A Building Regs’ Guide to Part L — Getting Energy Efficiency Right in New Homes
The Building Regulations have plenty to say about how energy-efficient new homes should be. Paul Hymers sets out the essentials, from the initial SAP calculations to the final Energy Performance Certificate
Building control officer Paul Hymers explains how to comply with the Regs surrounding energy efficiency when building a new home
Building a new home comes with some extra energy efficiency requirements under Part L1A of the Building Regs, compared to extending an existing home or converting an existing building. (Many such schemes also require SAP calculations; this article covers new builds only.)
It’s tempting to think that you can simply apply the maximum U values to the elements (the walls, roof, windows) of your new build and assume that the completed house will comply with the Regs. Instead, you have to examine the house more holistically. For that, SAP ( Standard Assessment Procedure) calculations – the government’s chosen assessment method for measuring energy efficiency in homes – are used. Two requirements must be met: 1 The Fabric Energy Efficiency Standard (FEES) 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 and 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. 2 The Dwelling Energy Rating (DER) is much more holistic and looks at energy use per
unit of floor area, a fuel-costbased energy efficiency rating ( the SAP rating) and emissions of CO ( the 2 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.
Starting Early is Key
For new builds, SAP calculations are made at the design stage to arrive at a Predic ted Energy Assessment (PEA), which 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 building works are complete, the real air leakage rate will be discovered by on-site testing”
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 at: epcregister.com.
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.
Of course the air leakage rate can only be speculated at this point, but 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 6 m3/hr/m2 are used and 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 only two exceptions: 1 If the same builder has produced an identical construction in the last 12 months and successfully passed an airtightness test. (This does not usually apply to self-build homes); 2 Where a high default value of 15m3/hr/m2 has been used in the SAP calculation. (If you would like to avoid the worry of site testing, you can choose to enter a rate of 15 for the air leakage rate — the equivalent of leaving a large window open during the test and hence 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 test 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 during the test — which is looking for air leakage where it shouldn’t be and not where it should. Traditionally this means gaps around the window and door openings or the gaps left from holes made for plumbing and services — all of which leak air (and consequently heat) if they aren’t sealed correctly.
Carried out and certificated by licensed testers, an inflatable door with an electric fan in it is fitted to an external doorway. Sealed airtight, the fan blows in ( creating positive pressure) or sucks out (creating negative pressure) to achieve a pres sure difference between the internal and external air of 50mb. (Because of this, the test can’t be done on windy days when the air pressure outside is unstable.) Wired to a laptop the pressure sensor measures the loss (or gain) of pressure over a period of time and hence records the air leakage rate from the house. The speed at which it loses its internal air is the figure then inserted into the SAP calculations to work out whether compliance has still been met.
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 and 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 systems
can become essential in achieving compliance.)
Energy Performance Certificates (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. And, as always, your building control bodies should be sent a copy.
One of the most criticised features of EPCS is the financial assessment of how much money it will cost to run the house. This is not surprising, given that it quotes precise figures which rarely turn out to be anything close to accurate in real life. I don’t personally see this feature as helpful because it’s based on standardised assumptions rather than reality such the number of occupants, hot water usage, etc. The EPC costs account for energy used for heating, lighting and hot water, but do not include other energy uses in the home like cooking or electrical appliances, and these factors vary appreciably between different households and, of course, the degree of environmental exposure to things like north winds make a huge difference to heating.
So my advice is to ignore this figure. Like the band rating, the cost figure is just another way of comparing homes on the market.
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. In the case of new build homes this bit is much shorter, 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.
An Energy-efficient Build This self-build in Sheffield, designed by Paul Testa Architecture, was rated a B under SAP calculations, in line with around 80% of all new builds in England and Wales.
Trying to achieve extremely low air leakage rates of 1 or 2 comes with its own problems. We are carbon-based life-forms and we need to breathe air, so living in a hermetically sealed box is not exactly healthy for us, especially when you consider the high levels of indoor air pollution from cleaning chemicals and fire-retardant furnishings, but mostly when you consider our combustion appliances for heating and cooking. Open flued or flue-less appliances like log burners and cookers can become lethal in airtight buildings where they are starved of oxygen. Extra measures, such as trickle vents, are needed for new build homes because of their inherent airtightness, and sometimes what becomes necessary is a whole building ventilation system that provides background air mechanically (see page 117 for more details).
Next month: Part H — Connecting to and building near mains drains