Building a Passive House by Stefano Piraccini & Kristian Fabbri

Author:Stefano Piraccini & Kristian Fabbri
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

conductivity λ, measured in W/m K;

thermal bridges Ψ, measured in W/m K;

solar transmittance for non-scattering glazings (g) or windows with scattering glazing and/or solar shading devices (g gl + sh);

effective solar collecting area of glazed elements (m2) and effective collecting area of opaque building elements.

6.2.1 Thermal Transmittance (U)

The thermal transmittance, usually identified with the letter U and measured in W/m2 K, expresses the quantity of heat flow per second or the power exchanged (W) for each square metre of surface (m2) and for every degree of temperature difference (in degrees Celsius °C, or in degrees Kelvin K). For example, a transmittance of 1 W/m2 K, with an indoor–outdoor temperature difference of 25 °C, corresponds to the thermal exchange of 25 W per square metre of surface area.

The thermal flow is defined by EN ISO 7345 as the “quantity of heat transferred to or from a system, in the unit of time”.

The heat flow may be horizontal (from the inside to the outside), upwards (towards the ceiling) or downwards (towards the floor on the ground or on another room or outside).

The technical application of the laws of thermokinetics, for the construction sector, given that most of the technical solutions refer to elements where the surface area is the largest size, considers that the passage of heat occurs in one dimension (1-D). This “simplification” is applicable only in the case of homogeneous planar surfaces, while we cannot apply it for other geometric configurations where there is not a predominant dimension, such as the thermal bridges.

The thermal transmittance expresses the ability of the element or of the stratigraphy, to conduct heat; it is evident that the greater the transmittance value, and, at the same temperature, the greater the quantity of heat that flows through it. In contrast, the lesser the transmittance value, the lesser the quantity of energy exchanged. Transmittance allows us to measure the insulation capability of the technological solution, and its value depends on the building element. In the case of opaque closures, external walls, foundation or roof slabs, etc., the literature, the design practice and the technical legislation, indicates as reduced transmittance values those having U lower than 0.5–0.30 W/m2 K, they are “very poor” values (i.e. they allow the passage of a great deal of energy), U greater than 1 W/m2 K, referable to uninsulated buildings, while considered as “excellent” (allowing little passage of energy) are values of U lower than 0.20 W/m2 K, bearing in mind that the minimum reachable value is approximately 0.12–0.10 W/m2 K, i.e. difficult to obtain within the scope of the materials used in the construction sector.

The thermal transmittance is the result of the calculation of the sum of the resistances of the individual materials, and it is directly proportional to the thermal conductivity λ (ability of the material or materials to conduct heat) and inversely proportional to the thickness of the material or materials. The calculation is performed according to the EN ISO 6946 standard.

In the case of windows and transparent elements, the

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