Energy efficiency of timber buildings

Introduction to energy efficiency

When designing a building, certain choices about the outer parts of the building—like the roof, walls, floor, windows, and doors—can affect how comfortable and affordable it is to keep the building warm or cool. Key factors include:

• how well the roof, walls, and floor resist heat loss (thermal resistance),

• how much heat they can store and release over time (thermal mass and thermal response),

• how well the windows and doors perform in terms of insulation, natural light, and air flow.

These factors have a big impact on how much heat the building loses and how much sunlight it can collect and use to help heat the space. They also affect how much electricity is needed for lighting and running appliances.

Prerequisites

Before starting this topic, students should have:

• Basic knowledge of building construction and building components.

• A general understanding of heat transfer (conduction, convection, and radiation).

• Familiarity with basic physics and mathematics.

• Basic computer skills for using simulation software.

An interest in sustainable and energy-efficient building design.

Learning outcomes

By the end of this module, learners will be able to:

1. describe energy performance characteristics of timber buildings;

2. analyze heat transfer, thermal bridges, and moisture transport in timber buildings using respective software (THERM, UBAKUS);

3. understand renovation strategies using prefabricated timber panels;

4. simulate and optimize timber envelope performance in THERM, UBAKUS.

The importance of energy efficiency in timber construction

Thermal performance:

• Natural insulation: Timber has low thermal conductivity, meaning it resists heat flow better than concrete or steel, which helps keep buildings warm in winter and cool in summer. 

• Reduced energy bills: Due to its insulating properties, timber buildings have lower heating and cooling energy demands and can lead to significant cost savings. 

• Thinner walls: Timber construction can achieve the same thermal performance as concrete with thinner walls, which allows for more usable interior space and less material use. 

Environmental benefits:

• Carbon sequestration: Trees absorb carbon dioxide as they grow, storing it in the wood. When used in buildings, this carbon is locked away for the life of the structure, helping to combat climate change. 

• Renewable resource: Wood is a renewable resource, and responsible forestry practices ensure a continuous supply while also managing forests for long-term health and biodiversity. 

• Lower embodied energy: The production of timber requires significantly less energy compared to materials like steel and concrete, which are energy-intensive to manufacture. 

Modern engineered wood products:

• CLT and glulam: Engineered wood products like cross-laminated timber (CLT) and glue-laminated timber (glulam) have overcome previous limitations of timber, allowing for taller and more complex designs than traditionally possible. 

• Technological advancements: Modern timber construction relies on innovative technologies and engineered products that improve durability and resilience, matching or exceeding the performance of conventional materials. 

The concept of U-values

We calculate U-values to determine how much heat is transferred through building elements such as walls, roofs, floors, and windows. A lower U-value means better thermal insulation and lower heat loss, which improves the building's energy efficiency. In Ubakus, calculating U-values helps us evaluate different construction assemblies, compare materials, and verify whether a building meets thermal performance requirements. 

📄 Assignment: U-values of Timber Frame Structures

Objective: the aim of this assignment is to understand and calculate the thermal transmittance (U-values) of different building components in timber frame construction; the calculations may be done either manually or using an online tool, e.g. UBAKUS or THERM.

Tasks:

1. Build models of typical timber wall assemblies (UBAKUS: www.ubakus.com).

2. Modify wall layers to meet a target U-value (e.g. Passivhaus standard).

3. Moisture risk assessment for internal vs external insulation strategies.

4. Evaluate thermal bridges with temperature index at roof/wall and wall/floor junctions (THERM).

5. Prepare simulation report including screenshots of setups, outputs (U-values, isotherms, moisture curves), and design recommendations.

Structures:

1. Timber Frame Walls:

   • Model at least two different wall assemblies in UBAKUS.

   • Compare the impact of insulation thickness and type on the U-value.

2. Floors

   • Design a timber frame floor assembly.

   • Calculate its U-value and analyze how material layers (e.g., insulation, floor finish, air gaps) affect thermal performance.

3. Roofs

   • Model at least one pitched roof and one flat roof with timber structure.

   • Compare U-values depending on insulation placement (between rafters, above rafters).

4. Windows

   • Insert glazing and frame options provided in UBAKUS.

   • Compare at least two different window types and their U-values.

Deliverables: compile short report (3–5 pages) including:

• screenshots of each calculation;

• tabulated U-values of walls, floors, roofs, and windows;

• a short discussion on which elements perform best thermally and why.

📄 ASSIGNMENT: case study analysis

1. Select a timber wall assembly (from your coursework or provided drawings).

2. Illustrate and label key airtightness and vapour control layers (e.g., membranes, tapes, sealants).

3. Identify potential weak points for air leakage and propose detailing solutions.

Thermal and energy performance characteristics:

• Low Thermal Conductivity:  CLT panels have inherent insulation properties (λ ≈ 0.11–0.13 W/mK), reducing heat loss through the structure.

• Airtightness and Moisture Regulation: Proper joint detailing and vapor-permeable membranes ensure a tight building envelope, preventing energy losses and condensation.

• Dynamic Thermal Mass: Timber buffers temperature fluctuations, improving indoor comfort and reducing heating/cooling peaks.

Envelope design and insulation strategies:

• CLT + External Insulation (Hybrid Wall System): ETICS with mineral wool or wood-fibre insulation significantly improves U-values (e.g. 0.10–0.15 W/m²K).

• Thermal Bridge Reduction: Use of thermally separated connectors and minimized steel joints.

• Window-to-Wall Optimization:  Installation of high-performance windows (U ≤ 0.8 W/m²K) aligned within the insulation layer.

• Building-Integrated Photovoltaics (BIPV): CLT roofs and façades can integrate PV panels due to structural adaptability and lightweight support needs.

• Solar Thermal Collectors: Effective in low-energy housing typologies using CLT structures.

• Heat Pumps and Mechanical Ventilation with Heat Recovery (MVHR): Systems adapted for airtight CLT buildings further reduce primary energy demand.

Airtightness, moisture and fire performance:

• Airtightness Testing (n₅₀): Target ≤ 0.6 h⁻¹ for passive standards.

• Moisture Management: Hygroscopic nature of wood demands careful detailing at panel joints.

• Fire Resistance: CLT elements can achieve 60–90 min fire rating due to predictable charring behavior.

Case studies and measured performance:

• Passive CLT multi-storey buildings in Austria, Norway, and Lithuania show heating demands below 15 kWh/m²·year.

• Comparative LCA studies demonstrate up to 50% lower embodied carbon than reinforced concrete alternatives.

Conclusions:

• Technical solutions in CLT buildings—optimized envelopes, renewable energy integration, and precise detailing—enable compliance with nZEB and nearly carbon-neutral building targets.

• Advancing digital design tools (BIM, LCA, hygrothermal simulations) further supports lifecycle-based optimization of CLT systems.

🗨️ Questions for discussion

1. What are the main trade-offs between thermal performance and embodied carbon in CLT construction?

2. How can airtightness testing influence the final energy label of a CLT building?

Prefabricated timber panels are increasingly used in energy-efficient renovation projects, especially for upgrading existing masonry or concrete buildings. These panels offer:

• Fast installation with minimal disruption to occupants.

• Integrated insulation layers for thermal performance improvement.

• Factory-controlled quality ensuring airtightness and precision.

Such renovations can help existing buildings achieve higher energy efficiency classes and extend their service life sustainably.

Renovation with prefabricated timber panels is characterized by speed over quality. Nothing is left to chance or the builder's eye: a 3D measurement of the house is made using lasers, based on which panels suitable for a specific house are produced as a tailor-made job, with windows, ventilation pipes and insulation already inside. 

Production takes place in a warm and dry factory, and the panels are packaged in a weatherproof way before being transported to the construction site, which helps to avoid problems caused by the weather during construction. All that remains at the site is to lay the panels on the wall with a crane. Then the old windows of the house are removed and the window frames are finished inside the apartment.

Differences from factory renovation:

• 80% finished in the factory: A timber frame element is built in factory conditions (in a house factory) to at least 80% finished, including insulation, wind and vapor barriers, new opening fillings, new ventilation pipes and a finished facade.

• Measurements and 3D scanning: Before the work begins, the apartment building is scanned three-dimensionally (3D) to accurately determine the specific features of each building (curves, bumps). Tensile strength tests are also performed on existing structures to ensure the suitability of this construction method for a specific building.

• No scaffolding: The facade elements are transported to the site, where they are attached to the existing building. On-site lifting mechanisms (cranes and hoists) are used for this, and months of living under scaffolding (minimum 6 months in the case of conventional construction) are not necessary.

The Sara Kulturhus Center, Swedish Environmental Protection Agency Offices, and Fyrtornet represent leading examples of sustainable architecture in Sweden. All three projects emphasize energy efficiency, low carbon construction, and the use of environmentally responsible materials, particularly timber in the case of Sara Kulturhus and Fyrtornet. Together, they demonstrate how modern buildings can combine innovative design, renewable energy systems, and high environmental standards to reduce both operational and embodied carbon while maintaining high performance and user comfort. 

This section presents examples of best practices for energy-efficient building design. The images illustrate key solutions for improving the thermal performance of buildings, including well-insulated wall assemblies, the reduction of thermal bridges, effective airtightness strategies, and the use of mechanical ventilation with heat recovery (recuperation). 

• Gustafsson, A., & Thelandersson, S. (2019). Energy performance of timber buildings – A review. Energy and Buildings, 183, 418–432, https://doi.org/10.1016/j.enbuild.2018.11.025

• Sandberg, L., & Haller, P. (2018). Retrofitting with timber: Concepts and case studies. Journal of Building Engineering, 19, 198–208, https://doi.org/10.1016/j.jobe.2018.04.011

• https://dctech.com.au/products/dct-w41-1-clt/ 

Further references:

• UBAKUS Software (Official Site & Tutorials): https://www.ubakus.com

• UBAKUS Help Guides & Community Forum: https://forum.ubakus.com 

• Therm: https://windows.lbl.gov/therm-software-downloads