ISO 9869 – 1: 2014
Uddrag af Standard
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The committee responsible for this document is ISO/TC 163, Thermal performance and energy use in the built environment, Subcommittee SC 1, Test and measurement methods.
This first edition cancels and replaces ISO 9869:1994, which has been technically revised.
Annexes A, B and C form an integral part of this part of ISO 9869. Annexes D, E and F are for information only.
The thermal transmittance of a building element (U-value) is defined in ISO 7345 as the “Heat flow rate in the steady state divided by area and by the temperature difference between the surroundings on each side of a system”.
In principle, the U-value can be obtained by measuring the heat flow rate through an element with a heat flow meter or a calorimeter, together with the temperatures on both sides of the element under steady-state conditions.
However, since steady-state conditions are never encountered on a site in practice, such a simple measurement is not possible. But there are several ways of overcoming this difficulty:
a) Imposing steady-state conditions by the use of a hot and a cold box. This method is commonly used in the laboratory (ISO 8990) but is cumbersome in the field;
b) Assuming that the mean values of the heat flow rate and temperatures over a sufficiently long period of time give a good estimate of the steady-state. This method is valid if:
1) the thermal properties of the materials and the heat transfer coefficients are constant over the range of temperature fluctuations occurring during the test;
2) the change of amount of heat stored in the element is negligible when compared to the amount of heat going through the element. This method is widely used but may lead to long periods of measurement and may give erroneous results in certain cases.
c) Using a dynamic theory to take into account the fluctuations of the heat flow rate and temperatures in the analysis of the recorded data.
NOTE The temperatures of the surroundings, used in the definition of the U-value, are not precisely defined in ISO 7345. Their exact definition depends on the subsequent use of the U-value and may be different in different countries (see Annex A).
This part of ISO 9869 describes the heat flow meter method for the measurement of the thermal transmission properties of plane building components, primarily consisting of opaque layers perpendicular to the heat flow and having no significant lateral heat flow.
The properties which can be measured are:
a) the thermal resistance, R, and thermal conductance, Λ, from surface to surface;
b) the total thermal resistance, RT, and transmittance from environment to environment, U, if the environmental temperatures of both environments are well defined.
The heat flow meter measurement method is also suitable for components consisting of quasi homogeneous layers perpendicular to the heat flow, provided that the dimensions of any inhomogeneity in close proximity to the heat flow meter (HFM) is much smaller than its lateral dimensions and are not thermal bridges which can be detected by infrared thermography (see 6.1.1).
This part of ISO 9869 describes the apparatus to be used, the calibration procedure for the apparatus, the installation and the measurement procedures, the analysis of the data, including the correction of systematic errors and the reporting format.
NOTE 1 It is not intended as a high precision method replacing the laboratory instruments such as hot boxes that are specified in ISO 8990:1994.
NOTE 2 For other components, an average thermal transmittance may be obtained using a calorimeter or by averaging the results of several heat flow meter measurements.
NOTE 3 In building with large heat capacities, the average thermal transmittance of a component can be obtained by measurement over an extended period, or the apparent transmittance of the part can be estimated by a dynamic analysis of its thermal absorption response (see Annex B).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 6781:1983, Thermal insulation — Qualitative detection of thermal irregularities in building envelopes — Infrared method
ISO 6946:2007, Building components and building elements — Thermal resistance and thermal transmittance — Calculation method
ISO 7345:1987, Thermal insulation — Physical quantities and definitions
ISO 8301:1991, Thermal insulation — Determination of steady-state thermal resistance and related properties — Heat flow meter apparatus
ISO 8302:1991, Thermal insulation — Determination of steady-state thermal resistance and related properties — Guarded hot plate apparatus
ISO 8990:1994, Thermal insulation — Determination of steady-state thermal transmission properties — Calibrated and guarded hot box
3 Terms, definitions, symbols and units
3.1 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 7345:1987 apply.
3.2 Symbols and units
Symbol Quantity Unit
thermal resistance m2·K/W
total thermal resistance m2·K/W
internal surface thermal resistance m2·K/W
external surface thermal resistance m2·K/W
thermal conductance W/(m2·K)
thermal transmittance W/(m2·K)
heat flow rate W
density of heat flow rate =Φ/A W/m2
interior environmental (ambient) temperature °C or K
exterior environmental (ambient) temperature °C or K
interior surface temperature of the building element °C or K
exterior surface temperature °C or K
density of a material kg/m3
thickness of a layer m
specific heat capacity J/(kg·K)
thermal capacity of a layer: C=ρcd J/(m2·K))
correction factors calculated with Formula (8) to take into account the storage effects [J/(m2·K)]
operational error (of an installed HFM) which is the relative error between the measured and the actual heat flow –
NOTE The environmental (ambient) temperatures shall correspond with those used in the definition adopted for the U-value (see Annex A).
In the steady-state, the thermal properties of the elements have the following definitions:
R is the thermal resistance of an element, surface to surface and is given by
where Λ is the thermal conductance of the building element, surface to surface.
U is the thermal transmittance of the element, environment to environment and is given by
where RT is the total thermal resistance which is given by
where Rsi and Rse are the internal and external surface thermal resistances, respectively.
R and RT have units of square metres kelvin per watt (m2·K/W); U and Λ have units of watts per square metre kelvin [W/(m2·K)].
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