One of the main changes presented by the new version of the Technical Building Code is related to the Basic requirement HE 1: Conditions for the control of energy demand. This section has undergone a total transformation, no longer explicitly using the demand indicator in its justification.
The name change in the HE1 document can give us an idea of what is intended to be justified in it. In the CTE 2013, the HE1 requirement was titled Limitation of energy demand and was justified taking into account the heating and cooling demands of the building. In the 2019 version it is called Conditions for controlling energy demand and it is justified through indicators; overall heat transfer coefficient through the thermal envelope (K) of the building, the parameter solar control (Qsol;jul) and in new buildings for private residential use with a total useful area greater than 120 m², with the air change ratio with a differential pressure of 50 Pa (n50).
The fact that the demand values of the building are not verified does not mean that the demand is not key in the justification of the CTE DB HE1 2019 but rather that it has been chosen to take it into account through combine the limitation of Total Primary Energy Consumption and minimum quality conditions, mentioned in the previous paragraph.
In this Blog entry we are not going to focus on these new indicators and how to justify them, we are going to delve into a topic that has not been studied as much, Why has the demand indicator been stopped being used in the HE1 document?
To answer this question, it is necessary to go to the Bases Document for updating the DB-HE Basic Document (December 2016), where the bases that gave rise to the current document are established. Although many of the ideas expressed in this document have undergone changes, updates or modifications, in the sections; 23. Demand constraints (energy needs) and 2.4. Limitations related to demand (quality of the thermal envelope), they tell us the causes or reasons for the change in the justification of HE1 and because the new indicators are more reasonable.
Key aspects of the demand indicator
Below are the sections that are considered most important:
The demand indicator is not consistent in relation to ventilation
The justification of the new CTE had to be associated with a reduction in the building's energy needs, which includes more demanding conditions for its justification. One of the largest loads that affects buildings in the calculation of demand is ventilation, being especially relevant in cold climates. Since the demand calculation is independent of the systems, it is not possible to establish a level of demand that ensures optimal demands in terms of passive design without simultaneously imposing a fixed level of ventilation efficiency or, alternatively, assessing the use of ventilation efficiency measures without distorting the requirement in relation to the passive design of the building.
The demand indicator is not robust to evaluate passive building design and envelope quality
This CTE seeks building designs that demand little energy to achieve comfort conditions, first of all through a very careful envelope. In the current market we have efficient ventilation technologies available (heat recovery, free-cooling,...) that reduce energy demand, without taking into account criteria more oriented to design or passive measures (quality of the envelope, compactness of the building, solar collection or protection).
The variation in the use regime of mobile solar protection also has a significant impact on energy demand, so that it reflects not only geometric-constructive conditions of the building but also of use.
The demand indicator does not consider the relative energy importance of the services or their interaction
The separate consideration and specific treatment given to the demand for each building service no longer responds to either its relative importance in energy terms or the cost of combating these needs.
At the same time, the establishment of separate limits for heating and cooling (pooled demand has limited use) encourages a service-by-service approach rather than a more global one.
The prioritization and valuation of services implicit in this scheme is not adequate to satisfy the level of global efficiency currently required since it requires more complex strategies with a more integrated approach to services or there is a double risk of reducing the overall level of demand in relation to the optimal level and of establishing very different levels of demand between services.
The use of different demand indicators makes comparison between cases difficult.
The use of the limit values of the heating and cooling demand, of joint demand, of the demand of the reference building or of the percentage of savings of the joint demand, depending on the use and type of intervention in question, in addition to multiply the number of concepts and application procedures, makes it difficult to internalize normal values and energy costs of design alternatives.
The transmittance indicator (U) element by element
The transmittance indicator (U) element by element does not take into account the interaction between elements of the thermal envelope and is ineffective in terms of design.
The element-by-element consideration of the construction quality of the elements of the thermal envelope and not of the overall energy behavior omits the importance of the arrangement of the elements and its role in energy behavior (eg in Trombe walls) or the importance of thermal bridges.
In the case of existing buildings, where the U limit values of the envelope elements are more restrictive (in relation to demand or consumption), this approach is not very flexible and excludes equally or more efficient designs. adopting a more global energy approach but which may also be more interesting from a technical or economic point of view. That is why compensation between elements is allowed while maintaining the average transmittance.
In the case of new buildings, where the U limit values of the envelope elements are not so restrictive (compared to those derived from the demand or consumption limits), these are of little relevance from the design point of view. significantly deviate from effective values.
The effective design of solar protection is not evaluated
The demand indicator includes the effect of solar protection, but couples its effectiveness with its use regime (in the case of mobile protection), so that, indirectly, the level of demand for the rest of the demand components is affected by the user's judicious or fortunate choice of usage regime. The possible prescription of a solar factor for the gaps presents the same problems of decontextualization in relation to the rest of the thermal envelope as the establishment of minimum transmittance values and puts the focus on a technical solution for a problem that is generally a design problem.
It is true that the justification of the CTE DB HE1 with the new regulations is a point of conflict among technicians and although a priori it might seem logical to evaluate the demand for the building, the previous points do They give objective and quantifiable reasons for the change. The detractors of this change argue that there are certain types of buildings that can be penalized with this justification, for example homes with a very high percentage of openings, buildings with a low compactness... or, leaving aside the typology, buildings located in climatic zones with temperate climates.
Technicians must adapt to this change, knowing the reasons why it has occurred can help us better understand how to act and how to justify buildings according to the new regulations.
