CPD PROGRAMME

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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.

Determining U values for real building elements

In the May edition of CIBSE Journal, the CPD article ‘Variations in Thermal Transmittance’ considered some of the variables that can affect what might otherwise be thought to be ‘standard’ values of thermal resistance that are used when establishing the thermal transmittance (or ‘U value’) of an element in a building structure. This article will introduce some of the considerations that are frequently required when determining the U values of real building components. This includes those that are ‘non homogenous’ (made of individual layers that themselves are made of more than one material) as well as considering the effects of connections between different surfaces on their U value.

The U Value Calculation As any student of building services or architectural engineering will know, to determine the U value the individual thermal resistances, R (m2

K/W) of the

layers that make up the structure must first be determined from R = d/λ, where d = thickness or depth of the material (m) and λ = thermal conductivity of the material (W/mK). Example values of thermal conductivities (at standard moisture content and temperature) are given in Figure 1 and extensive tables of these are to be found in CIBSE Guide A 2006 – Section 3. The ‘n’ individual resistances that make up a structure are combined with the inside

and outside surface resistances, Rsi and Rse, (m2

element ∑R = Rsi + R1 m2

+ ….. + Rn + Rse

K/W) to give a total resistance, ∑R, for the + R2

K/W The U value is simply the reciprocal of the

total resistance, ie 1/∑R, and then the basic building fabric heat transfer coefficient is

www.cibsejournal.com Material

Walls (external and internal) Brick (exposed) Brick (protected) Dense concrete block (exposed) Light concrete block Mortar (exposed) Mortar (protected)

Surface finishes Plaster (dense) Plaster (light-weight)

Insulation Expanded polystyrene (EPS) slab

Density (kg/m3

1750 1700 2300 600 1900 1900

1300 600

15

Figure 1: Example of standard thermal conductivities ∑(A U) where the area, A (m2

) is the area of

each individual element that has a respective thermal transmittance of U (W/m2 And so it looks quite straightforward.

K).

However when considering the individual layers that make up the structure, very few of them are actually consistent across

) Thermal conductivity (W/m K)

0.77 0.56 1.87 0.20 0.94 0.88

0.57 0.18

0.040

the whole area of the structure. Take, for example, the simplified four layer wall in Figure 2. The outer brickwork is made up both of bricks and mortar and the blockwork is also a mix – this time of lightweight concrete blocks and mortar. Of course the bricks and blocks themselves are not totally

June 2011 CIBSE Journal 57

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