Evidence to support an update to the methodology for estimating infiltration rates in SAP: Gathering evidence to improve airtightness in the UK housing stock

<p dir="ltr">Executive summary</p><p dir="ltr">Existing infiltration estimation methods are unreliable when validated using existing datasets, with limitations. The new method proposed for inclusion in HEM was the most accurate and precise of the 12 tested and could be a promising improvement on the SAP method, subject to further validation work and method development. Infiltration influences predictions of overheating and energy demand, so accuracy is vital. </p><p dir="ltr">Research purpose Gather evidence to support an update to the method used to estimate infiltration rates in SAP. </p><p dir="ltr">Research methodology A Rapid Evidence Assessment reviewed 73 documents after relevance and quality screening. An empirical validation exercise evaluated the reliability of 12 infiltration estimation methods. Dynamic thermal and steady-state models of overheating and heating season energy demand were used to examine the effect of varying infiltration rates on model predictions. </p><p dir="ltr">Research findings There is high-quality evidence to suggest that current infiltration estimation methods produce unreliable estimates of infiltration rates in English dwellings, when validated using point-in-time measurements tested with small indoor-outdoor temperature differences. There is moderate?quality evidence to indicate the reliability of estimations of mean heating season infiltration. Empirical validation and inter-model comparison revealed that the new infiltration estimation method proposed for the DESNZ Home Energy Model (HEM) estimates infiltration with greater accuracy and precision than all other methods investigated in the case study dwelling under spring/summer weather. Yet, when wind speeds are low, the HEM method estimates infiltration less reliably. Therefore, the HEM infiltration estimation model could be improved with the inclusion of ΔT to account for the stack effect, which is the dominant driving force of infiltration at low wind speeds. The effectiveness of this has been demonstrated in other methods. All methods require further testing and validation in a range of dwellings under a range of weather conditions, before final conclusions can be made, however. Varying the infiltration rate of a house in a calibrated dynamic thermal model revealed that the predicted hours of overheating (Criterion A) were reduced from 5.8 to 2.3% in a bedroom when the value of infiltration was increased from 0.3 to 0.8 ach (the infiltration rate measured vs. estimated by the divide-by-20 method). This means the house would be compliant with Building Regulations on Overheating (Part O) using the estimated (higher) infiltration rate, but not compliant if the real, measured (lower), value for infiltration was used in the model.No description supplied</p>