Posidonia oceanica (seagrass) in the roof of a house: the process

We have used Posidonia oceanica (seagrass) dried as thermal insulation, just like it was used in traditional architecture, to build 14 social dwellings in Sant Ferran, Formentera, promoted by IBAVI. 

The use of Posidonia as thermal insulation is possible because of the indoor air bubble content. In Formentera, one of the most abundant types of residue are dead Posidonia leafs. The annual surplus volume of Posidonia, around 4000 m3, would be sufficient to insulate all new dwellings on the island.

Protected plant 

To clarify doubts, the Posidonia oceanica is a protected plant and its use must be authorised by the Ministry of Environment by applying to the Service of Protected Species.

It has been collected by the company awarded the contract for beach maintenance and has been limited exclusively to the areas in which the material accumulated on beaches exceeds the amount necessary to maintain the balance of the coastal dune ecosystem and extreme care has been taken not to affect this ecosystem, under the supervision of the environment technicians from the Council of Formentera.

The illegal collection of Posidonia is fined with 3,000 euros.

Tests with the UIB 

Posidonia does not require artificial treatment as sea salt acts as a preservative and a biocide. Despite this, in order to ensure its worthiness as insulation and to determine its thermal conductivity, three different tests were carried out in collaboration with the University of the Balearic Islands (UIB). The result was a success.

The first test involved the use of two PCE-HT 71 data logger sensors.

El segundo, mediante un horno de secado comparando los resultados con poliestireno expandido. 

The second, using a drying oven, sought to compared the results against expanded polystyrene.

The third, involved using a heat flow meter pursuant to UNE-EN 12664:2002.

Dry Posidonia insulation provides a U: 0.2936 W/m2°C and Λ: 0.044 W/mK for a density of 185 kg/m3 and a thickness of 16 cm.

The process

Once the permission to use them were acquired and the tests with the UIB were carried out, we could build the roof. The compaction process to obtain 185 kg/m3 was performed manually, with participants using their hands and feet to push the leafs between the pallets that form the substructure to which the OSB boards are nailed.

Afterwards, the boards were water- proofed with EPDM and protected with sandstone slabs the form of paving.

The smell of the sea was apparent on the site throughout the process!

The use of posidonia oceanica (seagrass) as a building material

Let us begin by asking you, do you know what seagrass was traditionally used for?

1- In agriculture, as a fertilizer and a material to increase bulking in excessively clayey soils for cultivation.

2- In stockbreeding for animal bedding.

3- In the past, as a building material and thermal insulation.

We focus on point 3. This is why our project is called #LifeReusingPosidonia. We use dry seagrass as a building material, as in the past, taking advantage of its thermal insulation properties.

Why do we believe that it is a good idea to use posidonia in construction?

First reason:

Because it makes us of the natural resources at hand, not having to transport it to the island. This means a reduction in the consumption of non-renewable raw materials and a reduction in carbon emissions.

Second reason:

Because we recover its former use as thermal insulation. Years ago, buildings were made with wooden beams and cross braces - usually juniper wood- on which a 20-30 cm thick layer of seagrass was laid. This was then cover with a mixture of ash and lime from the lime kilns and then finished off with a layer of clay. Nowadays traditional houses in Formentera made this way are still standing. Reusing and advancing.

Third reason:

Because we use the material from the controlled removal of seagrass without harming the environment, as it is essential to maintain beach dunes! In Formentera there is such a surplus that we have no problems with sourcing, and it is good to make good use of it.

These are the three reasons why we believe that using posidonia oceanica #isagoodidea. We have begun to use it in the 14 Public Housing homes in Formentera.

Help us to publicise the project and support efficient and responsible building!