Thermal conductivity hollow concrete masonry bricks

To limit the energy consumption of buildings, Belgian energy performance regulations impose, among other things, maximum heat transfer coefficients (Umax) on the outer shell of buildings. To assess whether an exterior wall meets the requirements, the thermal conductivity coefficients (λ) of the hollow concrete masonry bricks, among other things, must be known.

The standard for concrete masonry bricks, NBN EN 771-3, and its Belgian application rules PTV 21-001 state that manufacturers of concrete masonry bricks must declare the thermal conductivity λ 10.dry, masonry brick if the bricks are intended for building components subject to thermal requirements.

The value λ10, dry, masonry brick is not directly usable to assess the performance of a building component under EPB regulations. For that, according to PTV 21-001, an additional ë-value must be declared, i.e. this one for which 90 % of production, with a reliability of 90 %, has an ë-value less than or equal to the declared value (λ 10, dry, masonry brick, 90/90). 

Within the Belgian standards committee E125 "Masonry", new tables were drawn up based on the Belgian configurations. These tables will soon appear in the first national annex of NBN EN 1745. The λ-values in the tables were calculated based on the simplified method described in NBN EN ISO 6946. This method is permitted within the framework of the EPB regulations. Here, the concrete masonry bricks are divided into layers and sections, such that the element consists only of parts that are thermally homogeneous in themselves, i.e. consist of either concrete or air (Fig. 1). The tables give, as a function of the thickness of the transverse walls, longitudinal walls and upper shell of the concrete masonry bricks, the λ 10, dry, masonry brick for different values of ρnetto, dry, concrete. 

The thickness of the transverse walls, of the longitudinal walls and of the upper shell of concrete masonry bricks is not the same everywhere in practice, hence for simplicity the average value should be considered in each case, being half the sum of the smallest and the largest nominal value. Tables were prepared for different sizes of concrete masonry bricks with one, two and three cavities, with both plain aggregates and expanded clay pellets. By way of example, Table 1 and Table 2 give some values for concrete masonry bricks with two cavities.

The contribution of hollow concrete masonry bricks to the thermal resistance of the outer shell of a building is limited. Based on the new tables, the explanation of the thermal performance of hollow concrete masonry bricks is much simplified. Moreover, the values closely approximate reality. These values will find their way into the EPB product database and can then be applied in the context of EPB regulations.