The Department for Energy Security and Net Zero (DESNZ) has officially unveiled the new RdSAP 10 methodology, set to roll out on June 15, 2025. This release represents a major update from the previous version, RdSAP 9.94.
RdSAP 10 reflects the government’s ongoing drive to improve the UK’s housing stock—among the least energy-efficient in Europe—by providing more accurate and detailed assessments. These updates aim to support national goals such as reducing carbon emissions, tackling fuel poverty, and creating warmer, more comfortable homes.
Here’s a summary of some key RdSAP 10 changes that every Domestic Energy Assessor (DEA) should be aware of.
Date Built
Identifying the construction year of a property is often one of the initial steps in any energy assessment. With RdSAP 10, a new Age Band M has been introduced to enhance accuracy in categorising modern homes. This band applies to:
- Properties built in 2023 onwards in England, Wales, and Northern Ireland
- Properties built in 2024 onwards in Scotland
This update ensures that newly constructed homes are more precisely categorised right from the start.
Walls
Alternative Wall
RdSAP 10 allows assessors to select two alternative walls per building part. This is particularly helpful for assessing flats, which may have different wall exposures (e.g., one sheltered, one exposed). Previously, this required splitting the property into extensions. Now, assessors can define these directly within the software, streamlining the process and saving time.
Wall Insulation
Insulation options have been enhanced in RdSAP 10 to offer greater precision. Previously, selections were generally in 50mm increments, but RdSAP 10 introduces 25mm increments, plus 10mm/12mm as a minimum value. Even more usefully, if the insulation thickness doesn’t align with a preset option, assessors can now input the exact measured value.
Room in Roof
RdSAP 10 introduces two distinct Room in Roof (RiR) types to improve assessment accuracy.
Type 1 Room in Roof – True Room in Roof
Type 1 is a true room in roof. It is a room which is fully within the roof space. You will not see the actual common wall within the room as this will either be below foot level (therefore non-existent) or behind the stud walls.
This type is fully within the roof space and treated as a room in roof regardless of stud wall height. When using the ‘Simplified’ assessment, assessors will need to record:
- Area of the space
- Gable wall details (length, type, whether or not the U-Value is known)
Additionally, if applicable, they will also have the option to record:
- party wall length
- sheltered wall length
- connected wall length
- additional gable wall length
Type 2 Room in Roof – With Common Wall
This version involves a continuous external common wall from the floor below. To qualify as a room in roof (and not a separate storey), at least 50% of the common wall must be under 1.8m high.
For ‘simplified assessments’, details include:
- Area of the space
- Gable wall length and height
- Gable type and whether or not U-Values are known
- Common wall lengths and heights
It should be noted that assessors will still have the option to select the ‘detailed assessment’ option for roof rooms when differing levels of insulation have been identified at the various junctures.
These new roof room options in RdSAP 10 may take some getting used to for assessors, as they involve a few extra measurements and more detailed data entry. However, they significantly improve the accuracy of EPCs. By capturing more precise details, assessors can now ensure that roof rooms are assessed correctly – something that can have a notable impact on the final EPC rating.
Windows
One of the most significant changes in RdSAP 10 is the updated approach to window data collection.
In previous versions, only atypical glazing required full measurement. With RdSAP 10, every window must be measured individually. For each window, assessors are required to record:
- Length and height (or total area)
- Orientation
- Frame type (e.g., PVC)
- Glazing type (single, double, triple, secondary)
- Glazing gap (6mm/12mm/16mm)
- Age of glazing
- Draught proofing
- Presence of shutters
- Location within the property
This change will likely have the greatest impact on data collection time, requiring DEAs to be more thorough during site visits.
Ventilation
Ventilation entries are also expanding. Previously, assessors only needed to record open fireplaces. RdSAP 10 introduces several new categories:
- Blocked chimneys
- Open flues
- Closed fire flues/chimneys
- Solid fuel boiler flues
- Flues for other heaters
- Extract fans
- Passive stack vents
- Flueless gas fires
For each category, assessors are required to count the units present within the dwelling. This approach enhances the accuracy of calculations, as it allows for a clearer understanding of the ventilation requirements.
Air Pressure Testing
Air pressure tests are becoming more commonplace. Not only are they now a requirement of buildings regulations, they are often completed alongside whole house retrofit projects at the start and end of the process. With more homes having air pressure tests done, RdSAP10 includes the option to enter the results. If assessors choose to include this figure, they will need to support this entry with documentary evidence.
Hot Water Tank Size
RdSAP 10 introduces greater precision in assessing hot water cylinders. Previously, assessors were limited to selecting a general size category – normal, medium, or large.
Now, they can enter the exact cylinder capacity directly into the software resulting in more accurate calculations of hot water losses. Additionally, assessors can input the specific heat loss value (kWh/day) for a cylinder, typically found on the data plate. When this value is available, it overrides all other hot water cylinder data inputs, streamlining the process and improving accuracy – particularly for modern encapsulated cylinders where this information is commonly provided.
Lighting
In RdSAP 10, lighting data entry has been expanded. Assessors now need to record four pieces of information:
- Total number of bulbs
- The number of LEDs
- The number of CFLs
- The number of incandescent bulbs
This change accounts for the significant difference in efficacy between bulb types, allowing for more accurate lighting energy calculations.
In Summary
The move to RdSAP 10 is a significant one for Domestic Energy Assessors. It provides more detailed and accurate options for assessing a property’s energy performance. While these updates mean assessments will take longer, the enhanced detail aims to produce more accurate and actionable EPCs, better reflecting each home’s true efficiency.
RdSAP 10 introduces several other moderate updates that are important for assessors to be aware of. While the key changes highlighted above are among the most significant, there are additional updates that will also impact the assessment process. We will be reviewing these further in upcoming posts to ensure that all Domestic Energy Assessors are fully informed and prepared for the transition.
DEAs should take time to familiarise themselves with the RdSAP 10 changes to stay compliant and confident in their reporting. Energy Trust will be providing further guidance and resources to ensure that our DEA learners are fully equipped to comply with RdSAP 10 when it comes into effect. Be sure to follow our social media to keep up to date with our new releases.
