What is numerical thermal modelling?
We all have a stake in the planet on which we live and would desire to leave a world that provided fresh air for our children and grandchildren. There has been much discussion as to the impact we humans have had and measures have been taken to help the construction industry play its part to improve the situation.
In 2007 the government introduced a policy for all new homes to be constructed to meet a zero carbon standard from 2016 onwards. Through a progressive tightening of the Building Regulations (Part L) the aim was to help the UK do its part in reducing the environmental impact of greenhouse gas emissions.
The need to know
Whether a building is being refurbished, re-clad or built from scratch it is important to know how much energy it will use; 70% of the energy required for a dwelling will be used on heating and climate control. This means that accurately assessing its ability to retain heat is important in our aim to comply with the regulations and reduce our carbon footprint.
There are a few ways to asses the thermal properties of a building:
Introduction
Infra-red thermography
Infra-red thermography uses a special camera to photograph a building in the infra-red spectrum to see how the heat is distributed across the surface. This is good for showing areas that leak more heat than others. However, the infra-red camera cannot be used to measure U-values. So although we may be able to spot areas that are leaking a lot of heat we have no way of knowing how much heat is being lost (the U-value or Psi-value). Also it is impossible to test a building until it has been built or until after it has been refurbished- a time consuming and expensive way to make any changes in design that are needed!
Hot box method
This method is often used for light, technical elements such as windows. It involves constructing specimen sections of the building and sending them off to be hot box tested and then compile the Dwelling Emission Rate (DER) from these results. Hot box testing is fundamentally a method of measuring the heat flow through an element by separating a hot chamber from a cold chamber with the specimens. The temperature is measured using many sensors which enables the thermal resistance of the specimen to be measured. This is rather expensive, requires the manufacture and transport of sections to be tested and any change in the design requires a complete rerun of the entire process. It also cannot practicably be used when re-cladding an existing building as the elements that will be reused cannot be sent off for testing.
Numerical thermal modelling
Thermal modelling takes CAD drawings and through several processes builds a virtual specimen section of a detail which is then used to calculate heat flow from the warm interior to the cold exterior. This iterative mathematical analysis often requires millions of calculation nodes to best represent what would happen in the real world. It is also possible to build models and run them to analyse time-dependent temperature and water vapour flows. It is possible to calculate the interstitial and surface condensation risk. This is a well established and reliable method which is regulated by national and international standards, such as BS EN ISO 10211 and, in the case of windows and doors, BS EN ISO 10077-2. These standards clearly establish the software compliance method and rules of operation.
How does it work?
Thermal modelling is a process whereby a building design is represented in a computer simulation program and is run to show how it will function thermally. Various elements of the design can be tested in this way and the results can be seen as graphic output as well as text. If the results show that the building will fail the design specification it can be redesigned until the simulation shows that it will comply. These results are then presented to show that the building complies with the design specification.
What are the benefits?
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Computer numerical modelling is significantly less expensive than constructing a sample specimen and transporting it to a facility to be hot box tested.
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Thermal modelling can be used to calculate the U-value of existing walls and façades prior to a refurbishment, even photographs are sufficient if they show the wall/façade build-up in order to construct a virtual model.
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The ability to assess a current building’s thermal performance prior to change-of-use can be extremely beneficial as it will help to inform the technical requirements of the new work. This is important as often times the change of use brings upon the building a more stringent application of Part L of the building regulations, e.g. change from 1980’s office block to 2016 residential.
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With thermal modelling the design can be altered relatively easily and the simulation rerun to test the design change, this can be repeated many times until the design change that will give the best performance is found.
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The ability to test a design before a single aluminium rail is brought on site helps companies asses their design when changes can be easily made to ensure compliance.
Condensation Risk
As buildings are increasingly improving in heat retention and draught exclusion, the potential for condensation increases. A surface condensation is unsightly, can stain the surface and, on porous surfaces, encourage mould growth- indeed a surface relative humidity level of 80% or more is sufficient for mould growth.
Where the surfaces are porous the atmospheric vapour can seep into a wall or ceiling and there condense inside the structure. This is called interstitial condensation and is a problem for numerous reasons: it can degrade the performance of structural elements, cause other materials to break down and reduce the effectiveness of insulation layers.
Through thermal modelling it is possible to run models to predict if there is a risk of condensation. This then informs the design and allows for changes to the design to either remove the risk of condensation altogether or to mitigate it (by use of a vapour control layer etc.) if the complete removal is not possible due to other design constraints.
