Thatching with seaweed for greener construction

Materials World magazine
7 Aug 2019

A dissertation project building on traditional Viking methods of thatching eelgrass is helping make this seaweed a popular choice of modern building material.

An old method of thatching seaweed, used by Vikings on the Danish island of Læsø, has been updated by Copenhagen School of Business and Design architectural technology student, Kathryn Larsen. Larsen believes the technique could be adopted more widely as a modern and environmentally friendly option in construction projects.

The project, Seaweed Thatch Reimagined, looks at using eelgrass to make prefabricated panels for either roofs or façades. Larsen told Materials World she chose this seaweed because of its many natural properties and to raise awareness of how it is used in Denmark.

‘I wanted to create a solution inspired by the old, but for a new and modern building industry,’ she said.

‘If it is prepared correctly – laid on a field shortly after harvesting for two weeks to two months to remove the microalgae – it is rot-resistant. It is naturally fire-resistant because of the salt impregnated in it, and it insulates comparably to modern mineral wool. It is also carbon-neutral. If you are using it locally, it can reduce the carbon footprint of your building.’

Eelgrass is not immediately waterproof but becomes so after approximately one year outdoors. Therefore, it is not suitable for all climates, however, it has proven successful on the island of Læsø and would have similar potential in other regions with a dry climate.

Water is the way

Larsen tested several combinations of seaweed with natural binders to achieve the best results.

‘I found that [water] was the best option to create this loose, organic shape. The idea of using binders came from Studio Seagrass’ work with eelgrass, and I had hoped it might waterproof the construction faster. My testing outside has shown that this is not the case, with the specific binders I used,’ she told Materials World.

Larsen tried other binders including casein glue and agar-agar, but they caused undesirable effects. ‘The bone and hide-based glues I used tended to make the eelgrass brittle and prone to breaking. The eelgrass itself does not smell, but the animal-based glues smell terrible, especially outside. That isn’t very appealing for people to use in their homes,’ she said.

‘I found out later that they thatched the Læsø houses with dry seaweed, but I found that hydrating it helped keep the shape as I thatched it, and seemed to help the natural binders in the eelgrass itself to be released. Now the first tests have been outside for around 10 months, the eelgrass is nearly solid from repeat exposure to rain.’

It is becoming increasingly acceptable to use eelgrass as a material for insulation. Larsen sees her panels as a supplementary insulation option or being installed over existing surfaces of renovated buildings to reduce heat loss, as well as panels for roofs and façades. ‘I am currently working on newer prototypes of panels that I hope will be tested for a u-value [insulation rating] soon,’ she said.