DA DB-HE / 3 Puentes térmicos – Spanish Building Code Support Document for Thermal Bridges
The DA DB-HE / 3 Puentes térmicos is an official Support Document (Documento de Apoyo) issued by the Ministerio de Fomento through the Secretaría de Estado de Infraestructuras, Transporte y Vivienda in Spain. This document, dated January 2014, serves as a technical supplement to the Código Técnico de la Edificación (CTE), specifically supporting the Basic Document for Energy Saving (DB-HE). It provides the theoretical foundations and calculation methodologies required to evaluate the hygrothermal behavior of thermal bridges in accordance with the regulatory demands for building energy performance and surface condensation prevention.
The scope of this regulation encompasses the characterization of common thermal bridges and the description of calculation methods used to assess their impact on energy demand and consumption. It defines a puente térmico (thermal bridge) as an area of the building's thermal envelope where the thermal resistance is significantly reduced due to variations in construction uniformity, such as changes in thickness, material conductivity, or the difference between internal and external areas. The document includes a comprehensive Atlas de puentes térmicos which provides pre-calculated linear thermal transmittance values for a wide range of constructive solutions.
The following sections detail the classification of thermal bridges, the specific calculation procedures for linear and point transmittance, and the criteria for limiting the risk of surface condensation and mold growth.
Classification of Thermal Bridges in the CTE
The Documento Básico HE establishes a typological classification for thermal bridges based on their position and integration within the thermal envelope:
- Integrated thermal bridges: Includes columns integrated into facade walls, the perimeter of openings and skylights, and roller shutter boxes (cajas de persianas).
- Bridges formed by intersections: Covers facade slab fronts (frentes de forjado), roof-to-facade junctions, and junctions with ground-contact elements such as slabs on grade or basement walls.
- Corners or facade junctions: Subdivided into re-entrant corners (esquinas entrantes) and salient corners (esquinas salientes).
- Cantilevers and partitions: Includes junctions of balconies/cantilevers with facades and internal partitions meeting external enclosures.
Calculation Methods for Thermal Transmittance
The document defines the formulations for determining heat flow through the thermal envelope, accounting for both linear thermal transmittance (ψ) and point thermal transmittance (χ). Several calculation approaches are permitted depending on the required precision:
- Detailed Methods: Utilizing three-dimensional or two-dimensional numerical modeling as per UNE-EN ISO 10211 to simulate heat flow and coupling factors.
- Simplified Methods: Estimating the two-dimensional component of heat transfer through tabulated values or representative estimations of ψ and length (L).
- Correction Factor Method: Estimating the total two-dimensional component as a fraction (α) of the one-dimensional component (U).
For these calculations, specific surface resistance values (resistencia superficial) are defined. For temperature calculations, the internal surface resistance (Rsi) varies: 0.13 m²K/W for glass, 0.25 m²K/W for elements in the upper half of a space, and 0.35 m²K/W for elements in the lower half.
Surface Condensation and Mold Growth Risk
To protect building health and structural integrity, the document establishes methods to limit the risk of surface condensation and subsequent mold formation. This risk is assessed using the factor de temperatura de la superficie interior ($f_{Rsi}$).
- Condensation limit: Condensation occurs when the air temperature falls below the dew point (temperatura de rocío).
- Mold risk limitation: Risk increases significantly if relative humidity exceeds 80% on a surface for several days. Compliance is verified by ensuring $f_{Rsi} > f_{Rsi,min}$.
- Hygrometry Classes: Minimum temperature factors are defined for different space categories, ranging from class 5 (high humidity) to class 3 or lower.
Atlas of Thermal Bridges
The document provides an extensive catalog of pre-calculated values for constructive solutions. The most critical factor identified for improving performance is maintaining the continuity of the insulation.
- Columns: Includes cases with and without insulation continuity, showing that ψ values are negligible when insulation is continuous.
- Openings: Provides data for lintels (dinteles), sills (alféizares), and jambs (jambas), categorized by their proximity to the insulation layer.
- Roof and Ground Junctions: Characterizes flat roofs, slabs in contact with outside air, and floors in contact with the ground.
Frequently Asked Questions
How is a thermal bridge defined according to the CTE?
According to DB-HE, it is an area of the building's thermal envelope where construction uniformity varies due to thickness changes, different material conductivities, or differences between internal and external areas, leading to reduced thermal resistance.
What is the threshold for mold growth risk on interior surfaces?
The risk is considered significant when surface relative humidity is maintained above 80% for several days; therefore, the interior surface temperature factor ($f_{Rsi}$) must exceed the minimum acceptable value ($f_{Rsi,min}$).
Which dimensional system does the Atlas of Thermal Bridges use?
The Atlas primarily adopts the sistema de dimensiones interiores (interior dimensions), as it is the standard for regulatory energy efficiency evaluation tools in Spain.
What happens to the thermal bridge value when insulation is continuous over a column?
When insulation continuity is maintained, the linear thermal transmittance (ψ) effectively disappears, resulting in negligible values (often 0.00 W/mK).
How do metallic lintels affect thermal bridge performance?
Non-protected metallic lintels can significantly aggravate the cold spot effect, increasing the risk of condensation at the junction between the window frame and the wall.
What are the standard surface resistance values for energy demand calculations?
For heat flow calculations, Rsi values include 0.10 m²K/W for upward vertical flow, 0.13 m²K/W for horizontal flow, and 0.17 m²K/W for downward vertical flow.
