Summer outside
Daily mean temperature
Cooled inside
Outside surface temperature, θeo
The ‘swing’ or deviation from the mean value drives the non-steady state heat flow
Inside surface
Daily mean temperature
Solid wall
Midday Time lag
that the admittance is normally defined by the characteristics of the inner 100 mm of the surface (or, better still, the value can be determined using a spreadsheet). Published values of thermal admittance
Midnight Figure 3: Daily temperature profile at points through a concrete wall bridging).
Decrement factor, f, is a ratio that accounts for the thermal dampening that reduces the magnitude of the swing in temperature imposed on one face of the fabric as the temperature wave passes through the structure – the effect of which is shown in Figure 3. And the time lag, φ (hours), is how long it takes for the heat passing into one side of a structure to get to the other side (and, of course, the magnitude will have been reduced, due to the decrement factor). So, for example, the careful selection of materials can ensure that high solar irradiance striking the outside of a wall is not felt inside the space until after the building’s use has ended for the day. The final pair of factors are Surface factor,
F, that is the ratio of the swing in heat flow from the internal surface of the element to the swing in heat flow received at the
Ref Construction
A 105 mm brick, 50 mm airspace, 100 mm dense concrete block, 13 mm dense plaster
B 105 mm brick, 50 mm airspace, 105 mm brick, 13 mm dense plaster
C 105 mm brick, 50 mm airspace, 100 mm light concrete block, 13 mm dense plaster
D Internal wall – 12.5 mm plasterboard, timber studding, 12.5 mm plasterboard
E Internal wall – 13 mm lightweight plaster, 100 mm lightweight concrete block, 13 mm lightweight plaster
F Internal wall – 13 mm lightweight plaster 105 mm brick, 13 mm lightweight plaster
internal surface of the element (such as the gain from localised sunshine through a window), and an associated time factor, ψ (h), that defines the time delay. Calculated admittance data for some example constructions are given in Figure 4. For example, comparing the external walls (A, B and C) clearly shows the impact of the structures to improve the heat loss (by reducing the U value) can make significant alterations to the admittance and decrement factor that may adversely affect the resulting room cooling loads. The selection of internal walls (for example, D, E and F) can make a major difference to the average room admittance. If seeking out a particular thermal admittance value that is not specifically listed in the CIBSE tables, it should be possible to find a similar structure as an approximate proxy, considering
Transmittance Admittance U
Y
(Wm-²K-1 1.77
1.44 1.06 1.7 1.11 1.69 )
(Wm-²K-1 5.37
4.38 2.72 0.61 2.27 3.76 ) ω
(h) 1.2
1.5 2.6 5.7 3.8 2.2 Midday
are calculated on a basis of sinusoidal variation of heat input and temperature. In practice, these conditions rarely occur. Theoretically, it would be possible to calculate thermal admittance of periodic conductance for any pattern of heat gain, but to retain the simplicity of a thermal admittance method, standard thermal admittance values are generally used. The follow-on CPD article will apply the thermal admittance method to undertake some concept ‘what-ifs’ to help reduce the heating and cooling loads. © Tim Dwyer, 2012.
• Thanks to Dr Tony Newton and Tom
DeSaulles for contributing information for this CPD.
Further reading See the relevant appendices CIBSE Guide B3 and CIBSE Guide A5 – they are a mine of information.
References: 1
Loudon, A.G., Summertime temperatures in buildings without air conditioning, JIHVE, 1970.
2 CIBSE Guide A3, Appendix 3.A6, CIBSE, 2006.
3 Rogers and Mayhew, Engineering Thermodynamics, Work and Heat Transfer, Prentice Hall, 1992.
4 Millbank, N.O. and Harrington-Lynn, J. , Thermal
Decrement-factor Surface-factor f
ϕ 0.34 0.35 0.53 1 0.81 0.52
(h) 8.1
8.8 7.4 0.5 3.6 5.4 F 0.4 0.53 0.76 1 0.88 0.65 ψ
(h) 2.2
1.7 1.1 0.3 1.1 1.6 Figure 4: Thermal properties of example constructions taken from the CIBSE Guide A3 (the external constructions are not suitable for modern buildings – used for illustration only)
72
CIBSE Journal January 2013
www.cibsejournal.com
Temperature
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