CPD PROGRAMME
Professional development
The CIBSE Journal CPD Programme
Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.
Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).
CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.
Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at
www.cibsejournal.com/cpd while the information they contain remains current.
You can also complete the questionnaire online, and receive your results by return email.
Variations in thermal transmittance
The ‘U value’ of building elements are at the heart of all building models. But is the humble U value as straightforward as it appears? What might affect its value, and by how much? Some of these often-neglected issues are discussed below
T
here are great opportunities to reduce the energy use and carbon footprint of buildings through careful system selection, the use
of renewable energy sources and the education of occupants. However, the best way to reduce the environmental impact of buildings through heating and cooling is to create constructions that provide the best opportunity to minimise the building loads. Whether it be cooling or heating that dominates energy use (although heating predominates in the vast majority of the domestic and small commercial buildings in temperate zones such as the UK), one of the fundamental factors is the thermal transmittance, or ‘U value’, of the structures that make up the building envelope. This CPD article will look at how this much-used value of thermal performance may actually vary in practice from the ‘standard’ calculated value.
The U value in context In a building there will be a complex pattern of heat flows between not only the
www.cibsejournal.com
internal and external environment, but also between the walls, ceilings, floors, and so on, that make up the internal environmental space. Even in the simplest of buildings there is a requirement to examine the pattern of heat flows so that a proper judgement may be made on issues such as condensation risk, optimising insulation, heat emitter and plant sizing and increasingly as part of the information needed to model the energy use of the individual spaces and the complete building. The U value is a measure of the ability
for heat to flow from one space, through a solid structure, and into another space and assumes that the conditions stay the same both in the wall and in the spaces. It is used in both simple ‘steady state’ calculations – as would typically be employed to determine design heat losses, and in conjunction with other parameters (including principally the ‘admittance’, Y) when evaluating dynamic loads – as would be used for cooling loads when the indoor and outdoor conditions vary relatively quickly.
As the U value increases so does
the potential for heat flow. And so, for example, in the England and Wales Building Regulations Approved Document Part L1A for houses, there are maximum acceptable ‘back stop’ values for the U value of 0.2 W/m2 0.3 W/m2
K for a roof and 2.0 W/m2
K for external walls, K
for windows. However the reality of the predicted U value is not always straightforward and can vary significantly from day to day, as well as over its lifetime. A U value can only be calculated if the materials and thicknesses are known and, importantly, the actual location of the complete structure. It’s orientation, whether it is horizontal or vertical, the speed of the air flowing across it and the potential for radiant heat exchange will affect the value. Most building structures are made from more than one layer, and normally from a variety of materials. Each of the layers will have its own thermal characteristics and, to determine the U value, the thermal conductivity and thickness of each individual layer will be required.
May 2011 CIBSE Journal 67
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84