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Evaluation of wall energy performance by means of 3 complementary methods

Within the scope of environmental approaches (COP 21) and the set of 2012 Thermal Regulations, the thermal performance of insulating materials proves to be a critical element to consider in projects aimed at producing more energy-efficient buildings.

Determining the energy performance of building walls is viewed as the criterion most representative of reality for this type of characterization, given that the presence of thermal bridges can drastically degrade results. The methodologies devised to access this information appear to be pivotal, as do their associated uncertainties.

Background, challenges and objectives

Two-thirds of France's housing stock were built prior to enactment of the first round of thermal regulations issued in June 1975, and slightly less than 50% of them have undergone renovation since then (insulation of attics or walls mainly from the inside, change of windows, etc.).

The number of poorly-insulated buildings among the existing stock thus remains very high in France, and such will be the case for many years to come. Consequently, an extensive renovation program must be planned, in making the quality of both insulating materials and their installation a top priority.

The ACERMI certification validates the intrinsic characteristics of heat insulators. Yet the thermal performance of a building wall depends first and foremost on application of insulating materials and all associated thermal bridges. It therefore proves necessary to characterize thermal performance at the building wall scale, in noting that meteorological variations complicate the use of in situ methods. Reliance on a test bench designed at the wall scale and operated within a controlled environment offers a sensible path forward.
LNE has conducted research towards advancing these issues in the aim of proposing to its clients a more effective approach within the scope of evaluating the performance of their products.

The study conducted has been intended to determine use conditions for the various methods described in the literature, such as co-heating, introduction of flux meters or infrared thermometry methods, and then associates them with an uncertainty level.

LNE's REBECCA platform (acronym for Research and Testing on Buildings and Heat Sources within an Artificial Climate), which makes it possible to study the coupling between a heat source and a room (in addition to establishing boundary conditions), was deployed in order to execute this project. The thermal enclosure is composed of an internal cell with the dimensions of a room in a dwelling (3.8 m on a side by 2.6 m high) surrounded by 4 climate-controlled chambers. These chambers serve to adjust temperature from –7° to +35°C in standard mode, or they can generate any type of transient sequence (hot-cold, cyclic, etc.). The advantage of this set-up lies in the fact that the internal cell is both structurally and climatically independent of the four other adjacent climate chambers (front cell, guard, floor and ceiling). A simulation phase (COMSOL software) was inserted early on in order to validate both the development of this testing platform and the placement of flux meter sensors.

This study has confirmed that the use of flux meters can be anticipated for perfectly homogeneous structures provided certain implementation rules are being followed. The wall's global thermal resistances, as obtained by means of standardized measurements using a Guarded Hot Plate, by energy consumption (co-heating) or by the flux meter approach, are thus all similar with an associated uncertainty lying between 6% and 15%. In the case of complex structures leading to the presence of thermal bridges (most notably correlated with the application of vacuum panels or the presence of an air knife), the use of flux meter sensors must be supervised in order to evaluate the global thermal resistance of this type of building wall. Flux meter placement actually exerts a predominant influence on the results obtained. The proximity of a thermal bridge or the presence of an air knife disrupts the thermal flux, which in turn leads to significant deviations across the three methods. The energy consumption method (co-heating) thus appears to be the best suited approach for measuring the thermal performance of an insulating system.

Scientific and industrial impacts

  • Comprehensive development of a method for characterizing the energy performance of building walls, as validated by modeling, and capable of integrating both product implementation and the uncertainty budget.
  • Possibility of conducting thermal measurements on a complex product at full-scale and in the presence of predefined boundary conditions.
  • Determination of potential heat transfers in the case of complex systems.

Publications and papers presented

Koenen et al., Wall thermal resistance measurement with an Energy Room method. Uncertainty analysis of different approaches, 33rd International Thermal Conductivity Conference (ITCC), 15-18 May 2017, Utah, USA

Networks

Associated projects

  • NRA (National Research Agency) project: RESBATI
  • H2020 project: InnoVIP
     

Enceinte REBECCA du LNE

Enceinte REBECCA du LNE

Illustration maison énergie

Méthode flux-métrique

Méthode flux-métrique

Méthode par consommation d'énergie

Méthode par consommation d'énergie

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