IN Brief:
- The Structural Timber Association has published more than 1,500 free construction details with associated psi-values.
- The initial library covers five timber-frame wall systems, trussed rafters, I-joists, and metal web floors.
- The resource is intended to support energy assessment and better matching of junction calculations to building designs.
The Structural Timber Association has launched a free digital library containing more than 1,500 thermal junction details and associated psi-values for timber-frame construction. The resource brings together calculations for five generic wall systems, roof arrangements, and engineered timber floors, giving designers and energy assessors a common set of information when preparing new housing specifications. It has been developed for single-family homes and multi-occupancy apartment buildings, where several junction types can affect the overall heat-loss calculation.
The Structural Timber Thermal Details platform addresses the points at which otherwise insulated building elements meet. A wall may have a defined thermal transmittance, or U-value, across its main area, but the connection to a ground floor, intermediate floor, roof, window, or another wall can behave differently. Geometry and the position of insulation alter the heat-flow path at these interfaces. In an energy model, the linear thermal transmittance of a junction is represented by its psi-value, generally expressed in watts per metre kelvin.
A building heat-loss calculation combines heat transmitted through the main fabric areas with linear losses at junctions. The latter depend on both the psi-value and the length of each interface, so a recurring wall-to-floor connection can influence a whole housing scheme. A calculated junction value therefore needs to correspond to the insulation layout and structural geometry in the working drawings. Energy assessments based on a different detail can misrepresent the installed envelope even where the nominal wall or roof U-values are correct.
Five generic timber-frame wall systems sit alongside non-habitable and attic trussed-rafter arrangements, I-joists, and metal web engineered timber floors in the first release. Wall-to-floor and wall-to-roof junctions vary as floor depth, structural members, insulation continuity, and roof construction change. For example, moving insulation around a joist zone changes the thermal path through the connection, while an attic truss introduces junctions that differ from those in a conventional unoccupied roof space. The library provides matching calculation data for the different generic constructions, allowing a detail to be selected by assembly rather than by a broad timber-frame label.
Each detail is accompanied by a calculated psi-value, while the project partners say the library also includes temperature-factor information. A temperature factor helps assess how cold an internal surface may become relative to internal and external conditions; it is relevant to the evaluation of surface condensation and mould risk. Psi-values address linear heat loss, whereas the temperature factor addresses surface-temperature behaviour. They are related outcomes of the junction geometry but serve different checks during design and compliance assessment.
The work is a collaboration involving Timber Development UK, Swedish Wood, and Knauf Insulation, supported by technical input from Studio Partington Architects and Briary Energy. The Structural Timber Association says the calculations were independently prepared and reviewed with industry involvement. Studio Partington contributed design expertise, while Briary Energy was involved in producing the thermal calculations. The library is hosted through the association, with the technical material available for project teams to consult rather than commissioned separately for each generic junction.
Paul Newman, technical director at the Structural Timber Association, said: “This platform provides a significant library of independently calculated thermal bridge details, together with the information needed to support compliance and design optimisation.” His comments accompany an industry effort to make repeatable junction calculations accessible across housebuilding design teams. The published information gives assessors a common starting point for junction evaluation, while project teams remain responsible for matching a selected detail to the construction they will specify and install.
A published generic detail still needs to match the proposed building. Thermal bridging can change when insulation thickness, continuity, member dimensions, materials, cavity configuration, or assembly interfaces differ from the reference construction. Designers therefore need to verify the assumptions supplied with a detail and determine whether the proposed project remains within its intended scope. A library of repeatable values can reduce repeated calculation work, but it does not replace the coordination required between architectural drawings, factory-built panels, and site-installed junction components.
A suitable calculated junction value may also produce a more representative energy assessment than a broad default. Briary Energy says the published details can improve performance over current SAP default values, while designers must still choose the value for the specific construction being modelled. Construction teams then have to preserve insulation continuity and the designed junction during manufacture, installation, and inspection. Deviations at the interface can alter the heat flow and internal surface temperature that the published calculations describe.
The current website covers the listed timber-frame assemblies, and the association has designed it to accommodate further construction methods in future. Structural insulated panels, mass timber, and proprietary systems are among the possible additions identified by the project partners. Their thermal junctions would require calculations based on the relevant structural and insulation arrangements before values could be used in project assessments. The launch therefore provides a defined initial catalogue, with future expansion dependent on further technical details being developed and published.
The launch establishes a reference base for repeated details across housing plots and apartment blocks, where wall, floor, and roof junctions appear many times in the same construction programme. Design schedules can identify the selected detail and its psi-value together, enabling the energy assessment to be checked against drawings used for manufacture and installation. The website has been designed to accept further structural timber systems as new details are developed. Its current catalogue comprises more than 1,500 thermal details for the stated timber-frame assemblies.
