Thermal Performance Standard for Residential Buildings: what is it and what changes in the new version?

When it comes to single- or multi-family residential buildings, the comfort of residents inside housing units is something that must be considered from the modern design stage; after all, most of our lives are spent in houses and apartments. To this end, construction professionals must promote design solutions in order to provide thermally, luminously and acoustically comfortable environments for their users.

In general, to be in thermal comfort, the individual must be in an environment with a temperature between 21 and 27ºC. Values below or above this range characterize, respectively, thermal discomfort due to cold or heat. However, these temperature limit values can vary according to the particular characteristics of each person. In addition, many other factors influence the sensation of thermal comfort by the individual in the environment, such as relative humidity, the incidence of winds, their clothing and the activity they perform in that location. Therefore, it is important to evaluate each building on a case-by-case basis, according to its location, orientation, use and other design features. The building envelope, for example, plays an important role in the thermal comfort of its users – which, as seen, are the medium between the external and internal environments – and each material that composes it has thermal properties and its own characteristics that determine the influence of external conditions on the interior of the building. Therefore, such materials found in the walls, roofs and openings of the building must meet the minimum performance requirements, stipulated in ABNT NBR 15.575, recently updated. 2013 for all new residential buildings, establishing minimum performance requirements for housing environments throughout the country. In September 2021, the new version of the Performance Standard for thermal and acoustic comfort was published. For the assessment of lighting performance and for new buildings registered up to 180 days after the publication of the new Standard, the 2013 version remains in force. For accommodation buildings, compliance with the Standard is recommended, but not mandatory.

The new Standard presents two methods for assessing the thermal performance of building envelopes: the simplified method – which replaces the old prescriptive method – and the computer simulation method, which allows for a more precise assessment of the building parameters. Using the simplified method, it is possible to obtain only the minimum level of performance. Using the simulation method, however, it is also possible to obtain intermediate and higher levels of compliance with the Standard. In this case, two scenarios must be considered – with and without natural ventilation –, taking into account, in the second, the artificial cooling or heating of areas for prolonged stays. To comply with the minimum level, however, simulation is only necessary under the condition of the presence of natural ventilation. This arises in a context of increasing global warming, enabling thermal comfort for building users when natural ventilation is not sufficient.

In the simplified method, in general, there were changes in the requirements for internal and external vertical sealing systems and roofing. In addition to the percentage of openings for ventilation already required in the prescriptive method, the percentage of transparent elements in the facades must also be considered, which consists of the ratio between the glazed area and the opaque wall area. High-performance glass can now be used in cases where the percentage of transparent elements is greater than the maximum percentage for compliance with the Standard. With the addition of these variations, buildings are assessed with greater limitations, since glazed areas are also considered, and not just as openings. The comparison between these construction characteristics and reference values described in the Standard determines whether or not the building meets the minimum level of performance required.

Figure 1: Changes in the Simplified Method of NBR 15.575/2021 compared to the Prescriptive Method of the 2013 version. Source: Own elaboration (adapted from LabEEE).

There is no longer a simulation method; the results are now obtained by comparing the parameters of the real building and a model of the same volume with reference characteristics. The new evaluation criteria are: the inclusion of the complete building environment, operable frames, and the concept of thermal bridges. Another change is the use of the annual climate file instead of climate data from only two days of the year – a typical summer day and a typical winter day – as was the case in the old version. In addition, ventilation, previously a fixed rate, now depends on wind speed and direction, making it closer to the real conditions of the building's implementation.

Figure 2: Simulation Method of NBR 15.575/2021. Source: Own elaboration (adapted from LabEEE).

Figure 3: Changes in the Simulation Method of NBR 15.575/2021 compared to the 2013 version. Source: Prepared by the author (adapted from LabEEE).

Previously, only critical housing units could be considered – generally located on the top floor of the building, below the roof – in order to simplify the process. In the new method, all units, including standard floors and other typologies, must be included in the simulation. In addition, only bedrooms and living rooms were considered in the old Standard, while kitchens and integrated circulations were planned for evaluation, even if they were integrated into the analysis environments. In the new version, these environments are also mandatory considered in the evaluation, if they are integrated.

Figure 4: Performance Indicators and Levels of NBR 15.575/2021. Source: Prepared by the author (adapted from LabEEE).

What can we expect for the future of the Standard?

As occurred in 2021, the Standard may undergo further revisions over the years, given the proven need to adapt to climate change and new technologies available, also changing the improvement of assessment methods. The future of the Performance Standard involves the revision of the item related to lighting performance, the standardization for buildings of other typologies - such as, for example, commercial buildings -, the revision of Brazilian bioclimatic zoning and the integration with the metamodel of the Brazilian Building Labeling Program (PBE Edifica). In this way, the aim is to design increasingly sustainable and energy-efficient buildings, aligned with the growing needs of cities to promote thermal comfort for their population with minimal environmental impact.