The Performance Gap in New Construction : Evaluation of UK Passivhaus Dwellings

: (Alternative Format thesis).

Abstract

It is becoming clear that many new homes are using more energy in-use and overheating to a greater extent than predicted by building models. This performance gap has implications for the credibility of the construction industry and leaves building owners bearing the cost. In addition, as homes become more energy efficient to reduce carbon emissions, the existence of the performance gap means national underreporting of greenhouse gas emissions from this sector, which impacts on managing climate change. There is also emerging evidence linking increased dwelling energy efficiency with increased overheating risk - with the causes uncertain.
The direction of new housing in the UK is currently out for consultation through the Future Homes Standard. This suggests that a large-scale evaluation of the measured performance of existing low energy building standards would be timely, to help inform both future housing policy and our understanding of the performance gap.
Hence, this thesis aims to evaluate the key metrics of space heating demand and internal temperature data from UK homes, certified to the widely adopted low-energy Passivhaus standard, looking for evidence of the performance gap in both energy use and overheating risk. Since a performance gap can only be evaluated through access to both the predicted and observed parts of the problem, due consideration is first given to obtaining reliable predictions and then obtaining large-scale observed data for both winter and summer. The research is centred around three key questions.
Can a simplified method for temperature and weather normalisation be developed? There are many reasons for differences between design and measured energy use. In steady state building models such as Passive House Planning Package (PHPP), used to design and certify Passivhaus buildings, and the UK Standard Assessment Procedure (SAP), design internal temperatures are fixed. In reality, there will be differences between these design assumptions and user preferences. Being able to account for this, especially in low-energy homes, is essential, but is complicated by the fact that the original assessment may not be accessible at the time of a post-occupancy evaluation. Hence, a method was developed for these two routinely used building assessment models, to allow for temperature, solar and internal gains corrections to be made, without access to the original assessment. The results showed that measured internal temperature has the greatest impact on space heating variation, compared to solar and internal gains, thus reducing the level of data collection needed on-site. Applying these findings allow internal temperature normalisation to be undertaken more frequently, to eliminate this element of the energy performance gap.
Is there a performance gap between internal temperatures and the overheating risk methodology in PHPP and how does this prediction compare to other methods? Dry bulb internal temperature data from 82 certified Passivhaus homes, with different tenures, from varying locations, was analysed using Passivhaus (fixed temperatures) and CIBSE TM59 (adaptive comfort) overheating risk methodologies. Results showed that while most homes met both standards, the single zone approach of Passivhaus had the potential to mask overheating risk in individual rooms, especially bedrooms, where high internal temperature impacts more on health and comfort. TM59 focuses on the summer months only and could miss overheating outside of this season. When applied to bedrooms only, comparison of the two standards showed similar results, especially when using Passivhaus good practice levels (-50% of the maximum allowable hours). This showed that either assessment could be applied to measure overheating risk in domestic homes.
How do Passivhaus dwellings in the UK perform once occupied, compared to the space heating prediction in design models (PHPP)? Space heating data was collected from 97 certified Passivhaus homes (this sample included the 82 homes analysed for overheating risk). Using three different collection methods (i) heat metering(ii) monthly meter readings and (iii) bi-annual meter readings, which reflected the levels of data available, the results showed no evidence of the energy performance gap for space heating. In fact, despite using a cautious approach, which overestimated rather than underestimated the heating demand, on average the homes used less heating than predicted. This negative gap further increased when the normalisation technique developed in research question one was applied. Analysis of the data collection methods showed that minimal monitoring can yield useful results for estimating space heating demand.
This thesis demonstrates that homes certified to the Passivhaus standard do not show the energy performance gap, contrary to the findings in homes constructed to other standards. In addition, overheating risk can be managed using both the Passivhaus method and CIBSE TM59. These findings are then discussed in the context of the Future Homes Standard and the benefit of adopting a verified design is considered.

Date of Award	18 Nov 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	David Coley (Supervisor) & Sukumar Natarajan (Supervisor)