Post occupancy evaluation of 12 retrofit nZEB dwellings: The impact of occupants and high in-use interior temperatures on the predictive accuracy of the nZEB energy standard

The Europe Union needs to reduce its carbon emissions. To achieve the proposed 55% emission reduction target by 2030 it has embarked on a Renovation Wave which aims for more, and deeper, building renovation. Considering this ambition and the scale of the challenge, there remains a surprising paucity of documented post occupancy evaluation studies of deep retrofit projects, particularly those related to the new nZEB standards and of group housing schemes. This paper reports on the post-retrofit performance of a community of 12 single story, one bedroom social houses located in the southeast of Ireland, occupied by retired and elderly tenants. The deep retrofit works included the upgrade of the building fabric, ventilation and heating, all with a view to transforming the living standards of the occupants. They in turn responded, when surveyed, with near unanimous satisfaction. The upgrade and the addition of onsite microgeneration ensured these houses were transformed from lowly F and G national building energy ratings (BER) to A rated homes as calculated by the national energy rating software, DEAP. However, a performance gap is reported between the expected A performance (<75 kWh/m2 /yr) and the actual performance, with some homes consuming more than twice the predicted energy, while in one extreme case the mean winter indoor temperatures are more than 7C above the operating temperatures assumed by the DEAP software. The higher than expected indoor temperatures are directly correlated with the higher than expected energy consumption, consumed by a heat pump which in itself exhibited inefficiencies in operation. This post occupancy evaluation, post retrofit, provides evidence therefore of high occupant satisfaction, but a satisfaction based on significant energy ‘underperformance’. This case study provides evidence and insights that can help guide future retrofit practices as Ireland progresses towards the transformation of its building stock to near Zero Energy Buildings (nZEB). It also opens up serious questions around the aims and expectations of our national nZEB standard and generates some key insights into the energy consumption of nZEB dwellings and the assessment methods necessary to measure them accurately.