Chris Hocknell, Eight Associates’ Technical Director shares his views on how to evaluate and improve the quality of energy strategies at planning.
Energy strategies require careful evaluation at planning. An effective energy statement provides a roadmap for those working on the strategy post-planning approval – ultimately dictating the building’s energy performance at practical completion and in use.
What makes a good energy strategy? At its most basic, an energy strategy must achieve the Part L Building regulations 2013 baseline, follow the energy hierarchy, be appropriate to its setting and adhere to the local authority’s planning policies and management plans.
It is still common to receive planning permission without conditions that expressly state that the carbon emissions reduction in the energy strategy is demonstrated at practical completion. Yet, by the very nature of the iterative design process, specifications evolve and are refined throughout the design and construction phase. An explicit requirement to demonstrate the carbon emissions reduction retains energy conservation as a priority – and ensures, or at least tests, that the proposed energy strategies are feasible.
An effective energy strategy should be:
The operational carbon footprint of the development after each stage of the energy hierarchy should be clearly identified, including both regulated emissions and those emissions associated with uses not covered by Building Regulations.
Misunderstandings and inconsistencies are commonly found in the following sections of an energy strategy. The guidance below will help to identify these common deficiencies.
Passive design measures, such as optimising orientation and site layout, natural ventilation and lighting, thermal mass and solar shading, and active design measures, such as high efficacy lighting and efficient mechanical ventilation with heat recovery, underpin demand minimisation.
Check that consistent assumptions and benchmarks have been used across the strategy’s inputs. For example, it is common for the energy strategy and overheating report to not share the same inputs; the overheating modelling may make assumptions that limit the risk of overheating by increasing mechanical ventilation flow rates, while the energy modelling decreases mechanical ventilation flow rates to reduce carbon emissions. Similarly for refurbishments, it should be clear if the baseline is existing specifications, or Building Regulations Part LB.
Check there is enough detail about the thermal mass estimation, preferably simple table with indicative values for different structure types which specifies the ranges being used.
Thermal bridges are ‘weak spots’ in the building envelope’s insulation. Check whether an average y-value been assumed in the thermal bridging calculations. The common assumption of an average y-value of 0.08 (or even lower) comprises reducing the fabric’s thermal bridges by 50% over the default value – and this may not be feasible in practice. Thermal bridging is a function of heat loss psi-values multiplied by the linear length of the junction, so the linear lengths should always be measured, otherwise the average y-value cannot be accurately ascertained. A good energy strategy should include a simple table with junction lengths and method used.
The specified airtightness design targets should be in line with best practice, not higher or lower, to avoid air quality issues. The Energy Saving Trust recommends 5m3/hr.m2 is best practice for natural ventilation, and 3m3/hr.m2 for mechanically ventilated spaces. For example, if mechanical ventilation with heat recovery (MVHR) is specified yet the airtightness design target is greater than 3, the benefit to the development’s carbon emission reductions is negated by air leakage through the fabric, rather than through the MVHR. Similarly, the ventilation strategy must not present an air quality risk. It is common to see residential MVHR units with very low specific fan powers specified. The smaller amount of ventilation provided from lower power-consuming MVHR units might mean that the dwelling does not meet its required air change rates for air quality and overheating.
The space heating and cooling infrastructure must be accurately assessed and selected. This could include for example, connection to district heating networks or consideration of onsite combined heat and power (CHP) engines, in line with the local core strategy.
CHP oversizing – to meet carbon targets on paper – only means that in use the CHP will either not run or over generate and dump energy. As such, a load profile analysis with all basic assumptions and calculations that have determined the hot water demand should be presented. Typically, CHPs are viable on domestic schemes of more than 100 units or larger commercial schemes, such as hotels, that have large hot water requirements.
The cooling hierarchy informs the development’s cooling strategy, when the use of natural and/or mechanical ventilation is not enough to guarantee the occupant’s comfort. Details must be included of the active cooling plant being proposed, including efficiencies and the ability to take advantage of free cooling and/or renewable cooling sources. Overheating analyses should always be presented. Even a slight risk of overheating, as identified by SAP analysis, may result in a high risk when it is properly analysed; SAP is a very simplified tool based on average monthly temperatures, rather than peak temperatures.
The cooling hierarchy aims to reduce the need for air-conditioning and it should be designed out where possible. However, if the outside temperature is greater than 28 degrees, it may be necessary to specify A/C as comfortable levels cannot be achieved through ventilation levels alone.
Be aware how much glazing affects the cooling demand. As a ‘rule of thumb’, a façade with greater than 25% glazing needs close attention. The glazing solar factor and Seasonal Energy Efficiency Ratio (SEER) need to be reviewed where cooling demand for a building is high. These values have a significant impact on the results even though they are not specified in Building Regulations Part L documentation produced by software models.
Very high luminous efficacy of lighting is often specified in non-domestic settings, to lower the lighting demand. However, there is often little evidence to support the values. Where luminous efficacy exceeds 75 lumens per circuit watt, more information is needed as this will be difficult to achieve in practice, once all colour rendering and aesthetic demands are incorporated.
The use of renewable energy and renewable technologies needs to be fit for purpose, not ‘bells and whistles’ to achieve targets.
If an air source heat pump (ASHP) is specified to provide space heating, it alone cannot provide sufficiently high water temperatures for the avoidance of legionella. An additional gas or electric boiler is needed to heat the water to 60oC. While a gas-fired heat pump is a good solution to provide the required higher temperatures, be aware that their efficiency levels drop as the temperature output increases.
If the specified ASHP plays a significant role in carbon reduction improvements for the development, it’s worth comparing performance to a gas boiler, given the current carbon intensity of electricity. In some commercial settings however, the small heating/DHW demand tends to result in negligible differences between gas and electric ASHP.
Check that the proposed allocation of photovoltaic systems on the roof is feasible and realistic. Moreover, PV is often specified without analysis of the shadow effect on the roof – yet even a small amount of PV panel shading reduces its output. Shading analysis, with an assessment of the height of existing buildings and any permission granted for buildings near the application site, should also be included – and is required by the GLA.
Finally, up-to-date generation/export tariffs and technology costs are useful to demonstrate that the proposal provides a reasonable payback.
The efficacy of an energy statement is only really realised once the development is complete. The interconnectedness of the multitude of complex variances needs to be balanced and maintained during the design and construction process. It is still common to receive planning permission without conditions that expressly state that the carbon emissions reduction in the energy strategy is demonstrated at practical completion. If this was an explicit requirement, it would serve the dual purpose of keeping energy conservation as a priority, and secondly making consultants accountable for their energy strategy assumptions and inputs, in particular how feasible they are in practice. Perhaps such an explicit requirement would, more than any other single action, improve the quality of energy strategies submitted at planning.