THERMAL MODELLING
Thermal Modelling:
Changes to Building Regulation requirements over the last decade have given rise to increased levels of thermal performance in the fabric of buildings. Heat losses due to thermal bridging have become even more proportionally significant. This increase in insulation has put more focus on the building fabric junctions in terms of a change in direction in both plan and elevation, changes in material conductivity or material thickness. These junctions can be analysed using computer thermal modelling techniques for both Building Regulation compliance for heat loss and surface risk condensation. As Registered NSAI Thermal Modellers, NZES have successfully demonstrated our expertise to NSAI in a series of tests and have therefore been included on the NSAI Register of Approved Thermal Modellers. Our modeler uses the latest computer software technology to model building elements and building junctions in 2D and 3D. We can model and calculate U- values of complex 3D building elements in accordance with EN ISO 6946 as well as modelling and calculating the linear thermal transmittance of thermal bridges at building junctions in accordance with EN ISO 10211-1 and BR 497.
What is a thermal bridge?
Thermal bridges occur within the building fabric where, because of the geometry or the presence of different conductivity materials, heat flows at different rates. For many situations simple calculations are no longer enough to correctly determine thermal performance and it is necessary to analyse the construction using numerical modelling. This is completed by specifying the geometry, materials and boundary conditions of the model in 2D or 3D as appropriate. The development of thermal models is now an integral part of the design process, to demonstrate compliance with the Building Regulations.
Why is thermal bridging important?
A linear thermal bridge is essentially a 2D concept and can be modelled in 2D. However, if the plane elements are not uniform in the third dimension - i.e. they contain repeating thermal bridging such as steel studs, then a 3D model must be used. Thermal modelling also allows the minimum internal surface temperature at the thermal bridge to be found. This is necessary in order to check that the thermal bridge does not pose an unacceptable risk of surface condensation and mould growth. Modelling can provide accurate energy consumption data for a set of defined input criteria. It is important therefore that thermal models are subject to rigorous quality control to mitigate this risk, reduce design faults and provide evidence in the event of future disputes or claims.
What is a Ψ (psi) value
The Ψ (psi) value represents the extra heat flow through the thermal bridge over and above that through the adjoining plane elements. The thermal transmittance, Ψ, of the 2D junction is the residual heat flow from the internal to external environment after subtracting the one-dimensional heat flow through all flanking elements.
What is the y - value concept?
To account for the heat loss through non-repeating thermal bridges at building junctions, for windows and door perimeters, ground/intermediate floor to external & party walls, corners and junctions of roofs to external wall. For domestic projects, DEAP allows a user defined y-value to be entered. The y-value is an additional U Value applied to the sum of all the exposed fabric element areas of the building. It has the same units as the fabric U value of W/m²K. The heat loss through the non-repeating thermal bridges that is added to the heat loss through the exposed building fabric is called the transmission heat transfer coefficient, HTB and is calculated as follows; the sum of the exposed building fabric area for the roof, walls (including windows & doors) and ground floor exposed areas. A default y=0.08 W/m²K can be used if all details are Acceptable Construction Details ACDs prepared by the Department of Housing, Planning and Local Government. Target Ψ values for ACD’s are given in Table D1 of TGD Part L. However, using this approach for NZEB standard houses may not result in a true reflection of the construction on site and additional construction costs.