Building Physics is the study of the movement of heat and moisture in buildings.
The objectives are usually to help design buildings which are comfortable, energy efficient, and healthy. This involves using a variety of simulation and calculation tools to understand and predict the behaviour of whole buildings, sub-assemblies and specific materials in terms of heat and moisture, and where the results are not acceptable, to support design teams to improve or refine their proposals.
The UK building regulations incorporate several requirements that are assessed using some element of building physics, in particular Part L where whole-building models are used to assess the energy efficiency and carbon emissions before a building is constructed, and Part C where calculations or simulations are used to avoid the accumulation of moisture. The Passivhaus methodology is strongly underpinned by building physics.
But building physics is not just about desk-top theory – we pride ourselves on a deep understanding of how buildings new and old are put together, and bring this knowledge to bare in our analysis work. This experience enables us to deliver more value to our clients by discussing potential problems at an early stage, and by ensuring the design is buildable, and what gets built matches the design. We like to get our hands dirty on site and in our lab – we believe getting up close and personal with buildings and materials is essential to a proper understanding and meaningful calculations and simulations.
We operate several different whole-building simulation tools, which are selected according to which is best for a given situation. The Passivhaus Planning Package (PHPP) is simple but robust, transparent, and in the right hands, a powerful tool. Building pursuing Passivhaus Certification must of course be modelled using this tool, and meet a number of criteria which it assesses. IES-VE is a dynamic simulation tool, which means it can simulate the conditions and energy flows in each room of a building on an hourly basis. DSM tools require many different input variables to be correctly set, and generate large amounts of output data that must be properly analysed, so they tend to be more time consuming and need an experienced user. The increased complexity is useful for certain situations such as assessing overheating risk in multi-zone buildings without active cooling. SAP and SBEM are the tools that must be used for demonstrating compliance with Part L of the building regulations. They are relatively simple, and arguably less flexible than some other tools because they are primarily intended for compliance.
The insulating properties of Walls, floors, roofs can be calculated quite easily, and mechanisms for doing so are built into the whole-building tools. The junctions between these elements need more sophisticated analysis; where a wall, floor or roof can be modelled in 1D, their junctions are inherently 2D or 3D in nature, which requires a different mathematical approach. We use LBNL Therm, Psi-therm and Comsol, depending on the situation to assess thermal bridges.
The movement of moisture through building materials can be very complex, because it often involves water vapour, liquid water and the interaction of water with heat and even pollutants such as salt. For certain simple situations, the ‘Glaser Method’ is adequate, however this only considers water vapour and temperature across an element such as a roof. Where the materials present have the capacity to absorb and re-distribute water as a liquid, it is usually more appropriate to adopt a more sophisticated approach using a tool such as Wufi or Delphin. All of these methods are used to assess whether moisture is likely to accumulate anywhere in or on a building element, and the subsequent risk of failures such as mould growth, rot and corrosion.
Measure and Monitor
We use a variety of measurement and monitoring methods and tools to inform our simulation and calculation work. This can either be before or alongside a simulation exercise, or afterwards as part of a learning or research plan. When dealing with existing buildings, its often useful to undertake analysis of some materials to establish how they behave in relation to moisture and heat. Our in-house lab has the ability to quickly establish various properties such as density and moisture content, absorption coefficient and hygroscopicity. These and other parameters feed into our simulations to refine them and improve the reliability of the conclusions and recommendations we make.