G-06 Distinction between the declared unit and the functional unit

Aspect G-06 Distinction between the declared unit and the functional unit
The functional unit represents the quantified performance of a product system for use as a reference unit for the LCA study. It is the unit of scale or reference on which the LCA results are based, and relates to the given function of the product. In other cases, the functional unit should be defined according to the future use of the building. A functional unit comprises a function, a quantity, a duration and a quality. The declared unit is used instead of a functional unit when the precise function of the product at the building level is not stated or known, or when the LCA does not cover a full life cycle. It is necessary to distinguish between these two concepts for product LCA, and to give operational guidance.

related study objective

stand-alone LCA comparative assertion

related study phase

goal and scope definition inventory analysis (LCI) impact assessment (LCIA) interpretation reporting

relevant for

new buildings existing buildings construction products screening LCA simplified LCA complete LCA
Provisions The practitioner should refer to EN 15 804 and the ILCD Handbook for the correct definitions of the functional unit and the declared unit.

The declared unit should be used for raw materials that are not implemented directly in the building, and for non-application-specific products.

In other cases, the functional unit should be defined according to the future use of the building. A functional unit comprises a function, a quantity, a duration and a quality.

Under special conditions (e.g. if a building product has a rather large number of possible applications in a building), then the declared unit, as defined in EN 15804, may be more appropriate than the concept of the functional unit, as the functions provided by a construction product are often closely related to the way that the product is incorporated in the building, and this cannot be known at the point at which the product is assessed.

Rules from:

EN 15804

6.3.1 Functional unit6.3.2 Declared unit


Provisions: 6.4 Function, functional unit, and reference flowI) SHALL – Identify system or process

II) MAY – Photos, specifications

III) SHALL – Identify function(s) and functional unit(s)

IV) SHALL – Functional unit, details

         IV.a) Function provided (what),

         IV.b) in which quantity (how much),

         IV.c) for what duration (how long),

         IV.d) to what quality (in what way and how well is the function provided (what)

V) MAY – Obligatory and positioning properties

VI) SHALL – Measurement methods

VII) SHOULD – Alternatives and complements to the functional unit

         VII.a) Materials and other application unspecific products

         VII.b) Monofunctional processes

         VII.c) Multifunctional processes

VII) SHOULD – Highly variable function

VII) SHALL – Comparative studies

ISO 14044 Function and functional unit
1) Specific guidance for defining the declared unitThere are two cases when it is relevant to define a declared unit. The first is for raw materials (e.g. cement, gravel) for which a functional unit does not really make sense. The second is for products that can be used in a wide variety of different or simultaneous functions in the building or construction works: for example, a timber section might be used in upper floor construction, in a partition wall, in fencing or in joinery. For each of these functions, the wastage in construction and the service life of the timber might be different. Similarly, a concrete block could be used in either an internal or an external wall, and with thin joint or normal mortar; each of these options will impact differently on the thermal performance of the block in the building or construction works. In this second case, a declared unit is used, allowing the relevant data to be provided and aggregated at the building level, where the function of the product as installed can be considered over the building life. Examples of declared units include:

  • by item, e.g. 1 brick, 1 window (dimensions to be specified), 1 lighting-fitting, 1 radiator;
  • by mass, e.g. 1 kg of cement;
  • by length, e.g. 1 m of pipe, 1 m of a beam (dimensions must be specified);
  • by area, e.g. 1 m2 of wall elements, 1 m2 of roof elements (dimensions must be specified);
  • by volume, e.g. 1 m3 of timber, 1 m3 of ready-mixed concrete.


2) Specific guidance for defining the functional unit

For product LCA (e.g. EPD), the functional unit has to be closely defined with the company that commissions the LCA or EPD study.

For cradle-to-gate and cradle-to-site product LCA data, the function of the product within the building or construction works is uncertain, and it is therefore not easy to find a functional unit, which needs to include information on the required functional performance of the product within the building or construction works over the full life cycle. Nevertheless, during discussions on the development of CEN TC 350 the recommended route was to identify reasonable scenarios and include information for one or more of these scenarios. In defining such a scenario, much of the required information to describe a post-gate life cycle stage would originate from the building, and would be related to the product being studied.

The functional unit has a quantity (e.g. 1 m²), a duration (e.g. ‘maintaining the function over 50 years’) and a quality e.g. ‘“to ensure a thermal resistance of 2 m²/W.K’). It is closely connected to the definition of the relevant service life of the product (cf. the corresponding aspect).

For comparative purposes, different, simultaneously applying functions may be relevant. With the definition of the functional unit, all relevant functions should be covered.

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