Carbon-Storing Acoustic Panels from Seagrass: Søuld’s Reinvention of a Forgotten Material — Cradle to Cradle Certified® Gold v4.1

A new generation of building materials is emerging - not only reducing environmental impact, but actively contributing to climate solutions.

Søuld, a Danish scale-up, has developed acoustic panels made from eelgrass that function as a carbon sink when installed in buildings, storing biogenic carbon within the material for as long as it remains in use. Following several years of research and development, the company brought its first commercial products to market in 2020. Today, its panels are recognised among a small group globally to achieve Cradle to Cradle Certified® Gold, Version 4.1 - one of the most demanding product standards.

This achievement signals more than product performance. It reflects a broader approach - one that connects material innovation, local value creation, and verified sustainability into a coherent system designed for scale. 

From Coastal Biomass to High-Performance Material

At the core of Søuld’s innovation is eelgrass - a marine plant that naturally washes ashore along coastlines. Traditionally overlooked, this fibre holds a unique property: it contains significant amounts of stored carbon, as documented in the EPD downloadable on the company’s website here.

When transformed into acoustic panels and installed in buildings, that carbon remains locked within the material.

This characteristic positions Søuld’s panels within a new category of materials - not only low-impact, but actively contributing to carbon storage in the built environment.

Crucially, this environmental performance is matched by technical and commercial viability.

“It made no sense for us to just be a sustainable alternative. We wanted to be a real alternative to traditional materials.”

From the outset, the ambition has been clear: to meet industry expectations on acoustics, fire safety, and usability, while operating within realistic market conditions. The result is a high-end product that competes not through compromise, but through equivalence - and differentiation.

Reinventing a Material Heritage

While Søuld’s panels are contemporary in application, their origins reach back centuries.

Eelgrass was used in Danish construction as early as the 1600s, most notably on the island of Læsø, where it formed the basis of distinctive “seaweed houses.” Thick, resilient eelgrass was used for roof thatching - a technique found nowhere else in the world.

Over time, this knowledge receded as industrial materials became dominant. Søuld’s work represents a deliberate return - not as replication, but as reinvention.

“We just reinvented what already was known, but somehow forgotten.”

By combining this heritage with modern processing and product design, Søuld demonstrates how traditional material intelligence can be translated into contemporary building solutions - and validated through today’s most rigorous frameworks. 

Creating Value from the Local Landscape

Equally distinctive is Søuld’s approach to sourcing.

Eelgrass is collected along Danish coastlines by local farmers - often those whose land borders the sea. The process is time-sensitive: the material must be gathered shortly after it washes ashore, while still fresh.

“It needs to be collected within very few days after it has washed ashore.”

Farmers then dry and prepare the eelgrass for use in production, transforming what was once an overlooked natural by-product into a valuable resource.

In doing so, Søuld is not simply sourcing raw material - it is establishing a new local value chain.

“There’s no world market for eelgrass - so we’re actually inventing that market.”

This model creates economic opportunities within coastal communities, while maintaining a close relationship between material origin and production. It also offers a blueprint for future growth: as the company scales, similar localised supply systems could be developed in other regions where eelgrass is present.

Verification as a Strategic Choice

For a material that challenges conventional expectations, credibility is essential.

Søuld’s decision to pursue Cradle to Cradle Certified® was driven by the need to substantiate its claims within a recognised, science-based framework - one that extends beyond single metrics to consider the full lifecycle and impact of the product.

“Cradle to Cradle Certified® is a way of documenting our product, making sure that what we say is verified by someone who is not ourselves.”

Achieving Gold level under Version 4.1 - among the most rigorous iterations of the standard  - places Søuld within a limited group of products globally that meet these criteria.

The certification also plays a practical role in the market. Its alignment with building standards and schemes such as LEED and BREEAM facilitates integration into projects, supporting architects and developers in meeting regulatory and sustainability requirements.

From Certification to Capability

The transition from Version 3.1 to 4.1 was not incremental. For Søuld, it represented a significant organisational step.

“It took almost a year to collect all the data and write all the documentation.”

The process required deeper engagement with suppliers, expanded data collection, and the formalisation of internal policies - from governance and human rights to supply chain management.

For a company of around ten employees, this was a substantial undertaking. Yet its impact extends well beyond compliance.

Cradle to Cradle Certified® has, in this sense, become more than a product label. It functions as an operational framework - shaping how Søuld makes decisions, engages partners, and prepares for scale. 

Circular by Design - and Dependent on Use

Søuld’s panels are designed for recirculation. The material can be reintroduced into new production cycles without significant degradation, and systems are being established to support take-back and reuse.

“100% of what is sent back to us can be reproduced into new products.”

This includes guidance for dismantling, as well as packaging designed to facilitate return flows.

While the long lifespan of building materials - estimated at over 50 years - means that large-scale returns are still to come, the infrastructure is being prepared in advance.

This is critical, given the material’s carbon-storing function.

“As long as it’s kept in the building, it works as a storage for carbon… but we need to make sure it doesn’t get burned.”

Here, circularity becomes more than a design principle. It is a condition for preserving climate benefit — ensuring that stored carbon remains embedded within the material rather than released at end of use.

Scaling a System, Not Just a Product

Søuld’s next phase is defined by growth — but also by consolidation.

The company is focused on expanding market presence, increasing eelgrass sourcing capacity, and further stabilising production processes. At the same time, it is working to transition from investor-backed operations to financial sustainability.

“We need to become a sustainable company in a financial way, by growing sales and becoming more well known.”

What distinguishes this trajectory is that growth is not being pursued in isolation. The systems established through certification - from supplier engagement to circular design -provide a structure within which expansion can take place.

A Material Model for the Built Environment

Søuld’s acoustic panels bring together several dimensions that are often addressed separately:

• carbon storage in use

• circular product design

• local, decentralised sourcing

• cultural heritage and innovation

• verified performance through third-party certification

Through Cradle to Cradle Certified® Gold, Version 4.1, these elements are not only articulated - they are assessed, aligned, and made actionable.

In a sector increasingly defined by the need to reconcile performance, compliance, and climate impact, Søuld offers a compelling example of what this integration can look like in practice.