A building component can be described as “climate-positive” if it is an effective carbon sink during its life-cycle – in other words, achieves an overall removal of carbon from the atmosphere. This can be shown only by a dynamic life cycle analysis.1, 2 In timber frame construction, this is the case when a very good thermal insulation performance (U < 0.15 W/m²K) is combined with a rapidly renewable insulation material (e.g. straw) and a low-carbon heating source, such as a heat pump. In addition, the components need to have a service life of 70–80 years.
With a circular building component, all its major elements can be reused at the end of their service lives or fed back into the manufacturing process at a rate of at least 50 %. Biogenic or mineral oil-based materials can behave as a long-term carbon sink in a material repository, thus avoiding combustion.
The interesting question is: Can both objectives be combined using customary building components and methods? To find this out, an interdisciplinary research team performed a series of practical experiments. The participating companies built, dismantled and rebuilt a facade mock-up with blown-in straw insulation and gypsum fibreboard sheathing. All processes during the life cycle were recorded using time-lapse photography, then played through and documented.
The use of a prototype was deliberately avoided for the experiment, which involved an only slightly modified standard construction. This allowed the manufacture of a load-bearing facade for up to building class 3 and a non-structural external wall for up to building class 5. It was important that no layers should be rendered, plastered or have glued-on components that would be difficult to detach, and that all fastenings remain accessible.
Staples or screws?
Several preliminary experiments focused on easy-to-release connections, the introduction and extraction of straw insulation, as well as the installation and non-destructive removal of windows without detriment to their building physics performance.
Blowing insulation into airtight compartments can cause failure of the sheathing boards due to pressure differences. It is more common with straw than with cellulose or wood fibre because of its higher bulk density. Pressure-relief openings and support battens reduce this risk. Blow-in plates are designed to avoid board failures.
Today’s ubiquitous stapled connections save time, cost and materials but cannot be non-destructively dismantled. Nail screws are cost efficiently installed using gas nail guns. However, they are currently not always suitable for dismantling. On the other hand, traditional galvanised woodscrews can be driven and removed without problem. The edge distances are greater, however, as are the required timber cross sections.
Disassembly and reconstruction
Dismantling and rebuilding were done in a day. While dismantling took only 2 hours 40 minutes, rebuilding required almost twice as long.3 Screw fastener connections were easily released. For the rebuilding, new screws were driven into the existing holes rather than into newly drilled holes, as had initially been the intention. The non-detachable foil flashing under the windowsill was retained and over-glued with new materials for the reinstallation. The vapour barrier and the breathable facade membrane were completely replaced. The gypsum fibreboards were removed without difficulty, put to one side and reinstalled.
In the course of the experiment, a board broke in a compartment because only the opposite side had support battens. That being said, extracting the straw insulation and re-blowing it into the wall element laid on the floor posed no major problems.4
Advantages of prefabrication
In the debriefings, it became clear that extensive prefabrication leads to much fewer difficulties than doing this work on site. For manufacturing the elements, blow-in plates were used for the straw insulation and an automatic screwdriving machine. Screw fasteners involve considerably more time than staples. Completely taking apart the timber frame construction therefore requires the development of cost-efficient and easily dismantled timber-board connections.
Stapled connections do not present a problem to the reuse of the core elements. There is much more of a need for a standardised design approach to storey heights, grid modules and regularised separation points to allow maximum flexibility for component reuse. Two-stage water and moisture barriers are required for core elements so that they will last far longer than 100 years.
The visibility of all fasteners leads to a new aesthetics of the made, in which the manufacturing process is visually disclosed. Ease of dismantling also plays a large role in the renewal cycles of the life cycle. For example, non-destructive window replacement, renewal of the facade cladding and retrofitting new electrical systems in a general building refurbishment after 50 years.
The project team plans to evaluate circular facade elements using already available (semi-) dynamic life cycle methodology, such as the GWP-bio index.1 This circularity index has recently been developed for the Assessment System for Sustainable Building (BNB) and is currently being tried out in practice.5 The method is of interest when comparing the optimised timber frame elements with those used in current standard practice. In this context, mock-ups could become generally acceptable as proof of the circularity capability of building components. An obvious next step would be the development of climate-positive circular building components to create equally climate-positive buildings.
Project partners
Vallentin + Reichmann Architekten, Huber & Sohn, James Hardie, pro clima, Isostroh, Climasonic, ITW Haubold, dad Gruppe, Energieinstitut Vorarlberg, RWTH Aachen, Norbert Hirschbeck, Markus Lieb/lop
Bibliography
1 Guest, Geoffrey, u. a.: Global Warming Potential of Carbon Dioxide Emissions from Biomass Stored in the Anthroposphere and used for Bioenergy at End of Life. DOI: 10.1111/j.1530-9290.2012.00507.x
2 Vallentin, Rainer: Wie kann der Holzbau zum Klimaschutz beitragen? Veröffentlichung unter rb.gy/fi0d0e
3 Vallentin, Rainer: Klimapositiver zirkulärer Holzrahmenbau. Veröffentlichung unter rb.gy/fi0d0e
4 Borsch-Laaks, Robert und Vallentin, Rainer: Einblasdämmung aus Stroh; In: HOLZBAUquadriga, Heft 1/2026, S. 21 ff.
5 Fortentwicklung und Evaluierung des BNB-Kriteriensteckbriefs 4.1.4 Rückbau, Trennung, Verwertung bit.ly/zirkularität
