SAP Calculations by Xpert Energy

SAP is essential to ensuring homes are energy-efficient, comfortable, and legally compliant. Whether you’re building from scratch, extending, or converting, early engagement with SAP assessors helps prevent costly delays and ensures your project meets Part L requirements.

Xpert Energy can produce Standard Assessment Procedure calculations for your new build, extension or conversion project. Our team includes approved On Construction Domestic Energy Assessors (OCDEAs) who are accredited and registered with Elmhurst Energy.

Our SAP Assessor reviews your plans and collaborates closely with the architect to ensure the specification meets Part L compliance, delivers thermal comfort, and remains cost-effective.

For information on overheating, click here.

SAP and Its Role in Part L Compliance

SAP, or the Standard Assessment Procedure, is the UK government’s approved methodology for assessing the energy performance of dwellings. It plays a vital role in demonstrating compliance with Part L of the Building Regulations, which focuses on the conservation of fuel and power.

What is SAP?

SAP calculations evaluate a dwelling’s predicted energy use based on key factors including:

• Construction + levels of insulation
• Glazing
• Heating and hot water system
• Air leakage and ventilation
• Thermal Bridging
• Renewable technologies (e.g. solar PV, heat pumps)
• Lighting efficiencies

The result is a SAP Rating, scored between 1 and 100+. A SAP Rating enables buyers and homeowners to compare the energy performance of individual properties. A score of 100 or above indicates zero energy costs, so anything over means you are exporting your surplus micro generated energy.

SAP for New Builds

For all new build dwellings, SAP is essential from the design stage. A ‘Design SAP’ is submitted before work begins to demonstrate that the dwelling will meet or exceed the required Target Emission Rate (TER), Target Primary Energy Rate (TPER) and Target Fabric Energy Efficiency (TFEE). These targets are defined by Part L1A of the Building Regulations.

Once construction is completed, an ‘As-Built SAP’ is submitted — updated with any changes and air pressure test results — and is used to generate the final EPC, which is required for building control sign-off. Building control also require minimum compliance for each fabric of the building, along with air tightness, heating solutions and low energy lighting.

SAP for Extensions

Extensions fall under Part L1B, which governs work to existing buildings. If an extension includes significant glazing (over 25% of floor area), SAP calculations may be required to prove that the overall energy performance won’t worsen the building’s carbon footprint.

There are multiple options for demonstrating compliance including:

• Calculating area weighted u-values of the fabric of the extension
• Carrying out a whole-house SAP assessment showing no overall detriment – this allows improvements to the existing house such as insulation or heating system to compensate for the excessive glazing.

SAP for Conversions and Change of Use

When converting a building into dwellings (e.g. a barn or office to flats), SAP assessments are also required under Part L1B.

Each resulting unit must:

• Have its own SAP rating and EPC
• Meet minimum U-values for upgraded elements
• Meet current efficiency standards for heating systems

Conversions can often involve both existing fabric retention and new elements, so careful consideration is essential.

Part O – Overheating

Part O of the Building Regulations, introduced in June 2022, focuses on the mitigation of overheating in new residential dwellings including:

• Homes
• Blocks of flats
• Care homes
• Student accommodation
• Residential schools

The aim is to ensure new dwellings are designed and constructed with a reduced risk of high indoor temperatures – particularly during summer months – without relying on mechanical cooling.

Mitigating the risk of overheating in new dwellings is of growing importance with multiple factors contributing to overheating risk including climate change and urban heat island effect, where the concentration of buildings and infrastructure can lead to a “heat island” effect, causing temperatures to be much higher in cities compared to the surrounding areas.

Buildings may suffer from overheating due to a number of building-specific factors:

• Part L Building regs reducing heating demand with increased insulation and improved air tightness, but unintended consequence is an overheating risk
• Poor building design – excessive glazing, lack of ventilation
• Heat gains from circulating heating, DHW, solar thermal systems – particularly affecting common areas of flats

Overheating can have significant consequences:

• Discomfort: High indoor temperatures can cause discomfort, sleep disruption, and lower productivity.
• Health Hazards: In severe cases, overheating presents serious health risks, especially for vulnerable individuals.

The Key requirements of Part O are separated into 2 parts:

• Limiting unwanted solar gains in summer
• Providing adequate means of removing excess heat

These requirements must be balanced against other regulations, such as security (e.g. not requiring windows to be left open in unsafe areas) and noise limits (e.g. not relying on open windows in noisy urban areas).

How compliance is demonstrated

There are 2 main methods for demonstrating compliance with Part O:

• Simplified Method
• Dynamic Thermal Modelling (Based on CIBSE TM59)

Simplified Method

Part O of the building regulations limits solar gains by restricting the amount of glazing in a building. The simplified method calculates this by assessing the total glazing area and comparing it to the floor area. The façade with the most glazing determines the maximum allowed glazing as a percentage of the floor area, helping prevent overheating from excessive solar heat gain.

For removing excess heat, the simplified method sets minimum free areas for windows to ensure proper natural ventilation throughout the dwelling, including bedrooms. These requirements are based on the floor area of the building, helping to maintain comfortable temperatures by allowing sufficient airflow.

The Dynamic Thermal Modelling (DTM) approach provides greater design flexibility compared to the simplified method, enabling more precise predictions of thermal performance and potential overheating risks. For compliance using the DTM method, the following requirements must be met:

• The CIBSE TM59 design methodology must be followed.
• All limits outlined in TM59 (Sections 2.5 and 2.6 of Part O) must be adhered to.
• As per Sections 2.7-2.11, any strategies aimed at reducing overheating are acceptable.

DTM software is used to simulate and assess the performance of a proposed dwelling by generating temperature profiles for specific rooms over an annual period, considering factors such as ventilation, solar shading, and load profiles for each zone. These simulations use localized weather data based on the building’s site to ensure accuracy.

To demonstrate compliance for homes that are predominantly naturally ventilated, the following criteria must be met:

• Criterion 1: For living rooms, kitchens, and bedrooms, the number of hours with a temperature exceedance (dT ≥ 1K) must not exceed 3% of the occupied period between May and September.
• Criterion 2: For bedrooms specifically, the operative temperature between 10 PM and 7 AM must not exceed 26°C for more than 1% of the annual hours. (This equates to a maximum of 32 hours; 33 or more hours will result in non-compliance.)

By ensuring that these criteria are met, dynamic thermal modelling can help to optimize designs that minimize overheating risks and enhance occupant comfort.