Starting point: an inefficient house
To illustrate what is required we shall start with an uninsulated house with single glazed windows, electric radiators, an electric immersion hot water cylinder and standard lighting throughout.
To comply with Part L1A every new dwelling must pass a SAP calculation. SAP is the Governments approved methodology for assessing the energy performance of a house, with calculated results expressed in terms of CO2 emissions. Unsurprisingly this house fails to achieve compliance with the calculated Dwelling CO2 Emission Rate (DER) being 327% above the Target CO2 Emission Rate (TER).
What is required to pass L1A?
Approved Document L1A and its supporting document, the Domestic Building Services Compliance Guide, list the minimum acceptable standards that all elements that go into constructing a house must achieve. We shall call these the “L1A minimum standards”. But as you will see, although it is acceptable to have one or two elements built to these minimum standards, a house built with all elements set to the L1A minimum standards will fail a SAP calculation with a DER much higher than the TER.
To provide clarity to designers and builders, Approved Document L1A lists a full specification for a new house that will achieve a pass with a DER below the TER. This specification will achieve a pass with a modern gas boiler central heating system and without the need to add renewable technology, such as solar PV panels. We shall call this the “L1A recommended specification”.
London Plan energy requirements
Developments built within one of the London boroughs may have to demonstrate that they will meet the energy performance standards of the London Plan. Such developments will need to achieve a DER that is at least 35% below the TER.
As a point of illustration we shall list out a specification for this example dwelling that will meet the energy standards of London Plan.
Insulation
Why is insulation important?
The main component of a Dwelling CO2 Emission Rate is primary heat demand. Anything that can reduce heat demand within a house will have a significant impact on the DER.
How much insulation is required?
The degree to which an element is insulated is quantified by its U-value. The lower the U-value, the better the level of insulation.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Floor U-value (W/m2K) | 1.00 | 0.25 | 0.13 | 0.10 |
| External wall U-value (W/m2K) | 1.60 | 0.30 | 0.18 | 0.15 |
| Party wall U-value (W/m2K) | 0.50 | 0.20 | 0.00 | 0.00 |
| Roof U-value (W/m2K) | 2.30 | 0.20 | 0.13 | 0.13 |
Windows and doors
Replacing draughty single-glazed windows and doors with draught-sealed highly-efficient double-glazed windows will result in a significant reduction in heating demand within the dwelling, and consequently a significant reduction in the calculated Dwelling CO2 Emission Rate.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Window U-value (W/m2K) | 4.80 | 2.00 | 1.40 | 1.30 |
| Door U-value (W/m2K) | 3.90 | 2.00 | 1.00 | 0.90 |
Thermal bridging
Thermal bridging is the heat loss that occurs where one element joins another. For example, where a wall meets a floor, or around windows and doors. For highly insulated houses the proportion of heat loss that occurs at these junctions can be significant.
To reduce the effect of thermal bridging attention needs to be given to how insulation is installed along these junctions. To assist designers and builders the Government has published a set of “Accredited Construction Details” that can be followed for most common construction methods.
In addition to the Accredited Construction Details many manufacturers of building components (for example insulation manufacturers or block manufacturers) have published their own set of construction details that can be followed to mitigate against the effects of thermal bridging.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Thermal bridging | No mitigation of effects of thermal bridging. (Default: y=0.15) | No mitigation of effects of thermal bridging. (Default: y=0.15) | Mitigation of effects of thermal bridging, for example: Approved Construction Details | Further reduction of effects of thermal bridging, for example: Enhanced Construction Details |
Air tightness (air permeability rate)
Having reduced heat loss by adding insulation, installing highly efficient windows and doors, and reducing the effects of thermal bridging, the final source of uncontrolled heat loss to tackle is via gaps and cracks in the construction (cold draughts).
The amount of heat loss that occurs due to cold draughts is quantified as the air permeability rate for the dwelling. This will be measured via an air leakage test, which will be carried out at the very end of the construction process. The lower the air permeability rate, the lower the level of heat loss via cold draughts.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Air permeability rate (m3/h.m2) | Draughty house: no quantification of air permeability rate | 10.00 | 5.00 | 3.00 |
Controlled ventilation
With uncontrolled ventilation (cold draughts) kept to a minimum it is very important that adequate levels of controlled ventilation are provided. Naturally ventilated houses will most often be fitted with openable windows, window trickle vents and intermittent mechanical extractor fans to extract moist stale air from kitchens and bathrooms.
Although this type of ventilation will work very well for most houses, the drawback lies in the fact that the air that is expelled will have been heated within the dwelling. The fresh, cold air, that comes in as replacement will then have to be heated again in order to maintain a comfortable indoor temperature. So the greater the level of natural ventilation, the greater the level of heating demand, and the greater the impact on the calculated Dwelling CO2 Emission Rate.
For improved efficiency mechanical ventilation with heat recovery (MVHR) can be installed. MVHR systems will mechanically draw cool fresh air into a house at the same time as extracting warm stale air. The efficiency lies in the fact that heat from the warm air that is being extracted is passed to the cool air that is being drawn in, thus reducing the requirement to reheat the supply air.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Method of ventilation | Natural ventilation and intermittent extractor fans | Natural ventilation and intermittent extractor fans | Natural ventilation and intermittent extractor fans | Mechanical ventilation with heat recovery |
Space heating
Having reduced the amount heating will be required we next need to consider what type of system is used to provide the heat within the dwelling. The efficiency of the heating system and the fuel it uses will have a significant impact on the calculation of the Dwelling CO2 Emission Rate.
As grid supplied electricity has a fuel CO2 conversion factor that is more than twice that of natural gas, any electric heating system will need to be at least twice as efficient as a gas heating system to achieve the same Dwelling CO2 Emission Rate.
Although electric heating does meet the minimum standards of Approved Document L1A, switching from an electric heating system to a modern gas central heating system will result in a significantly lower DER.
Houses with a floor area less than 150m2 need only be controlled using a programmer, room thermostat and TRVs. However, installing zoned heating controls can result in a significant reduction in the DER. Zoned heating controls have a function that enables at least two separate parts of the dwelling to be heated at different times and to different temperatures.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Central heating system | Electric radiators | Gas boiler (88% efficient) | Gas boiler (89.5% efficient) | Gas boiler (89.5% efficient) |
| Heating system controls | Appliance thermostat and controls | Programmer, thermostat and TRVs | Time and temperature zone controls | Time and temperature zone controls |
| Advanced controls | None | None | Weather compensation controls | Weather compensation controls |
Domestic hot water heating
The second largest contributor to a Dwelling CO2 Emission Rate is domestic hot water demand. Similar to space heating, the efficiency of the domestic hot water heating system and the fuel it uses are key to minimising the impact on the DER calculation.
Although electric domestic hot water heating does meet the minimum standards of Approved Document L1A, switching from an electric system to a modern gas DHW heating system will result in a significantly lower DER.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| DHW system | Electric immersion cylinder with loose jacket insulation | Gas combi boiler (88% efficient) | Gas combi boiler (89.5% efficient) | Gas combi boiler (89.5% efficient) |
Lighting
The minimum standard of Part L1A is for 75% of all lights within the house to be low energy. To be deemed low energy the light must have an efficiency greater than 45 lamp-lumens per circuit Watt. Most modern lighting will fall within this category.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Proportion of low energy lighting | 0% | 75% | 100% | 100% |
Renewables
Renewable technology systems are systems that generate heat or electricity on site. For example, a solar thermal system will generate heat for domestic hot water heating from a renewable source (the sun), and a photovoltaic system will capture energy from the sun and transform this into electricity.
There is no requirement to include renewable technology in order to pass a SAP calculation, but doing so would allow greater flexibility elsewhere in the design.
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| Renewable technology | None | None | None | 0.75 kWp PV system (3 panels). |
Results
| Inefficient house | L1A minimum standards | L1A recommended specification | London Plan | |
| DER (kg/CO2/m2/year) | 108.60 | 27.45 | 17.72 | 11.14 |
| TER (kg/CO2/m2/year) | 25.45 | 17.71 | 17.80 | 17.80 |
| Variance | Fail: 326.8% | Fail: 55.0% | Pass: 0.5% | Pass: 37.4% |
As you would expect, an uninsulated and energy inefficient house will fail to meet the standard of the current Building Regulations by a substantial margin (326.8%). What is more surprising is that a house built to all of the minimum standards of Part L1A will also fail to comply with a DER 55% above the TER in the example shown.
Houses designed and built to the L1A recommended specification should achieve a SAP pass with a DER less than the TER. However, there are many variables that feed into the calculation of a Dwelling CO2 Emission Rate. For the greatest level of design-flexibility and to have confidence your project will achieve full compliance, a qualified and experienced energy consultancy such as Energytest should be engaged from the outset.
ACCREDITATIONS