DA DB-HE / 1: Calculation of Characteristic Parameters of the Building Envelope
The DA DB-HE / 1 is a technical support document issued by the Ministerio de Transportes, Movilidad y Agenda Urbana of Spain. This January 2020 edition replaces the previous 2015 version and serves as a non-regulatory technical guide to assist in the application of the Documento Básico DB-HE Ahorro de energía, which forms part of the Código Técnico de la Edificación (CTE). It provides simplified methodologies for determining the thermal properties of building components required to satisfy energy efficiency standards.
The document's scope includes the calculation of thermal transmittance (U), solar energy transmittance for semi-transparent elements, and average accumulated solar irradiation. It details procedures for opaque enclosures in contact with outside air, ground-coupled elements, and internal partitions adjacent to unheated spaces. Specific sections address homogeneous and heterogeneous layers, air cavities, and complex openings such as windows and skylights, incorporating standards like UNE-EN ISO 6946, UNE-EN ISO 13370, and UNE-EN ISO 10077.
Professional users can reference this text for specific formulas, thermal resistance tables, and reduction factors needed for structural compliance and energy demand control. The document establishes the technical basis for evaluating the building's thermal envelope performance under various environmental and constructive conditions.
Thermal Transmittance of Enclosures
The calculation of thermal transmittance (U) for opaque components in contact with outdoor air is based on the total thermal resistance (RT). For components with thermally homogeneous layers, the resistance is the sum of surface resistances and individual layer resistances.
- RT = Rsi + R1 + R2 + ... + Rn + Rse: Formula for total thermal resistance incorporating interior and exterior surface resistances.
- Surface Resistances: Values defined in Table 1 based on heat flow direction (horizontal, ascending, or descending).
- Air Cavities: Classified as unventilated, slightly ventilated, or heavily ventilated based on opening area thresholds (e.g., 500 mm² per meter for vertical chambers).
Ground-Contact Components
Specific procedures are provided for floors and walls in contact with the ground. The thermal transmittance (Us) for floors depends on the characteristic length (B'), the insulation band width (D), and the insulation's thermal resistance (Ra).
- Characteristic Length B': Defined as the floor area divided by half of the exposed perimeter length.
- Case 1 (Slabs on Grade): Applied to floors at ground level or up to 0.50 m below grade.
- Case 2 (Buried Slabs): Applied to floors at depths greater than 0.5 m, utilizing Table 4 values.
- Buried Walls: Calculation based on depth (z) and wall thermal resistance (Rm) using Table 5.
Internal Partitions and Unheated Spaces
For partitions separating heated zones from unheated spaces (espacios no habitables), the document introduces a temperature reduction coefficient (b). This factor accounts for the insulation position and the ventilation level of the unheated space.
- U = Up · b: General formula where Up is the partition's transmittance calculated as an outdoor enclosure.
- Sanitary Chambers: Specific conditions for ventilated under-floor voids with height (h) ≤ 1 m and depth (z) ≤ 0.5 m.
- Ventilation Degrees: Air renovation rates (n) ranging from 0 to 10 h⁻¹ depending on the airtightness of the unheated space.
Openings and Solar Transmittance
The thermal transmittance of windows (UH) integrates the values for glazing, frames, and linear thermal bridges at the junctions. It also defines the total solar energy transmittance (ggl;wi) for various glazing types and mobile shading devices.
- Glazing Types: Includes values for single, double, and triple glazing with low-emissivity coatings.
- Mobile Shading: Calculation of monthly average effective solar transmittance (ggl;wi;m) including the time fraction the device is active (fsh;with).
- External Obstacles: Shading factors (Fsh,obst) for overhangs, side fins, and recesses based on geometry and orientation.
Frequently Asked Questions
How is the characteristic length B' calculated for ground floors?
The characteristic length B' is the ratio between the floor area (A) and half of the exposed perimeter (P), calculated as B' = A / (0.5 · P).
What are the surface thermal resistance values for vertical walls?
For vertical enclosures or those with a slope greater than 60° with horizontal heat flow, the interior surface resistance (Rsi) is 0.13 and the exterior surface resistance (Rse) is 0.04 m²K/W.
When is an air cavity considered unventilated?
An air cavity is unventilated if openings to the outside do not exceed 500 mm² per meter of length for vertical cavities or 500 mm² per m² of surface for horizontal cavities.
What depth threshold distinguishes Case 1 from Case 2 for ground slabs?
Case 1 applies to slabs at ground level or up to 0.50 m below it; Case 2 applies to slabs at a depth greater than 0.5 m.
How is the solar transmittance of a window calculated when shading is used?
It is calculated using the formula ggl;wi;m = (1 - fsh;with) · ggl;wi + fsh;with · ggl;sh;wi, where fsh;with is the reduction factor for the time the mobile shading is active.
What standard is used to calculate the thermal transmittance of window frames?
The document references UNE-EN ISO 10077-2 for the numerical method of calculating the thermal transmittance of frames.
