Introduction
The research group I belong to at Miguel Hernández University of Elche (UMH) has recently had a new scientific paper published in the journal Energy Efficiency. The study focuses on a building performance aspect that is often taken for granted: air infiltration.
Beyond measuring building airtightness through a Blower Door test, the paper analyses a less visible—but highly relevant—issue: how the mathematical model used to represent air infiltration can significantly influence the calculated heating demand.
What is the paper about?
Air infiltration refers to the uncontrolled entry of outdoor air through the building envelope (joints, connections, cracks, and gaps). Although its importance is widely recognised, in everyday practice it is often simplified or implicitly assumed in energy simulations.
The paper compares four widely used technical and normative models for estimating air infiltration from pressurisation test results:
- N-factor model (N = 20): a simplified approach assuming an almost constant airflow rate.
- LBL model (Lawrence Berkeley Laboratory): relates infiltration to wind and stack effect.
- AIM-2 model (Alberta Air Infiltration Model): a more advanced formulation for variable wind and temperature conditions.
- EN 16798-7 model: based on airflow network calculations, in accordance with European standards.
Important note: in Spain, official energy certification and simulation tools do not allow users to choose the infiltration model, as it is fixed internally within the software. This makes it especially important to understand which model is being used and how it may be influencing the results.
Case study and methodology
The analysis is based on a real single-family dwelling (“La Casa de la Tierra”, located in Murcia), energy-modelled using TRNSYS and experimentally characterised through a Blower Door test conducted in accordance with UNE-EN ISO 9972.
- The measured value was n₅₀ = 4.64 h⁻¹, which complies with the airtightness limits established in the Spanish Building Code (CTE DB-HE) for this type of building.
- This value was then used to estimate air infiltration under real operating conditions using each of the four models.
- The study was extended to eight Spanish cities (Almería, Málaga, Murcia, Alicante, Barcelona, Bilbao, Madrid, and Burgos), covering a wide range of climatic conditions.
This approach makes it possible to assess not only the effect of the model itself, but also the influence of climate and wind conditions.
Main results
- Air infiltration has a significant impact on energy demand
Even in a dwelling equipped with balanced mechanical ventilation and heat recovery, accounting for air infiltration leads to an increase in heating demand of approximately 27% to 32% compared to an idealised scenario without infiltration.
This clearly shows that neglecting or oversimplifying air infiltration can result in a substantial underestimation of actual energy consumption.
- The infiltration model does matter
When comparing the different models, differences in heating demand of up to approximately 6% to 12% were observed, depending on the city and climatic conditions.
These differences are not negligible and are comparable to the effect of certain moderate envelope improvement measures.
- Climate plays a key role
The results indicate that:
- In cold climates with lower wind intensity, differences between models become more pronounced.
- Under these conditions, the AIM-2 and EN 16798-7 models provide more balanced estimates.
- The LBL model tends to predict higher infiltration rates—and therefore higher heating demand—in more severe winter scenarios.
Why is this research relevant?
Because in professional practice the infiltration model is not a neutral assumption, even when it is “hidden” within the software. Its influence can affect:
- heating demand estimates,
- comparisons between design alternatives,
- the evaluation of energy efficiency measures,
- and, in borderline cases, compliance with specific energy performance requirements.
Understanding the assumptions behind simulation results is essential for correctly interpreting energy assessments.
Reference
Ícaro Vera Alvez, Manuel Romero Rincón, Juan A. Tudela y Pedro Martínez Beltrán
Comparison of models for estimating air infiltration in dwellings and their impact on heating demand.
Energy Efficiency, 19:10. Enlace DOI