1 CI/SfB. (4-) Rh2. NOVEMBER 2006. Conventions FOR Calculating . U-VALUES, f-VALUES AND -VALUES. FOR metal cladding SYSTEMS USING. TWO- AND THREE- DIMENSIONAL. THERMAL CALCULATIONS. MCRMA Technical Paper No. 18. Contents Page Summary 1. Introduction 3. Heat loss and surface temperatures 4. Heat loss 4. U-values 4. Thermal bridges 4. Internal surface temperatures 5. Multi-dimensional modelling 6. Details that should be included in models 6. Fasteners, brackets etc that cross the insulation 6. Panel joints 7. Purlins and cladding rails 7. Air cavities 8. Profiles 8. Fasteners 9. metal cladding meeting other systems 9. Point thermal bridges 10. Parapets and overhangs 10. Size of the model 11. Material thermal conductivities 12. Effective thermal conductivity of air cavities 13. Use of CEN values 13. Standard cavity resistances 14. Surface heat transfer 15. Assignment of features to plane elements and to thermal bridges 16.
2 Defining the grid points 16. Calculation of U-values from 2D and 3D models 18. 2-D models 18. 3-D models 18. The effect of joints in composite panels 19. Point thermal bridges 19. Linear thermal transmission - -value 20. Two dimensional models without profiles 20. Three dimensional models and models with profiles 21. Detail needed to calculate -values 22. -values of junctions including ground floors 23. Minimum internal surface temperature or f-value 24. Worked example 25. Appendix 29. References 44. Grateful acknowledgement in the preparation of this publication is made to Mr C Sanders, Centre for Research on Indoor Climate & Health, School of Engineering, Science and Design, Glasgow Caledonian University; For up to date information on metal roof and wall cladding , including downloadable construction details in 3D, visit The metal cladding & Roofing Manufacturers Association Limited.
3 November 2006. Summary This guide provides the information necessary for f-value as they affect the surface area and the the calculation of heat loss through plane areas, the effective insulation thickness. If the software U-value, heat loss through thermal bridges, the - used cannot model the sloping sides of the value and the lowest internal surface temperature, profiles, the heat flow can be approximated the f-value, for twin skin or composite panel metal very closely by a model with a series of steps, constructions using 2- or 3-dimensional heat flow provided the overall dimensions of the profiles calculations. It is designed so that users of different are the same. software packages can achieve consistent results Fasteners and other point fixings, which do when starting from the same construction data. not cross the insulation layer have a negligible effect on heat flows and can be ignored.
4 In many construction types, masonry or timber Air cavities more than 5mm thick, should be framed for example, because the lowest internal included, and allowance made for whether they surface temperatures are associated with the are ventilated. thermal bridges, the -value and f-value are When determining the size of the model, the calculated together. In twin skin and composite most accurate results will be obtained by panel metal cladding systems, although there ensuring that the more important thermal are some details where the lowest surface bridges are most accurately represented, temperatures are at the thermal bridges, there therefore fixing systems that cross the insulation are many where the f-value is associated with the should be given priority over profiles, and liner spacers or panel joints that are part of the plane profiles, which affect the insulation thickness, wall or roof.
5 In these cases, the U-value and should be given priority over outer profiles, the f-value are calculated together. To obtain an which do not. accurate U-value it is necessary to take account of spacers, panel joints and profiles; a detailed three 2) Define the thermal conductivity of each of the dimensional model is therefore necessary. In most materials present Section 6. Manufacturers data cases, however, the -value of a junction may be should be used, where possible, otherwise data obtained from a simple two dimensional model, are available in CIBSE Guide A or BS EN ISO. which does not incorporate spacers or profiles. 12524:2000. The following steps are necessary: 3) Allow for features such as air cavities in the structure - Section 6. The standard cavity 1) Decide which features of the construction should resistances quoted in BS EN ISO 6946 or CIBSE. be modelled Section 5.
6 When constructing a Guide A should be used. model of a structure for Calculating U-values, - values or f-values the following features must be 4) Specify appropriate internal and external surface considered: thermal resistances - Section 8. The standard Include all metal features that cross the surface resistances quoted in BS EN ISO 6946 or insulation. CIBSE Guide A should be used. Purlins or rails have negligible effect on the U-value of plane walls or roofs, but raise the f- 5) Decide which features are part of a plane value slightly. If purlins or rails are connected to element and which are part of a thermal bridge a severe thermal bridge, such as a gutter base Section 9. It is important to decide which features crossing the insulation, they must be included affect the heat flow through the plane areas and as they increase the severity of the thermal should be assigned to the U-value, and which affect bridge and raise the f-value.
7 It is recommended heat flow through the thermal bridge and should that they are included. be assigned to the -value. Otherwise there is a Profiled liner and outer sheets should be danger that some features will be included in both included in the calculations of the U-value and and their effect on the heat loss counted twice. 1. 6) Specify and input a two- or three-dimensional should never be assumed, but always calculated grid and position each material present within this for each plane element from the output of the grid and subdivide the grid to give accurate results model. In many cases reliable -values can be within a reasonable time- Section 10. The best derived from simplified models without profiles or results are achieved by the following procedure spacer systems. Define the minimum grid necessary to specify the materials present. 9) Calculate the f-value - Section 14 from the Divide all the spaces between the grid points temperatures output by the software.
8 Into two. A worked example is included in Section 14 to draw Identify all areas where the metal components the various ideas together. cross insulation and add extra grid points as shown in Figure 22. The procedures to be used in Calculating - and Calculate the resulting heat flow and lowest f-values and typical values for a range of details internal surface temperature. built as recommended on the MCRMA website are Divide all the grid elements into two and provided in the Appendix. recalculate the results. If the heat flows calculated from stages d) and e) differ by less than 2% and the minimum surface temperatures by less than C, the calculation is complete. If the change is greater, go back to stage c) and investigate adding further grid points in sensitive areas and repeat stages d) and e), until good agreement is reached. 7) Calculate the U-value of the plane elements adjacent to the bridge Section 11.
9 It is important to distinguish between: the U-value of the plane elements, necessary for the calculation of the overall loss from the building, and the U-values of those parts of a model used to calculate the -value - see Section 12. In cases where the structure is uniform so that there is no variation in surface temperature along the inside surface, the U-value can be calculated from the inside surface temperature. Otherwise the U-value has to be derived from the heat loss and dimensions of a three dimensional model. 8) Calculate the -value from the heat flows and temperatures output by the software - Section 12. The -value depends on the difference between the heat loss through a thermal bridge, and the heat loss through the adjacent plane areas. It is therefore very sensitive to the U-value used. Therefore, the U-value of the plane elements 2. Introduction We are all now aware of the need to save energy This guide gives the information needed to carry use in buildings to limit the production of the CO2 out these calculations, so that different users of that adds to global warming.
10 That this is very the same package and users of different packages much part of the government agenda is reflected can obtain consistent answers. It has been in the successive changes to Part L of the Building developed on the basis of a series of calculations, Regulations in England and Wales, with similar which examined the importance of individual changes to the equivalent sections in Scotland and features of the fabric and the validity of a range of Northern Ireland. The latest changes that came approximations. 1,2. into force in 2006 are also driven by the Energy It should be recognised throughout that the Performance Directive of the EU. methods discussed in this report will allow The introduction of more highly insulated structures designers and manufacturers of metal cladding that has resulted from this need to save energy systems to demonstrate compliance with the has also led to a need for more sophisticated Regulations by using standardised calculations methods for Calculating the heat loss and surface that will give consistent answers.