3.8.1 Admittance procedure
There are several methods available for assessing the non- steady state or dynamic performance of a structure. One of the simplest is the admittance procedure(42) which is described in detail in chapter 5: Thermal response and plant sizing. The method of calculation of admittances and related parameters is defined in BS EN ISO 13786(43)and a summary is given in Appendix 3.A6.
This procedure requires the calculation of three parameters in addition to the thermal transmittance:
admittance, surface factor and decrement factor. These parameters depend upon the thickness, thermal conduc- tivity, density and specific heat capacity of the materials used within the structure and the relative positions of the various elements that make up the construction. Each of these parameters is expressed as an amplitude and an associated time lead/lag.
3.8.1.1 Thermal admittance (Y-value)
The most significant of the three parameters is the admittance. This is the rate of flow of heat between the internal surface of the structure and the environmental temperature in the space, for each degree of deviation of the space temperature about its mean value. The associated time dependency (ω) takes the form of a time lead.
For thin structures composed of a single layer, the admittance is equal in amplitude to the U-value and has a time lead of zero. The amplitude tends towards a limiting value for thicknesses greater than about 100 mm.
For multi-layered structures, the admittance is primarily determined by the characteristics of the materials in the layers nearest to the internal surface. For example, the admittance of a structure comprising heavyweight con- crete slabs lined internally with insulation will be close to the value for the insulation alone. However, placing the insulation within the construction, or on the outside surface will have little or no effect on the admittance.
Table 3.31 Indicative U-values (/ Wãm–2ãK–1) for windows and fully- glazed doors with metal frames (4 mm thermal break) (Crown copyright, reproduced with the permission of the Controller of Her Majesty’s Stationery Office and the Queen’s Printer for Scotland)
Item Indicative U-value (/ Wãm–2ãK–1) for stated gap between panes
6 mm 12 mm 16 mm
or more
Single glazing 5.7 — —
Double glazing (air filled): 3.7 3.4 3.3
— low-E, εn= 0.2[1] 3.3 2.8 2.6
— low-E, εn= 0.15 3.3 2.7 2.5
— low-E, εn= 0.1 3.2 2.6 2.4
— low-E, εn= 0.05 3.1 2.5 2.3
Double glazing (argon filled[2]): 3.5 3.3 3.2
— low-E, εn= 0.2 3.0 2.6 2.5
— low-E, εn= 0.15 3.0 2.5 2.4
— low-E, εn= 0.1 2.9 2.4 2.3
— low-E, εn= 0.05 2.8 2.3 2.1
Triple glazing: 2.9 2.6 2.5
— low-E, εn= 0.2) 2.6 2.1 2.0
— low-E, εn= 0.15) 2.5 2.1 2.0
— low-E, εn= 0.1) 2.5 2.0 1.9
— low-E, εn= 0.05) 2.4 1.9 1.8
Triple glazing (argon filled[2]): 2.8 2.5 2.4
— low-E, εn= 0.2 2.4 2.0 1.9
— low-E, εn= 0.15 2.3 1.9 1.8
— low-E, εn= 0.1 2.2 1.9 1.7
— low-E, εn= 0.05 2.2 1.8 1.7
[1] and [2]: see footnotes to Table 3.30
Table 3.32 Adjustments to U-values in Table 3.31 for frames with thermal breaks (Crown copyright, reproduced with the permission of the Controller of Her Majesty’s Stationery Office and the Queen’s Printer for Scotland)
Thermal break / mm Adjustment to U-value (/ Wãm–2ãK–1) Adjustment for Additional adjustment thermal break for rooflights angled
< 70° to horizontal
0 (no break) +0.3 +0.4
4 +0.0 +0.3
8 –0.1 +0.3
12 –0.2 +0.3
16 –0.2 +0.3
20 –0.3 +0.3
24 –0.3 +0.3
28 –0.3 +0.3
32 –0.4 +0.3
36 –0.4 +0.3
3.8.1.2 Decrement factor (f)
The decrement factor is the ratio of the rate of flow of heat through the structure to the environmental temperature in the space for each degree of deviation in external tempera- ture about its mean value, to the steady state rate of flow of heat (U-value). The associated time dependency (φ) takes the form of a time lag.
For thin structures of low thermal capacity, the amplitude of the decrement factor is unity with a time lag of zero.
The amplitude decreases and the time lag increases with increasing thickness and/or thermal capacity.
3.8.1.3 Surface factor (F)
The surface factor is the ratio of the variation of heat flow about its mean value readmitted to the space from the surface, to the variation of heat flow about its mean value absorbed by the surface. The associated time dependency (ψ) takes the form of a time lag.
The amplitude of the surface factor decreases and its time lag increases with increasing thermal conductivity but both are virtually constant with thickness.
3.8.1.4 Internal structural elements
For internal structural elements, such as floors and partition walls, which are not symmetrical about their mid-plane, the dynamic responses will be different for the two faces and two sets of admittances and surface factors are required. The decrement factor is the same for heat flow in either direction.
Where internal structures divide spaces in which the thermal conditions are identical, the energy transfers can be simplified by combining the admittance and surface factor with the decrement factor to give modified admit- tance and surface factor respectively.
3.8.2 Heat capacity
The heat capacity per unit area of a building element, c (kJãm–2ãK–1), is a measure of the thermal response charac- teristics of the element. For a sinusoidal temperature variation it is defined in terms of the maximum change in heat stored within the element during a half cycle(43). It is used in simplified methods for calculating the energy use of buildings, such as that described in BS EN ISO 13790(44).
3.8.3 Effect of thermal bridging of dynamic characteristics
The presence of thermal bridges will affect the overall dynamic performance of a structure. However, since the admittance is mainly determined by the properties of the materials immediately adjacent to the interior spaces of the building, the presence of heat bridges within the structure will have little effect on the overall thermal performance. Therefore, it is only where the bridging material is at or near the surface temperature that it will affect the dynamic thermal performance. In cases where it
is felt necessary to account for the effects of thermal bridges, an area-weighted mean approach can be used.
References
1 BS EN 12664: 2001: Thermal performance of building materials and products. Determination of thermal resistance by means of guarded hot plate and heat flow meter methods. Dry and moist products of low and medium thermal resistance (London: British Standards Institution) (2001)
2 BS EN 12667: 2000: Thermal performance of building materials and products. Determination of thermal resistance by means of guarded hot plate and heat flow meter methods. Products of high and medium thermal resistance (London: British Standards Institution) (2000)
3 BS EN 12939: 2001: Thermal performance of building materials and products. Determination of thermal resistance by means of guarded hot plate and heat flow meter methods. Thick products of high and medium thermal resistance (London: British Standards Institution) (2001)
4 BS EN ISO 8990: 1996: Thermal insulation. Determination of steady-state thermal transmission properties. Calibrated and guarded hot box (London: British Standards Institution) (1996) 5 BS EN ISO 12567-1: 2000: Thermal performance of windows and
doors. Determination of thermal transmittance by hot box methods:
Part 1: Complete windows and doors (London: British Standards Institution) (2000)
6 Building and Buildings. The Building Regulations 2000 Statutory Instrument 2000 No. 2531 and Building Regulations (Amendment) Regulations 2001 Statutory Instrument 2001 No.
3335 (London: The Stationery Office) (2000 and 2001) 7 Conservation of fuel and power in dwellings Building Regulations
2000 Approved Document L1 (2002 edn.) (London: The Stationery Office) (2001) (www.odpm.gov.uk) (under revision) 8 Conservation of fuel and power in buildings other than dwellings
Building Regulations 2000: Approved Document L2 (2002 edn.) (London: The Stationery Office) (2001) (www.odpm.gov.uk) (under revision)
9 BS 8207: 1985 (1995): Code of practice for energy efficiency in buildings (London: British Standards Institution) (1985) 10 BS 8211: Energy efficiency in housing: Part 1: Code of practice for
energy efficient refurbishment of housing (London: British Standards Institution) (1988)
11 Building (Scotland) Regulations 2004 Scottish Statutory Instrument 2004 No. 406 (Edinburgh: The Stationery Office) (2004) (also available from www.sbsa.gov.uk)
12 Technical handbook: Domestic and Technical handbook: Non- domestic (Scottish Building Standards Agency) (www.sbsa.gov.uk) (under revision)
13 Building Regulations (Northern Ireland) 1994 Statutory Rules of Northern Ireland 1994 No. 243 (Belfast: The Stationery Office) (1994)
14 Conservation of Fuel and Power Building Regulations (Northern Ireland) 1994 Technical Booklet F (Belfast: The Stationery Office) (1998) (www.dfpni.gov.uk/buildingregulations) (www.buildingregulationsni.gov.uk) (under revision)
15 BS EN ISO 6946: 1997: Building components and building elements. Thermal resistance and thermal transmittance. Calculation method (London: British Standards Institution) (1997) (under revision)
16 BS EN ISO 13789: 1999: Thermal performance of buildings — Transmission heat loss coefficient. Calculation method (London:
British Standards Institution) (1999) (under revision)
17 Siviour J B Areas in heat loss calculations Building Serv. Eng.
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18 ISO 8302: 1991: Thermal insulation. Determination of steady-state thermal resistance and related properties. Guarded hot plate apparatus (Geneva: International Organization for Standardization) (1991)
19 BS EN 1745: 2002: Masonry and masonry materials. Methods for determining design thermal values (London: British Standards Institution) (2002)
20 BS EN 12524: 2000: Building materials and products.
Hygrothermal properties. Tabulated design values (London: British Standards Institution) (2000)
21 BS EN ISO 10456: 1999: Building materials and products and products. Procedures for determining declared and design thermal values (London: British Standards Institution) (1999) (under revision)
22 Heat transfer CIBSE Guide C, section 3 (London: Chartered Institution of Building Services Engineers) (2002)
23 Doran S M and Gorgolewski M T U-values for light steel-frame construction BRE Digest 465 (Garston: Building Research Establishment) (2002)
24 Ward T I Metal cladding: assessing the thermal performance of built-up systems that use ‘Z’ spacers BRE Information Paper IP10/02 (Garston: Building Research Establishment) (2002) 25 Metal cladding: U-value calculation — assessing thermal
performance of built-up metal roof and wall cladding systems using rail & bracket spacers Technical Information Leaflet P312 (Ascot: Steel Construction Institute) (2002)
26 Guide to good practice for assessing glazing frame U-values (Bath:
Centre for Window and Cladding Technology) (2002)
27 Guide to good practice for assessing heat transfer and condensation risk for a curtain wall (Bath: Centre for Window and Cladding Technology) (2002)
28 Conventions for U-value calculations BRE Report BR 443:
(Garston: Building Research Establishment) (2002) (under revision)
29 EN ISO 6946/prA2: Amendment 2, Annex D.3: Correction for mechanical fasteners (Brussels: Comité Européen de Normalisation) (available in UK through BSI) (2003)
30 BS EN ISO 10211-1: 1996: Thermal bridges in building construction. Heat flows and surface temperatures: Part 1: General calculation methods (London: British Standards Institution) (1996) (under revision)
31 Anderson B R The relationship between the U-value of uninsulated ground floors and the floor dimensions Building Serv. Eng. Res. Technol. 12 (3) 103–105 (1991)
32 Anderson B R Calculation of the steady-state heat transfer through a slab-on-ground floor Building and Environment 26 (4) 405–415 (1991)
33 BS EN ISO 13370: 1998: Thermal performance of buildings. Heat transfer via the ground. Calculation methods (London: British Standards Institution) (1998) (under revision)
34 Anderson B R The effect of edge insulation on the steady-state heat loss through a slab-on-ground floor Building and Environment 28 (3) 361–367 (1993)
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36 BS EN ISO 10077-1: 2000: Thermal performance of windows, doors and shutters. Calculation of thermal transmittance: Part 1:
Simplified methods (London: British Standards Institution) (2000) (under revision)
37 BS EN ISO 10077-2: 2003: Thermal performance of windows, doors and shutters. Calculation of thermal transmittance: Part 2:
Numerical method for frames (London: British Standards Institution) (2003) (under revision)
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components. Dynamic thermal characteristics. Calculation method (London: British Standards Institution) (1999) (under revision) 44 BS EN ISO 13790: 2004: Thermal performance of buildings.
Calculation of energy use for space heating (London: British Standards Institution) (2004) (under revision)
3.A1.1 Standard moisture content
While insulating materials are generally ‘air-dry’ (i.e. in equilibrium with the internal environment), this is not true for masonry materials in external walls. Research(A1-1)has shown that typical moisture contents of both the inner and outer leaves of external twin-leaf masonry walls are above air-dry values and the thermal conductivities used for calculating U-values should be corrected to take account of the presence of moisture.
The moisture content of the structural elements of occupied buildings varies widely depending upon many factors including climate, type of masonry, thickness of wall, whether or not the wall is rendered, standards of work- manship in construction, local exposure to rain (which varies across the building) etc. Therefore, it is convenient to base U-value calculations on thermal conductivities at standard values of moisture content.
Typical moisture contents for UK conditions are given in Table 3.2 for masonry that is ‘protected’ or ‘exposed’.
‘Exposure’ refers to the external climate (i.e. solid masonry or the outer leaf of cavity walls without protective cladding). ‘Protected’ refers to solid masonry or the outer leaf of cavity walls protected by cladding such as tile hanging or weather boarding, and to the inner leaf of cavity walls (whether or not the cavity is filled with an insulating material).
3.A1.2 Correction factors for thermal conductivity
The way in which the thermal conductivity of different materials increases with moisture content is shown in Table 3.33. For maximum accuracy, this variation is given in terms of either percentage by weight or percentage by volume, according to the characteristics of the particular material.