airtightness because the plaster will always need to connect somewhere, such as where there are electrical sockets or beams going into walls.
Createrra are helping to develop a system using Ecococon straw panels where the airtight layer is on the outside of the straw insulation. This is in contrast to the typical way of doing things, where the airtight layer should always be on the inside in cold climates. These panels are wrapped in a recyclable airtight membrane and covered with wood fibre panels / ventilated façade. Early blower door tests have found that they meet the rigorous Passivhaus Standard.
The Ecococon panels are a combination of straw with wooden construction which are used to create the structure.
It is Difficult to Achieve an Entirely Natural Building
There are areas where construction using natural materials can be achieved easily, but areas like the foundations are more challenging. When also considering the whole of the interior, things like electrical cables being wrapped in PVC shows where there is still so much more that can be done.
Bjørn says that only around 500 materials were used in the construction of buildings at the beginning of the 20th century, but now that figure is closer to 50,000. He sees the goal of having a building that at the end of its life can be put on the compost heap or recycled as being a greater challenge than solving the world’s energy crisis.
Createrra Have Completed 5 Passivhauses Almost Entirely From Natural Materials
The exceptions were the foundations and some of the roof membranes.
Feedback from their customers has been extremely positive, with perfect comfort levels maintained throughout the seasons. Bjørn thinks when people see the flat straw panels and the efficiency of the construction they see it as an attractive proposition, when perhaps they might not previously have considered straw.
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