The Modern Seaweed House by
Vandkunsten and Realdania Byg


Seaweed pillows were used as cladding for this holiday house on the Danish island of Læsø by architecture studio Vandkunsten and non-profit organisation Realdania Byg (+ slideshow).

The Modern Seaweed House by Vandkunsten and Realdania

The Modern Seaweed House revisits the traditional construction method in Læsø, where for many centuries trees were scarce but seaweed has always been abundant on the beaches. At one stage there were hundreds of seaweed-clad houses on the island but now only around 20 remain, which prompted Realdania Byg to initiate a preservation project.

The Modern Seaweed House by Vandkunsten and Realdania

The team enlisted Vandkunsten to design a new house that combines the traditional material with twenty-first century construction techniques.

The Modern Seaweed House by Vandkunsten and Realdania

"Seaweed is at the same time very old and very 'just-in-time', because it is in many ways the ultimate sustainable material," Realdania Byg's Jørgen Søndermark told Dezeen.

The Modern Seaweed House by Vandkunsten and Realdania

"It reproduces itself every year in the sea, it comes ashore without any effort from humans, and it is dried on nearby fields by sun and wind," he continued. "It insulates just as well as mineral insulation, it is non-toxic and fireproof, and it has an expected life of more than 150 years!"

The Modern Seaweed House by Vandkunsten and Realdania

Rather than just piling the seaweed onto the roof, the designers stuffed the material into netted bags and attached it in lengths across the timber-framed walls and roof of the house. More seaweed was enclosed in wooden cases to use as insulation behind the facade and beneath the floors.

The Modern Seaweed House by Vandkunsten and Realdania

"By using seaweed in the construction, we not only secure the continued supply of seaweed for use on the historic houses, we also reintroduce a material to the modern building industry which is CO2-reducing, environmentally friendly and sustainable in a broader sense," said Søndermark.

The Modern Seaweed House by Vandkunsten and Realdania

The interior walls are lined with wooden boards, framing a series of rooms intended to house two families. A double-height living room and kitchen forms the centre, while bedrooms are located at the ends and in the loft.

The Modern Seaweed House by Vandkunsten and Realdania

"Our project has demonstrated that seaweed has remarkable acoustic properties," added Søndermark. "This creates surprisingly comfortable rooms, while the ability to absorb and give off moisture contributes to regulate the indoor climate."

The Modern Seaweed House by Vandkunsten and Realdania

As well as building the new seaweed house, Realdania Byg has restored the seaweed roof of Kaline's House, a 150-year-old residence next to the site. The team hopes the two projects will inspire more seaweed architecture and restoration in Læsø.

The Modern Seaweed House by Vandkunsten and Realdania

"The seaweed houses on Læsø are physical testimony to the culture and the life that have characterised the building tradition on the island for centuries," said Realdania Byg director Peter Cederfeld. "It is our hope that others will embrace the experiences from this project and develop the ideas even further."

The Modern Seaweed House by Vandkunsten and Realdania

Seaweed has also been used in a few lighting designs recently, including for a series of laser-cut lampshades. See more stories about seaweed »

The Modern Seaweed House by Vandkunsten and Realdania

Photography is by Helene Høyer Mikkelsen and Realdania Byg.

Here's a press release from Realdania Byg:

The cultural heritage of Læsø: A resource in sustainable building

On the small island of Læsø in Denmark, a several hundred-year-old building style has formed the basis on which a new holiday house has been built – the Modern Seaweed House. The house is designed by Vandkunsten firm of architects and developed by Realdania Byg as a holiday house built in wood, covered and insulated with seaweed. The Modern Seaweed House is carefully adapted into the landscape and has a wonderful interaction with nature, the historic buildings and Læsø's unique cultural history. The Modern Seaweed House is now to be sold – but the ideas live on.

The Modern Seaweed House

The Modern Seaweed House is part of the Realdania Byg project 'Seaweed Houses on Læsø' that also includes 'Kaline's House' – a listed seaweed house from 1865, purchased and carefully restored by Realdania Byg in 2012. The seaweed houses on Læsø are an exceptional part of the cultural heritage of Denmark – and the world. Originally, several hundred of these seaweed houses were found all over Læsø while only approximately twenty remain today. The traditional seaweed houses were built using a timber frame construction with robust seaweed roofing – an abundant resource in the small and modest fishing community. 'Kaline's House' is one of these houses.

The Modern Seaweed House by Vandkunsten and Realdania

The Modern Seaweed House is not a replica of the building style of the past but a development inspired by the architectural history of Læsø. In contrast to the historic houses, on which the seaweed is stacked high on the roof, the Modern Seaweed House is more contemporary and tight in its expression. The visible seaweed has been stuffed into bolsters made of knitted nets attached to the façade in lengths. At the same time, seaweed is used invisibly for insulating floors, walls and ceilings enclosed in wooden cassettes. These prefabricated building modules comprise the framework of the house.

The Modern Seaweed House by Vandkunsten and Realdania

A sustainable resource

When seaweed was used in the past as a building material it was due to the fact that seaweed was found just outside the door, it was free, had a long-term durability, was very effective as insulation, naturally protected against vermin and putrefaction, and, finally, there was lots of it. These very preconditions make seaweed of current interest as a building material, especially in the light of the present attention to the topic of sustainability. The Modern Seaweed House fulfils expected 2020 demands, and, thereby, will have extremely low energy consumption.

The Modern Seaweed House by Vandkunsten and Realdania

At the same time, LCA (life cycle analysis) calculations have shown that the house actually has a negative carbon footprint. The almost exclusive use of organic materials, including seaweed used as both insulation and roofing material, causes the amount of CO2 accumulated within the house to exceed that which has been emitted during the production and transportation of the building materials.

The Modern Seaweed House by Vandkunsten and Realdania
Modern Seaweed House with Kaline's House

In a broader view

With the 'Seaweed Houses on Læsø' project, Realdania Byg wishes to focus on the unique tradition of Læsø using seaweed as a building material – both the immediate need to ensure the architecture of the past and the at least equally relevant need to develop the architecture in a sustainable approach. This way, seaweed is also ensured for restoring the historic houses.

Realdania Byg's project to develop and preserve seaweed houses on Læsø is one among a variety of existing projects that aim to secure the survival of the distinctive seaweed roofs on Læsø. The initiative is carried out in unison with enthusiastic inhabitants of Læsø, other foundations as well as the Danish Agency for Culture who are all involved in the effort to save this rather exceptional part of the cultural heritage of Denmark – and the world.

The Modern Seaweed House by Vandkunsten and Realdania
The restored Kaline's House nearby

The Modern Seaweed House has shown that eelgrass has a lot of qualities. Besides its excellent insulating property and long-term durability, which in itself offer a lot of potential, it has been discovered through practical application that seaweed has exceptional acoustic properties. This creates surprisingly comfortable rooms while the ability to absorb and give off moisture contributes to regulate a good indoor climate. The numerous qualities provide a wide range of applications in modern, sustainable building.

  • Ian Lapworth

    Looks similar to a piece of sushi I ate for lunch.

  • François

    It’s a great idea!

    But what happens to the seaweed if it rains? Do they swell? And the smell in the house – is it so strong? Does the seaweed repel insects or attract them?

    I know that’s a lot of questions, but in France we (a group of friends) seek ecological materials to build structures such as schools, houses and farms (farmacultures).

    Thank you for your answer.

  • Paul O’Brien

    I think that this is a beautiful little project. The covered entrance is fantastic.

    It will be interesting to see how the seaweed weathers and what the overall effect is after a year or so.

    The only let down for me is the internal roof soffit material. It looks so factory made in comparison to the rest of the scheme, which is finely crafted.

    Great stuff.

  • Jorgen

    Dear Francois, some answers:

    The seaweed does not swell as such when it rains; it will be wet though. In the beginning all the way through, but later on we expect the seaweed to form alayer that stops the water, as it does in the old seaweed roofs that you find on the island of Laesoe.

    There is no smell whatsoever. Also, this is the same in the old seaweed houses.

    Insects and larger animals such as mice are repelled; probably because of the content of salt. But birds some times build nests into the large roofs of the old house – we believe this roof is too thin for birds to enter it.

    Good luck with your eco build, thumbs up!

    Dear Paul:

    The interior is hand-crafted upholstery using traditional methods – the organic linen textile is placed over a ‘mattress’ filled with seaweed, and much care has been taken to make it look nice and wrinkle-free. They create great acoustics inside the house and is soft to your body when you sleep on the semi-floor.

    Jorgen, project manager of the Seaweed House.

    • Mario

      Dear Jorgen ,

      What supplier did you use for the bags of seaweed? I am doing an academic project in which I would like to use this material, and would like to get hold of a sample of one of these bags! If you could help me it would be great!! Thank you,


  • Alex

    It is a fascinating material with potential in many industries. But unfortunately seaweed is very ugly.

    I think what works well is the insulation panels. Attractive enough that they do not need to be plastered or finished over and they make good use of the material’s properties.

    Politically this is a good advert for seaweed construction but architecturally I think the exterior would have been more attractive as a simple log cabin.

    Nevertheless, this looks like a really fun experiment to be involved in!

  • Filipe

    Fantastic project. Congratulations, you are brave.

  • judy

    Beautiful house! What type of wood did you use for the floor and walls? It looks like pine?

    • Hi Judy,

      Thank you. All exterior wood is larch without any treatment of surfaces. Interior wall cladding, floors and structural members are pine. Interior surfaces are wiped with white soap, floors with lye and white soap.

      Søren Nielsen, architect, Vandkunsten

  • Tony R

    How do you imagine this sort of building fairing in West Africa?

  • Really it is an interesting concept to find a seaweed house. It is better to find more useful facilities in this home structure. It is economical, cheap and easy to find different methods of house building. And the number of different looks to the house adds to its beauty.

  • Beautiful! What are the knitted nets made of?

    • arkitekten

      The nets are knitted with sheep’s wool!

      Jorgen Sondermark, Realdania

  • Mark

    I am intrigued by the use of this as a potential ecological roofing material.

    Could you envisage a roof frame with only a layer of seaweed on top as the protective layer against rain? How about breathability?

    • arkitekten

      Hi Mark,

      Yes, this can be envisaged indeed! It already exists on the island of Læsø! The Modern Seaweed House builds upon inspiration from this unique cultural heritage that you find only on this small spot in Denmark. The roofs, however, need to be very thick -1.4m – before you can trust them to stay waterproof, and obviously this affects the cost of the roof.

      The roof in the old houses – and the two newly restored; one of which Realdania invested in – is made with a spread layer of for instance pine branches, upon which the seaweed is stacked. But note the crafted ‘shelf’ made of twisted bundles of seaweed; a sort of spinning technique only enlarged, this shelf needs to be in place just above the facades before the more ‘coincidental’ stacking takes place.

  • guest

    Nice project! I wonder what it smells like after a long rain, though. Well done, congratulations!

    • arkitekten


      As posted above; the seaweed used here does not smell. One has to know that the seaweed used is in fact a water plant named ‘Zostera marina’. When left to rot on beach in wet conditions, it will smell, but this seaweed is taken directly from the beach and dried on a nearby field by sun and wind. After this, it will not smell when when rained upon.


      Jorgen Sondermark
      Realdania Byg

  • Arantzazu Blanco

    Hello, I like this a lot and I have found your project very interesting but I think Seaweed are algae, and not phanerogams, which would be Seagrass. I mean that because we have Seagrass here in Mediterranean too: Zostera spp and Posidonia spp. Unfortunately, in my area (Murcia, Spain) it is not appreciated and considered as a waste, so it is removed from the beaches, even when they have such an important role in ecosystems :(

  • Marije Mulder

    Hello Jorgen,

    We are so inspired by the modern seaweed house
    that we wish to use the material on the HollandWikihouse. (Wikihouse is an
    open source way of building. This will be one of the first wikihouses being build outside on our allotment in Leiden, the Netherlands.)

    In January 2014 we will attend this fair in Rotterdam:

    We will use frame of the house to show different materials for the building envelope on. It would be great to show the seaweed as well!

    Please let me know if you like me to get into contact with you.

    Best, Marije Mulder

  • RobinH

    Could you give me the specifications of the stairs?

  • marliana

    Hi. Really nice house. I’m an architecture student, can you tell me the detail construction of the roof? Like how it’s stacked, what material beside the seaweed is used and how long it takes? Thanks.