York Passivhaus Building Performance Evaluation

The York Passivhaus is a 3-bed home in York, North Yorkshire, that achieved Passivhaus certification on completion in 2015. The project aim is to evaluate the building fabric and system performance of the home seven years post-completion against design targets and initial performance tests. Areas of interest are energy consumption, ventilation and air quality, thermal comfort, airtightness and building fabric.

Looking at these in turn, fuel bills were used to explore how gas and electricity consumption had changed since occupation in 2016. Gas use was higher during the first year postcompletion in 2016 but has steadily declined since. Electricity use has remained relatively constant. The annual energy consumption in 2023 was 2467kWh for gas (20kWh/m2/year) and 1652kWh (13kWh/m2/year) for electricity, which is between 60 and 74 per cent less for gas and between 9 and 39 per cent less for electricity than the average UK house.

The mechanical ventilation heat recovery (MVHR) system was not balanced when flow rate test results were compared against commissioning figures, as extract air flow rates were higher than intake air flow rates. This meant that the system no longer satisfied Passivhaus requirements.

Air quality was monitored inside and outside of the home over 12 months. For CO2, a high level of IAQ was recorded, with an average of less than 872 ppm. CO2 levels dropped when the MVHR filters were changed coupled with the onset of warmer weather. Higher noise levels associated with the MVHR system ceased following a service. Higher levels of particulate matter (PM) were recorded at the front of the house, close to a car parking area. Three peak periods were examined to see how particulates generated externally or internally rose and fell over time. Spikes in internal PM levels were generally due to cooking or use of the woodburning stove and dissipated quickly. Elevated PM level patterns recorded outside were often mirrored inside but at a much lower level.

Twenty internal sensors monitored temperature and humidity levels. Temperatures remained constant above 15°C throughout winter with all sensors staying within a 3-4°C range, indicating a low level of thermal variation across the home. However, internal temperatures were quite low – usually under 20°C, despite the space heating system defaulting to set points of 24°C during the day and 15°C at night during the winter months. This suggests that the space heating system was undersized for the current occupancy level, as design calculations were based on higher occupancy assumptions. It was assumed at the design stage that the wood-burning stove would meet 30 per cent of the home’s heating demand when during the monitoring period it was rarely used. During warmer weather, higher temperatures were recorded across the two southwest facing first-floor bedrooms. There was no evidence of overheating when the home was occupied during warmer weather.

In general, the house is still extremely airtight with a mean permeability of 0.86 m3/(h.m2) @50Pa. However, this is a significant increase in air leakage in relative (rather than absolute) terms since certification was carried out in October 2015, where a mean permeability of 0.39m3/(h.m2) @ 50Pa was recorded. The little air leakage detected appears to come from window seals at casements, the boiler flue, plus some air movement behind plasterboard in the upstairs rooflights, and at wall-to-ceiling, or wall-to wall-junctions. The air leakage area has increased only slightly – from around 73cm2 to 104cm2. Therefore, after seven years the home now satisfies EnerPHit rather than Passivhaus airtightness requirements.

A QUB test was used to measure fabric performance. First, a design-stage heat transfer coefficient (HTC) for the home was calculated, which was 69.5 W/K and then tested against. Three tests were done in the summer/autumn of 2022 and two in the winter of 2023. The
average measurement was 76.3 W/K. This is a low HTC but 10 % greater than the designstage performance calculation.

Overall, as a seven-year-old Passivhaus, the home’s performance is still exceptional compared to current-day new-build homes. Some performance aspects have deteriorated since completion, such as the airtightness and MVHR performance, which could be associated with wear and tear. It is not possible to compare changes to air quality, thermal comfort and HTC, as they were not monitored post-completion. The only area of note is thermal comfort in winter depending on the temperature sought by occupants, as the space heating system is not designed for the current occupancy level and could be considered on the cool side of comfortable.