As has already been mentioned in other forum entries, the changes and modifications that affect “Royal Decree 732/2019, of December 20, which modifies the Technical Building Code, approved by Royal Decree 314/, will be discussed. 2006, March 17.”
One of the documents that has undergone the greatest change is the Section HE 1 “Conditions for the control of energy demand”. As we already discussed, among the modifications in this section the disappearance of energy demand as an explicit indicator stands out, although this does not reduce its importance for the design given that maintaining a reduced energy demand is a necessary condition to be able to meet consumption requirements.
As indicated in the CTE 2019 DB HE1: Buildings must have a thermal envelope that limits their primary energy needs depending on the climatic zone, their use and their compactness.
To comply with this section it is necessary check five aspects:
- The global transmittance of the thermal envelope (K) and transmittances by elements (Ulim)
- Solar control of the thermal envelope (qsol;jul)
- The air permeability of the thermal envelope (Q100 and n50)
- Limit imbalances between use units (Ulim interior partitions)
- Control of condensation.
Let's focus the topic on the first point: The global transmittance of the thermal envelope (K) and transmittances by elements (Ulim).
Before starting to see the checks in detail, it is important to be clear about other related aspects:
- Geometric definition of the envelope:
- Surface of the enclosures: The total surface of an enclosure is made up of its opaque part plus that of the gaps, if any.
- Envelope composition:
- Thermal transmittance [U] of each enclosure element belonging to the envelope or interior partitions. It is obtained from the characteristics and composition of the enclosures. Value of [U] expressed in W/m2 ·K.
Normally opaque enclosures already belong to the envelope or interior partitions, will be composed of several “sheets” and different materials. The calculation of the transmittance of these elements is described in detail in the DA help document DB-HE / 1. Calculation of characteristic parameters of the envelope.
For example, in the calculation of the thermal transmittance of enclosures in contact with the outside air such as façade walls, roofs and floors in contact with the outside air:
being,
RT the total thermal resistance of the construction component [m2 K/W].
The total thermal resistance RT of a component made up of thermally homogeneous layers is calculated by the expression:
being,
R1, R2…Rn the thermal resistances of each layer defined according to the expression [m2 K/ W];
Rsi and Rse are the surface thermal resistances corresponding to the interior and exterior air respectively, taken from table 1 according to the position of the enclosure, direction of the heat flow and its location in the building [m2 K/ W].
Regarding the transmittance in gaps, the calculation methodology is again developed in the DA help document DB-HE / 1. Calculation of characteristic parameters of the envelope.
Linear thermal transmittance in thermal bridges.
- Volume contained in the ET:
For this section, the volume we are going to refer to is the total Interior volume (used, for example, to calculate compactness). This would be the one that completely encloses the thermal envelope, including slabs and roof thickness.
- Compactness:
Establishes the relationship between the volume enclosed by the envelope and its surface (V/A) (m3 /m2 = m). This is a fundamental characteristic of the “form” of the building. If we take into account that the heat flows between the interior of the building and the exterior are carried out through its skin, this relationship is decisive when evaluating its behavior.
Compactness is defined in the Annex A Terminology in the following way:
- Compactness (V/A): Relationship between the volume enclosed by the thermal envelope (V) of the building (or part of the building) and the sum of the heat exchange surfaces with the outside air or the ground of said thermal envelope (A = ΣAi). It is expressed in m³/m².
Therefore, to calculate compactness, The calculation of the area of the enclosures and interior partitions is excluded. in contact with other buildings or with adjacent spaces outside the thermal envelope.
Therefore, the computable area of the thermal envelope is exclusively that which is in contact with the outside air or ground.
Transmittances by elements (Ulim)
These limit transmittances ensure a minimum quality of the envelope and avoid imbalances in the thermal quality of the building spaces.
Thermal envelope conditions
Each element of the thermal envelope must comply with the limit values marked in Table 3.1.1.a – HE1 if it is a new building (except in the excluded cases: Protected buildings, Temporary constructions (<2 years), Industrial Buildings, defense or agricultural with low energy demand, Insulated buildings Subtle < 50 m2), an extension, a change of use or a renovation where more than the 25% of the envelope is renewed.
In the case of reforms where the intervention is less than 25% of the envelope, The table applies to:
- Elements that are replaced, incorporated or substantially modified
- Elements that see their exterior or interior conditions modified as a result of the intervention, resulting in an increase in the building's energy needs.
Limitation of imbalances between units of use
In the case of vertical partitions, the limit values marked in Table 3.2 – HE1 must be met if it is a new building (except in excluded cases), an extension, a change of use or a renovation where more than the 25% of the envelope.
In the case of reforms where the intervention is smaller than 25% of the envelope, the table applies to:
- Elements that are replaced, incorporated or substantially modified
- Elements that see their exterior or interior conditions modified as a result of the intervention, resulting in an increase in the building's energy needs.
However, the individual compliance of the elements does not guarantee the overall compliance of the building, limited by the global transmittance of the thermal envelope (K).
Overall transmittance of the thermal envelope (K)
The objective of the global thermal transmittance indicator (K) is ensure the efficiency of the thermal envelope in relation to heat transmission, taking into account the protected habitable volume and its thermal exchange surface with the outside.
The indicator integrates the characteristics of the elements that make up the thermal envelope, their proportion, the care of thermal bridges (except in the case of element-by-element interventions) and modulates its requirement depending on the winter climatic zone, the compactness of the thermal envelope (V /A) and, in existing buildings, the scope of the intervention.
The global thermal transmittance indicator (K) It is based on the overall heat transfer coefficient (Htr,adj, section 8.3.1, eq. 17 of UNE EN ISO 13790:2008 and section 6.6.5.2, eq. 108 of ISO/FDIS 52016-1) affected by the exchange surface with the outside.
It measures the overall ability to avoid heat exchange by conduction.
In a simplified way, this parameter can be calculated from the thermal transmittances and surfaces of the elements of the thermal envelope and an adjustment factor:
Heat transmission through the thermal envelope (gaps, opaques and thermal bridges)
Thermal envelope exchange surface
Indicator value:
Where:
btr,x
TOx,i
ORx,i
lx,k
ψx,k
xx,j
It is the adjustment factor for the envelope elements. Its value is 1 except for elements in contact with buildings or adjacent spaces outside the thermal envelope, where it takes the value 0;
It is the exchange area of the thermal envelope element considered;
It is the value of the thermal transmittance of the thermal envelope element considered;
It is the length of the thermal bridge considered;
It is the value of the linear thermal transmittance of the considered thermal bridge;
It is the punctual transmittance of the thermal bridge considered
The limit values (KLim) to justify the global transmittance indicator of the thermal envelope, it is differentiated depending on the use of the building, the winter climatic zone and whether it is a new building, an extension, a change of use or a renovation:
Residential buildings:
Buildings for use other than private residential:
As can be seen in the tables, the value of KLim varies depending on the compactness of the building. For example, a new building, for private residential use located in the winter climatic zone:
As percentages, up to a 30% difference for Klim values that we must justify, based on the compactness of the building.
