SAP (which stands for Standard Assessment Procedure) is the method we use to demonstrate a new build dwelling’s compliance with Part L of the UK Building Regulations 2021 – Conservation of Fuel and Power.
In SAP, the performance of the dwelling we are assessing is measured against the performance of a ‘Notional’ dwelling. The notional dwelling is the same size and shape as the dwelling we are assessing but built to the what’s called the ‘Notional Dwelling Specification’ as detailed on Table 1.1, Page 12 of the Part L 2021 document.
As long as the performance of the actual dwelling is equal to, or better than, that of the notional dwelling, the building will pass SAP, that is to say that it is compliant with Part L. A summarised version of the notional specification is reproduced below.
*Correction factor for angle applies
**See further explanation in section 4
There are three separate criteria within SAP that the actual model needs to meet to be considered compliant.
DFEE/TFEE The Dwelling Fabric Energy Efficiency vs Target Energy Efficiency, this is the energy efficiency of the actual dwelling’s fabric (it’s external thermal elements as described in section 4) versus that of the notional model. This is measured in kilowatt hours per square meter of floor area per year (kWh/m²/yr).
DER/TER Dwelling Emission Rate vs Target Emission Rate, this is the carbon dioxide emissions of the actual dwelling versus that of the notional model. This is measured in kilogrammes per year per square meter of floor area (kgCO2/yr/m²).
DPER/TPER Dwelling Primary Energy Requirement vs Target Primary Energy Requirement, this is the primary energy requirement of the actual dwelling versus that of the notional model. Primary energy, also known as regulated energy, is the energy required for heating, hot water, lighting and ventilation. It does not include energy used for anything else e.g. cooking or other appliances. This is measured in kilowatt hours per square meter of floor area per year (kWh/m²/yr).
Fabric, in the context of SAP, means the external thermal elements of a dwelling, so floors, walls, roofs, windows and doors. The performance of these elements are measured as u-values which are expressed as W(m².K).
The u-values you should be aiming for in your dwelling should be those of the notional model. If any element falls short of that level, it will need to be made up for elsewhere otherwise the dwelling is unlikely to achieve compliance.
The notional specification is not to be confused with the ‘Limiting U-Values’ on Table 4.1, Page 24 of the Part L 2021 document. If a new dwelling was built entirely to the limiting u-values, it would not comply with Part L.
The ground floor of a dwelling should ideally have a u-value of 0.13W(m².K), for a block and beam or ground bearing concrete slab floor, this could be achieved with 150mm PIR (Celotex or similar) insulation. Other types of floor need to meet the same u-value for example an exposed floor over an unheated space such as a bedroom over a garage.
An external wall should ideally have a u-value of 0.18W(m².K), so for masonry cavity wall construction with a lightweight block inner leaf this could be achieved with 150mm high performance cavity batts. Alternatively, if a 100mm cavity is preferred, 90mm PIR (Celotex or similar) cavity insulation could be used. A wall that separates heated space from unheated internal space is also considered an external wall, for example walls separating flats from unheated corridors or stairwells.
A roof should ideally have a u-value of 0.11W(m².K), so for a flat ceiling insulated at joist level (cold roof) this can be achieved with 400mm mineral wool or, for a roof insulated at rafter level (warm roof) 150mm PIR insulation between rafters with 60mm PIR insulation fixed beneath. A flat roof will need around 180mm of PIR insulation on top of the joists.
Solid or part glazed doors should ideally have a u-value of 1.0W(m².K) and windows and fully glazed doors should ideally have a u-value of 1.2W(m².K). Good, modern UPVC doors and windows can achieve this sort of performance. It’s also important to consider g-value, this is the amount of solar radiation that can pass through the glass. Low g-value helps prevent overheating but also restricts solar gain which has a negative effect on the SAP results, the g-value in the notional model is 0.63 so anything lower than this will make it harder to comply.
For Part L 2021 there has been a major change affecting the way that thermal bridging is assessed withing SAP. The government’s Accredited Construction Details have been deemed too inaccurate so it’s no longer permissible to use them.
Linear thermal bridges occur where thermal elements meet, for example floor meets wall or wall meets roof. These junctions are identified and measured as part of the SAP assessment. The performance of a thermal bridge is described as a psi value (the Greek letter Ѱ). There are default psi values within the assessment procedure however, these values are very conservative and it’s not possible to get to a compliant level of fabric energy efficiency if these are used exclusively. It’s therefore necessary to do one of the following.
The first option has significant costs associated, it costs around £300 to model a junction and there are usually at least 8-10 junctions in a dwelling, quite a few more if it’s complex, so we usually steer people towards the second option wherever possible. Further guidance on thermal bridging is available on request.
All new dwellings are required to undergo an air permeability test to determine its air tightness. As air is heated it expands and escapes the dwelling, therefore the less airtight the dwelling, the harder the heating system has to work to maintain its temperature. Typically a design air permeability of 5m³/(h.m²)@50pa is sufficient to meet compliance but if other fabric elements are weaker than that of the notional model it can be necessary to reduce this target. However, building regulations state that mechanical ventilation systems should be specified where the design air permeability is set lower than 5m³/(h.m²)@50pa.
All new dwellings are required to undergo an air permeability test to determine its air tightness. As air is heated it expands and escapes the dwelling, therefore the less airtight the dwelling, the harder the heating system has to work to maintain its temperature. Typically a design air permeability of 5m³/(h.m²)@50pa is sufficient to meet compliance but if other fabric elements are weaker than that of the notional model it can be necessary to reduce this target. However, building regulations state that mechanical ventilation systems should be specified where the design air permeability is set lower than 5m³/(h.m²)@50pa.
In the vast majority of cases, it’s not necessary to install anything other than normal intermittent extractors to meet compliance. In terms of SAP results and EPC rating there is no benefit in specifying mechanical ventilation unless the intention is to reach a very high level of air tightness.
The easiest way to meet compliance with the regulations is to specify an air source heat pump, the regulations have been set out in a way to encourage this. It is still possible to meet compliance with a gas boiler, however a very significant number of photovoltaic panels (PV) are required to offset the associated CO2 emissions. If the dwelling is not connected to a mains gas supply, an air source heat pump is usually the only option as the amount of PV required to meet compliance with an oil or LPG boiler is much higher and not usually feasible.
In the vast majority of cases, where an air source heat pump is specified, it is not necessary to install PV to meet compliance, but there is quite a variance in efficiency between the various makes and models of heat pumps so that’s not always guaranteed. In can be tricky with bungalows as the amount of PV in the notional model is based on ground floor area only, not total floor area.
The design flow temperature should be no higher than 45°C and time and temperature zone control is recommended.
For maximum efficiency the hot water cylinder should be located within the heated space of the dwelling with a cylinder stat, independent time control and fully insulated primary pipework. The heat loss factor of the cylinder should be within the specification detailed on Table 4.5, Page 32 of the Part L 2021 document. Wastewater heat recovery is not usually required where an air source heat pump is specified.
The minimum lighting efficacy for a new dwelling is 75 lumens per circuit watt (lm/W) but if the efficacy can be higher than this it will have a positive effect on SAP results and EPC rating. An efficacy of 100lm/W will usually result in an extra point on the EPC rating over the minimum.
Air conditioning obviously requires energy to run so it can have a negative effect on the SAP results and EPC rating, however as long as an efficient unit is specified, the effect is negligible.
An EPC is produced directly from the SAP software at the completion stage. When the dwelling is approaching the final stages, the developer should contact the SAP assessor who will provide a list of the documentation required as evidence that the dwelling has been constructed in accordance with the design stage specification issued.
An EPC rating is based purely on running costs, not CO2 emissions or anything else. It’s a common misconception that specifying an air source heat pump will result in a better EPC rating than a gas boiler. That is not the case, without PV a house with a gas boiler will have a better EPC rating than a house with a heat pump. That’s because the price of electricity in the UK is currently about 4 times the price of gas. The best thing you can do to improve an EPC rating is add PV. The fabric u-values that are required to meet compliance currently are already very good so there’s not much to be gained by improving them.
New for Part L 2021 is the requirement for the developer to provide photographic evidence of various items to the SAP assessor at completion stage. These photos must show the type, thickness and thermal continuity of insulation at various points as well as plant associated with heating, hot water, ventilation and renewable technologies. Further guidance on this is available on request. There are a number of smartphone/tablet apps available to assist with this process.
Buildings undergoing material change of use from non-dwellings to dwellings, are not required to meet the three performance criteria described in section 3. To achieve compliance with Part L for conversions the existing fabric must be assessed and improved where necessary in accordance with Section 11 of Part L 2021 (from page 67). An air tightness test is not mandatory and there are no targets or limits associated with emissions or energy requirements. A SAP assessment will still need to be undertaken though in order to produce a compliance report and an EPC at completion stage.
Part O was added to the building regulations as part of the 2021 update. All dwellings must be assessed to demonstrate compliance with this regulation, the intention of which is to reduce the risk of overheating in hot weather and increase occupant comfort. It’s therefore important to have a Part O assessment undertaken before construction starts to ensure that the completed dwelling is compliant. Further guidance on Part O is available on request.
As Part L has developed over the years it’s become more and more important to engage with a SAP assessor from the very early stages of a residential development project. In years gone by it was possible to carry out a compliant retrospective assessment on a completed dwelling where a design stage assessment was overlooked, but with the more stringent targets, changes to the assessment process and the requirement for photographic evidence that’s no longer the case.
We can undertake an initial assessment from the basic planning drawings to inform the architect’s final specification or to provide data for an energy and sustainability statement to support a planning application.
Once the construction phase is underway it’s important to consult with the SAP assessor if any changes at all are required to the design stage assessment. At homeenergymodelling.com we have a huge breadth of experience that we can call upon to make this complex process as smooth as possible and help developers to deliver efficient, comfortable and, most importantly, compliant new homes.
We are based in Norfolk, but SAP is a desktop assessment – no site visit is necessary, so we can offer these services anywhere in England or Wales. We are unable to provide these services in Scotland and Northern Ireland as they have their own separate regulations.