Modern Seaweed House
Læsø island
2012 - 2013

Ar­chi­tec­tural sea­weed

A remarkable material

On the Danish island of Læsø there is an old building tradition of using seaweed or more precisely eelgrass for roofing their houses. Seaweed roofs last for centuries. In 2010 the Realdania Byg Foundation bought the 150 year-old listed seeweed house “Kaline’s House” and did a gentle restoration. Nearby was a modern summer residence design by architect Hanne Kjærholm. Realdania Byg Foundation bought the natural plot between the old and the new house. In 2012 Vandkunsten won the competition to design a small summer house that interprets the old traditional seaweed house, while also fitting the natural site with a contemporary vacation home.

The Modern Seaweed House became an experimental build. The solutions of the house gives a new suggestion to the use of seaweed as a renewed and modern construction material, that also respects the old local traditions of Læsø.

”The Modern Seaweed House on Læsø is not only a tale of the renewed use of an remarkable material on a unique site with an extraordinary building history. It acts like a crystal ball that catches and illuminates many of the most important issues the construction industry are facing today.”

- Søren Nielsen, arkitekt MAA, partner at Vandkunsten

Keeps CO₂

Life cycle analysis shows that the Modern Seaweed House bind CO₂ in the order corresponding to CO₂ emissions by ten years of use. This is a noticeable reduction in the collected energy use compared to most traditional buildings. The question is, what happens if the building is torn down and the materials incinerated – will then the entire CO₂ deposit be exposed out into the athmosphere? Yes, but in that case the CO₂ binding materials will replace other kinds of fuel and maintain a positive effect on the climate.

Traditional design with untraditional materials

Experimental construction – just like classical scientific experiments - does not include too many variables. If the experimental setup is too complex it will simply be hard to conclude exactly what went well or went wrong. The Modern Seaweed House is an experiment that involves materials and construction – it is not a social experiment. Most people should feel at home in the house and for this reason the house is designed somewhat traditionally with a pitched roof, a large and central living room with picked ceiling and sleeping rooms at each gable. The building is a winged house with a pitched roof and follows the local building tradition in regards to size as well as the east-west orientation. The building should not stand out in its overall design or the interior, but primarily through its materials and its details. Two restrooms are placed between the living room and the sleeping rooms. The sleepover capacity has been increased by placing lofts on top of the sleeping rooms.

Insulation like mineral wool

Eelgrass insulates and its insulation value is close to comparable to mineral wool. Eelgrass is used to insulate the building and are placed between loadbearing structures in the floor, the facade, and the roof. The eelgrass is also used as facade cladding.

Renewal with technology

Warm interiors with planed larch and spruce

Where there is no seaweed, there is wood. Wood boards from larch, pine, and spruce are used for structures, floors, interior cladding and kitchen tops. The wood is the neutral background for the unusual seaweed claddings.

Designed for disassembly

The building is designed using principles of ‘designed for disassembly’ to make sure that materials of the Modern Seweed House ends up in incineration as late as possible.

Effective construction process with prefabrication

Despite the modest size of the house, the use of prefabrication has contributed to keeping down the cost of construction. This allowed some more costly elements of the house – for example energy efficient installations and the bespoke upholstered elements covering the ceiling.

Upholstered elements with eelgrass

Project facts


Project name: Modern Seaweed House

Category:Housing, R&D, Wood

Client: Realdania Byg

Location: Læsø island

Gross area: 90

Date: 2012 - 2013

Status: Finished

Number of units: Summer house

Cost of construction: 2 mio DKK


Program: Zero Energy Summerhouse

Activity: Architecture, components design, mockups, material tests, process developments, LCA

Job type: 1st Prize in invited competition

Construction system: Wood with eelgrass elements on the facades, roof, insulation as well as upholstery.

Contact: Søren Nielsen,

Project group: Katrine West Kristensen, Nel Jan Schipull, Anne-Mette Manelius Greisen, Søren Nielsen


Architect: Vandkunsten

Landscape: Vandkunsten

Engineer: Moe

Contractor: Greenhouse