Skip to main content

Meeting Part L: Conservation of Fuel and Power

Oak Frame Passivhaus Self Build
This high-spec eco home on the Yorkshire coast is the first oak frame Passivhaus-certified home in the UK (Image credit: Dave Burton/Joseph Holder)

Building a new home comes with some extra energy efficiency requirements under Part L1A of the Building Regulations (which looks at the conservation of fuel and power), compared to extending an existing home or converting an existing building.

Here we look at the assessments and certifications that are required to ensure compliance with the regulations.

What is a SAP Test?

New dwellings have to have a Standard Assessment Procedure (SAP) test (the government’s chosen assessment method for measuring energy efficiency in homes) which looks at the overall energy efficiency (once at design stage and once as-built).

Essentially it’s a pass/fail to allow Building Control to see if your specification will meet Part L, with a score out of 100.

It’s tempting to think that you can simply apply the maximum U values to the separate elements of your new build including walls, roofs and windows and assume that the completed house will comply with the Regs. Instead, you have to examine the house more holistically. For that, SAP calculations are used.

During the testing process, two requirements must be met:

This is the proposed maximum space heating and cooling energy demand. This is the amount of energy which would normally be needed to maintain comfortable internal temperatures. Since this is influenced by the external fabric U values, for outside walls, floors, roofs, windows and doors, this is all about thermal insulation to the external envelope of the building.

The FEES must not exceed the Target Fabric Energy Efficiency (TFEE), which is based on a notional dwelling of the same dimensions with maximum permitted U values.

This is much more holistic. It looks at energy use per unit of floor area, a fuel-cost-based energy efficiency rating (the SAP rating) and emissions of CO2 (the Environmental Impact Rating). These indicators of performance are based on estimates of annual energy consumption for the provision of space heating, domestic hot water, lighting and ventilation. Other SAP outputs include the estimate of appliance energy use and the potential for overheating in summer with solar gains and the resultant demand for cooling the home. 

The DER must not exceed the TER (Target Energy Rating) which is based on a notional dwelling of the same dimensions, etc.

How to Meet Part L1A of the Building Regs for New Builds

Follow these step-by-step guidelines for meeting Part L1A of the Building Regs for new builds:

  • Choose a local EPC assessor (find one here)
  • Provide your assessor with architectural drawings and specification in full detail
  • Include ACDs (Approved Construction Details) as part of the design if you can. These are large-scale section details of elements like window openings and eaves details. They aim to ensure airtightness and reduce cold bridging from key parts of the structure and help towards achieving compliance
  • SAP assessor inputs the plans and information and completes the design calculation before work starts
  • Designer agrees SAP input or reviews it after advice if the design does not pass. Once the design has been amended and meets requirement L1A, the design assessment and SAP calculations are submitted to building control before work starts on site
  • Construction begins; any changes from specification should be notified to the SAP assessor during the build to ensure it still remains compliant
  • Near completion, an air pressure test is carried out and as-built confirmations are provided to the SAP assessor, including declarations of the ACDs used during the build
  • Assessor produces the as-built SAP calculations and lodges an Energy Performance Certificate against the new home’s postal address
  • As-built SAP calculations and EPC submitted to building control body

What are the Regulations for Existing Buildings?

Part L1B covers renovation and extensions to existing buildings. It recognises that it is not always possible to meet new build standards. But the regulations state that if a thermal element (roof, wall or floor) is to be replaced or renovated then it must be done to Part L1A standard.

Part L1B provides more room for negotiation than L1A but the overriding requirement is still to reduce potential CO2 emissions by the same amount.

The best investment is in the fabric of the building, rather than in renewable energy technologies or energy saving devices.

SAP Calculations: Where to Start?

For new builds, SAP calculations are made at the design stage to arrive at a Predicted Energy Assessment (PEA). This provides a rating of energy performance based on the design of the house and shows the building inspector that the home will meet the requirements under Part L of the Building Regulations.

When the build is nearing completion, airtightness testing will be undertaken in order to establish the ‘as-built’ airtightness (or air leakage rate). Finally, the SAP assessor will create an EPC (energy performance certificate); an EPC is required for homes when built, sold or let.

Authorised SAP assessors are needed to produce these calculations and ultimately your completed home’s Energy Performance Certificate (EPC). As such, you will need to find a consultant and these are referred to as ‘on build’ SAP assessors. (The procedure and licensing of these assessors is different to that of the assessors who process SAP calculations for existing buildings.) You can search for a registered assessor or check the credentials of one on the EPC Register.

The calculation process has to take place early in the design stage, so that the design is specified correctly for the thermal insulation levels and heating and hot water products.

Testing for Air Leakage

Of course the air leakage rate can only be speculated at this point. Nevertheless, an assumed value has to be entered at the design stage. Part L1A of the Building Regulations set minimum requirements for testing at a back-stop value of 10m3/hr/m2 but often the rate needed to hit the TER is much lower.

Typically, design figures of between 4 and 6m3/hr/m2 are used. These can easily be achieved with some attention to detail during the build. Either way, when the building works are complete, the real air leakage rate will be discovered by on-site testing.

All new dwellings need to be tested on completion, with two exceptions:

  • If the same builder has produced an identical construction in the last 12 months and successfully passed an airtightness test.
  • Where a high default value of 15m3/hr/m2 has been used in the SAP calculation. If you want to avoid the worry of site testing, you can choose to enter a rate of 15 for the air leakage rate. It’s the equivalent of leaving a large window open during the test and it exempts you from testing at all. In this case, you will be heavily compensating for it with super-thick insulation and other measures.

Ideally the airtightness test (carried out and certified by licensed testers) should be undertaken when the building is as close to completion as possible. As a minimum, the building envelope needs to be completed but does not need to be decorated or carpeted.

Vents are sealed or taped over to help identify areas of air (and therefore heat) leakage where gaps and holes have not been sealed correctly.

With the real air leakage rate known and a certificate issued, the figure can be entered into the as-built SAP calculations. These figures are then run again to produce evidence of compliance and the final EPC.

If the house fails due to air leakage, the test engineers may use coloured smoke and positive air pressure to reveal where the gaps are. If it fails due to a change of design during the build – perhaps a boiler was changed for a less efficient one or the glazing areas were increased – the remediation may be harder to achieve. It may result in the need for some solar-power installation or heat recovery ventilation. Armed with heat recovery to steal back the heat from stale air before it’s exhausted out, these heat recovery ventilation systems can be essential in achieving compliance.

What is an Energy Performance Certificate (EPC)?

The EPC summarises the energy efficiency of your home using a sliding/rating scale.

The scale is colour coded and alphabetised, running from A to G. A (dark green) is highly efficient; G (red) is low efficiency. 

New build homes today mainly fall in A or B categories.

Once your new home is complete, a full EPC and recommendation report will be needed for present and future owners. As always, your building control bodies should be sent a copy.

An EPC also takes into account the price of the fuel used (per kWh) as well as the efficiency of the heating system. Electricity is much more expensive per unit than mains gas, which is one of the cheapest forms of fuel. For example, if a home has a mains gas boiler, it will cost less to run than an electric boiler or electric storage heaters.

The Energy Efficiency column informs the consumer about the heating system purely from a cost perspective. The ratings in the table will vary depending on the fuel used and the efficiency of the heating system. Even though an electric heating system may be 100% efficient at the point of use, turning all the electricity used into useful heat, it will still be more expensive to run than a 65% efficient mains gas boiler. This is something to consider if you are considering underfloor heating choices.

EPCs come with a recommendation report with suggested improvements to save money and energy. For new build homes, this bit is usually short, with suggested improvements such as solar panels.

If you are selling a home before it has been completely built, bear in mind that you will need to provide information about energy efficiency done in the Predicted Energy Assessment (PEA) carried out at the design stage.

Paul is a building control officer and has written eight books on home improvements and building homes.