Traditional foundations of oncrete, reinforced steel and insulation account for up to 40 per cent of carbon dioxide emissions when constructing new buildings with a timber frame. This was one of the reasons why Rise initiated the research and innovation project Sustainable Foundations for Industrially Manufactured Houses, funded by Vinnova – the Swedish innovation agency, the CBBT research foundation, the Swedish Federation of Wood and Furniture Industry, and Swedish Wood.
The aim was to explore the potential for reducing the climate impact of house foundations by providing a basis for decision-making for industrial timber-house manufacturers. In this way, the project hoped to find alternatives to traditional foundations and thereby achieve a lower carbon footprint.
The project ran from 2023 to 2025. Its coordinator was research and development engineer Irina Martynyuk.
“When the climate declaration was introduced, it became clear where the emissions came from. In very many cases, when we're talking about prefabricated timber houses, the foundation can account for between 20 and 40 per cent of the carbon dioxide emissions. It's a part of the building that house manufacturers can't actually influence themselves – they procure it through an external contractor. That made it low-hanging fruit to look at alternatives to concrete foundations,” she says.
The project consisted of several different work packages. In one of them, Rise carried out climate calculations to determine what carbon footprint four different foundations would generate if placed beneath two different reference houses: OBOS Kolonilotten (three storeys, with a timber stud frame in volumetric modules), and Masonite Beams' Flexibilitetshuset (four storeys with an I-joist frame in panel elements). The climate calculations were carried out according to Boverket's methodology for climate declarations, in collaboration with IVL.
The four foundations were a traditional concrete slab, a heated crawl space foundation, an optimised concrete foundation, and a solid timber foundation, each calculated separately for both houses. The purpose was to make the results comparable.
Rethinking the traditional foundation
The figures were clear. With a traditional concrete slab, Flexibilitetshuset produced 24 kilograms of CO₂ equivalents per square metre for the foundation alone, and Kolonilotten 20. With a solid timber foundation, the values dropped to 9 and 12 respectively.
For Kolonilotten, the optimised concrete foundation performed almost as well – 13 kilograms of CO₂ equivalents per square metre compared to the solid timber foundation's 12 – because the beams in that house had more scope for optimisation. For Flexibilitetshuset, where the traditional foundation was already relatively well-dimensioned, the optimisation yielded considerably less benefit: both the heated crawl space foundation and the optimised concrete foundation came in at 18. The concrete slabs had been optimised by reducing thickness, lowering strength grade and using climate-improved concrete at level 4.
The good result is primarily due to replacing concrete with timber.
“But it was also interesting to see that an optimised traditional foundation shows excellent values. It's possible to reduce emissions even when using concrete,” says Irina Martynyuk.
The project's main focus – and, according to Irina Martynyuk, its most interesting one – was on timber foundations.
“Timber foundations are sufficiently different and complex to deserve their own work package. It's a more untested and uncertain solution than the other alternatives, which required more in-depth verification,” she says.
In this work package, a cross-laminated timber (CLT) foundation was used as a direct replacement for the concrete slab. Klara Byggsystem, one of the industrial partners in the project, manufactures prefabricated foundation elements of CLT with multiple layers of insulation. CLT foundations are based on the same principle as a traditional slab-on-ground, but instead of concrete, panels of cross-laminated timber serve as the load-distributing layer.
“The elements are prefabricated and delivered directly to site. It's a change of material, not of system,” says Irina Martynyuk.
Timber as an alternative to concrete
A CLT foundation follows the same load principle as concrete: the CLT slab rests on reinforcement beams, where load-bearing walls and columns transmit their loads downwards. Where loads are low, the beams can be replaced with load-bearing insulation.
And then there's he matter of moisture – a question that often arises when it comes to building in timber. When it comes to foundations, it naturally becomes even more important. Consequently, moisture performance was given its own work package, on which the project spent the most time.
To investigate moisture performance, Rise used two different houses with timber foundations. Derome's Villa Klivet, where Rise was involved in the design from the outset, and the already completed Villa Zero, a collaborative project between Fiskarhedenvillan, Structor Byggteknik and Mondo Arkitekter. Both houses were equipped with moisture-monitoring sensors to track how the timber foundations behave in reality over time.
Villa Klivet showed good results. Relative humidity stayed at around 60 per cent, compared to a threshold of 75–80 per cent. The moisture content in the timber stabilised at approximately 12 per cent. During the construction phase, the foundation was left exposed to rain for over a week as a stress test. The moisture level rose – and then dried out rapidly.
Moisture – the critical challenge
“I wasn't really surprised. The foundation is well insulated, it's warm inside the house, and the structure is raised from the ground. The temperature gradient pushes the moisture downwards and away,” says Irina Martynyuk.
Villa Zero did not show the same good results. The sensors inside the house displayed excellent values, with a moisture content below 9 per cent, a stable temperature around 20 degrees and a relative humidity of 50–55 per cent. But on the north-west side, at the gable end, the picture is different. Three sensors show relative humidity of 75–100 per cent. Two of the three sensors show 100 per cent relative humidity during winter, when the risk of microbial growth is lower, according to Irina Martynyuk.
She describes how the construction differs from Villa Klivet's on a small but crucial point: a plastic layer between the first and second insulation layers.
The sensor is positioned at the edge of the edge beam, close to the plastic layer.
“Our theory is that the temperature difference between the two layers creates condensation against the plastic, and that the moisture is then unable to migrate away. It's a concerning result, and Fiskarhedenvillan are aware of it,” she says.
Rise are not entirely certain about the cause of the moisture accumulation in Villa Zero, since they were not involved when the house was built. Suspicions point to the damage – the presence of moisture or the elevated humidity – occurring during the construction period. And if moisture is built into a structure that is airtight and difficult to dry out, it has nowhere to go.
“The moisture content varies over the year but shows no gradually increasing trend. It repeatedly reaches high values in the slab at the gable end. If there were an ongoing leak at the vapour barrier, we would expect it to rise progressively. It doesn't. We believe the moisture is built in and cannot dry out, either downwards or to the sides.”
Sandra Tysell, Head of Product and Sustainability at Fiskarhedenvillan, explains that they raised questions about timber and moisture early in the project, since organic materials are more moisture-sensitive.
“That's why we've monitored moisture content, temperature and relative humidity in the foundation structure, both during the construction phase and in-use phase, with sensors placed at particularly critical locations,” she says.
When deviating values have occurred, they have supplemented with core samples. The analyses have not identified any microbial damage.
“Villa Zero demonstrates the importance of following up and evaluating new solutions over time. Identifying risks, learning from the results and applying that knowledge is an important part of development,” says Sandra Tysell.
Lessons for future foundation design
The results show that timber foundations can work from both load-bearing capacity and moisture perspectives, provided that the design, production and construction process are executed correctly.
“Timber foundations can be applied, but they will require measures depending on the ground conditions, such as reinforcement beams and more extensive drainage,” says Irina Martynyuk.
The project presents its results in, among other things, a decision matrix – an Excel document that helps structural engineers and house manufacturers choose a foundation type based on ground conditions, building systems and climate requirements. Irina Martynyuk emphasises that the matrix is a discussion tool rather than a definitive answer.
So what does a potential breakthrough for timber foundations look like?
“Several technical challenges remain, such as continued knowledge-building around load-bearing capacity, handling of lateral loads, odour and emission tightness, and moisture safety during the construction phase, particularly when exposed to precipitation and ground moisture. But CLT foundations are nonetheless a promising alternative for the sustainable construction of the future. Interest within industry and research is growing, which is driving development forward. But we need continued empirical validation, standardisation and development of technical guidelines. And information dissemination is important,” says Irina Martynyuk.
The segments where Irina Martynyuk believes timber foundations have the greatest potential are primarily smaller buildings and detached houses:
“Imagine not having to pour a slab? You lay out foundation blocks, erect the Attefall house, and within a week it's finished. It would be perfect for extensions, outbuildings and ancillary structures. That's the segment where I believe timber foundations have the greatest potential to establish themselves initially.”
Text: Mattias Boström AI-generated illustration: Intellecta
