This week we popped in to one of the homes that we retrofitted in 2010/11 as part of a Government-funded project, Retrofit for the Future, to find out how it was performing.
The headline finding is that the house is now using 9% of the energy it previously used for space and water heating; down from 12493kWh to 1133kWh. Overall energy use has been reduced by 75%, with the carbon emissions from the remaining use offset by a cost-effective investment in off-site renewables.
We visited after one year and found that actual energy use was 47% less than that predicted by SAP. There has been a further significant fall since then. Over the intervening years, the average energy use has been 30% of the use originally predicted for space and water heating, ventilation and lighting; and total average use has been 50% of use in that first year.
We expect heat usage to remain steady at this lower level (for the current occupancy patterns) as the fall can be attributed to one-off factors in that first year:
the building was drying out
the thermal mass had no heat stored as the build completed in early autumn
the winter of 2010/11 was particularly cold
Use of energy for appliances and cooking remains the largest influence on energy use, forming two-thirds of annual use on average. As highlighted in previous posts this is very dependent on working patterns and the number of residents. The final 3% of energy use is by the metering system itself – with metering on 8 rings in the house to enable this analysis.
Last, but certainly not least, was the residents’ feedback. The most notable problem was a rain sensor on the automated Velux windows in the sunspace, but that has been repaired quickly enough, and the occupants continue to enjoy the comfort of their old but cheaper-to-heat home.
Have we ever mentioned how thermal mass keeps our homes warm in winter and cool in winter?
No doubt if you have visited us you’ve heard the stats and felt the benefit, but a day like today makes the benefits all the more evident.
The thermal comfort of our homes is met through the application of three key design principles:
Thermal mass to store heat in the summer months to keep the home cool in summer and warm in winter
Passive solar gain to reduce the need for space heating and artificial lighting
Super-insulation and buffer zones to provide a reduced temperature gradient between the inside and outside of homes.
So on a day like today we shut the triple-glazing between the living space and buffer zones, along with curtains and shutters if we are out and about, to keep out warm air and solar gain; and let the thermal mass soak up the heat when we are around the house. We have built passive ventilation into our buffer zones – otherwise known as skylights in the conservatory and porch area – which are vital to keeping the temperatures in those spaces comfortable.
Given the warnings yesterday from the Committee on Climate Change, we think cooling, or overheating, is an issue that needs addressing as part of the government’s home energy strategy. It should not be an add-on as there can be a conflict between approaches that keep heat in during winter and keep it out over summer.
The positioning of insulation in a construction element was completely disregarded by SAP up to and including SAP2005, and continues to be ignored by RdSAP. But it is essential if you want to get the heat storage benefits of thermal mass throughout the year for cooling, heat storage & release. Our walls, floor and roof could have the insulation placed on the inside, which would give exactly the same U-Values and hence RdSAP result, but completely different and appalling thermal performance of the house as a whole in warmer weather. Instead of being absorbed into the thermal mass, the passive solar gain would continue to raise the temperature until vented in some way.
Even though this is beginning to be recorded by SAP, RdSAP does still not differentiate between internal and external solid wall insulation. Neither assessment reflects the benefits in their overall assessment of thermal comfort. If the Government wants to prepare homes for the 21st century, and beyond, these tools will need to both recognise and reward the way external insulation can“lock in” the mass of the walls to deliver summer cooling and winter heating.
If this property’s share in a community-owned wind turbine were taken into account, it would have the lowest energy use and CO2 emissions of all properties in the recent Retrofit Revealedreport.
Two of HHP’s retrofit projects were included in a recent report by the Technology Strategy Board, and whilst the published results look good, they are not the full story due to the role played in our design by off-site renewable energy.
The Technology Strategy Board (TSB) funded the Retrofit for the Future competition to encourage innovation in the retrofit market and understand what actually works. 87 projects were awarded up to £150k each to retrofit social housing units, aiming to achieve an 80% reduction in CO2 levels compared to 1990 averages. HHP won funding for a project to retrofit 2 semi-detached houses in Newark, Notts, which have now been re-occupied for over 2 years.
The TSB has recently produced a report, Retrofit Revealed, providing the first analysis of data from the monitoring of 37 of the projects.
As we had split the (not inconsiderable) budget between two properties, we were pleased to see that one of our retrofitted houses (property number TSB023) still had the 8th lowest level of CO2/m2 (3rd best of the all-electric properties) whilst the other (property number TSB022) was a credible 26th. In terms of total energy use (per m2), our properties were 4th and 12th respectively. This shows the impact of being an all-electric property, as electricity has a much higher carbon intensity than gas; and the impact of resident behaviour, as the houses are built and retrofitted to identical specifications.
Our choice of going ‘all-electric’ was deliberate: it is not a finite resource like gas; and because our design off-set that electricity use through investment (from the project budget) in a local community-owned wind turbine.
The impact of this investment is not recognised by the TSB report but it has proven much more cost-effective and a lower maintenance approach than on-site renewables. Analysis of the energy data for Property TSB023, for which we have 2 years of meter readings, shows that if its share of SHOCK turbine generation were taken into account, it would have the lowest energy use and CO2 emissions of all properties. A £1,500 investment offset 43% of the annual energy use, and at the same time the social landlord has a regular income rather than a maintenance overhead.
This offsite offset would not be recognised in the properties’ Energy Performance Certificates (EPC) either. This matters because the Government said in their Energy Efficiency Strategy that it intends to make more policies conditional on energy efficiency. Onsite renewables would be recognised, but what about all those unable to install systems onsite due to property type, leasehold or planning restrictions? Or simply unable to afford an onsite system at higher upfront cost per kW?
Further key aspects of our design (passive solar gain, high thermal mass and buffer zones) are not fully recognised by SAP, the Government’s assessment tool, and so similarly the benefits would not be fully registered in the EPC.
Here’s hoping that TSB take a technology-neutral look at the results and feedback into SAP what really works for different properties, and their residents.
Yesterday the Government launched its Energy Efficiency Strategy, and we were particularly interested to see if any mention was given to the role of SAP, the Government’s energy performance assessment tool.
Hockerton Housing Project takes pride in its low-tech, low-cost approach, and the homes consume 15-25% of the energy used by homes built today. But the Government’s energy efficiency measurement assessment tools (SAP and RdSAP) cannot compute the benefit of homes like ours:
SAP cannot cater for our use of passive solar gain as our main heating system
Use of thermal mass to reduce heat demand through the year is not recognised (the thermal mass in our homes essentially stores the summer heat and keeps our homes warm in winter)
SAP assumes that an element of mass thicker than 100mm has no additional thermal capacity is flawed (SAP2009 Table 1e: Heat capacities for some common constructions), contrary to evidence at HHP. As long as the mass is well insulated (externally) the full thickness of the mass will be effective as a heat sink.
RdSAP does not differentiate between internal and external solid wall insulation, so the benefits of external insulation to “lock in” the mass of the walls, which can then aid summer cooling and winter heating, are not recognised for existing dwellings.
SAP assumes that thermally separate conservatories are not present, ignoring two benefits:
The sunspace provides sheltering of the dwelling from the external environment, therefore reducing heat losses.
The sunspace can be used to harvest passive solar energy which can then be brought into the main dwelling to top-up the heat stored in the thermal mass as required.
All this matters because the Government tells us in the Strategy that it intends to make more policies conditional on energy efficiency. Access to feed-in tariffs and the renewable heat incentive are already affected, and RdSAP or EPC ratings could also be used to introduce differential council tax or stamp duty. All this will mean that energy efficiency improvements will be made to meet whatever measure of energy efficiency is applied. Whilst a policy to drive up the value of energy efficiency in the property market would be very welcome, as this is potentially the simplest way to drive investment in existing homes, this must not be so broad-brush as to drive out innovative approaches and a process for ‘exceptions-handling’ must be incorporated into future policies.
Seeing is believing
On the upside, whatever documents come out of Westminster, here at Hockerton we’re enjoying ‘zero’ energy bills as our investment in additional solar PV starts to pay off and the summer heat stored in our thermal mass continues to keep our homes warm.
If you are interested in homes that are comfortable yet consume only 15-25% of the energy used by homes built today, this time of year is the best time to visit to truly feel the difference. There are some spaces left on the tour this coming Saturday 17 November so book your place on a tour of Hockerton Housing Project here.