BS 40104 aims to clarify retrofit assessment

UK housing stock is among the worst performing in Europe in terms of energy efficiency, primarily because of its age and the slow rate of replacement.

Given that almost 60% of emissions are caused by space heating, the focus for improving homes is typically either to decrease the heat loss or change to a more efficient or clean system through retrofitting.

However, despite being one of the key measures by which the government intends to achieve net-zero carbon by 2050, the quality of domestic retrofitting is far from assured.

Regulation in the sector changed significantly with the introduction of PAS 2035 in 2019, a standard that provides guidance on and sets out a process for retrofitting residential property.

Most publicly funded domestic retrofits are covered by PAS 2035. However, the standard does not cover the assessment process in significant detail, so training and the quality of assessments vary.

With this in mind, BSI decided to create a new standard specifically for retrofit assessment: BS 40104 Retrofit assessment for domestic dwellings – Code of practice.

Due to publish later this year, the standard has been written by experts in building surveying and pathology, installation of energy-efficiency measures, conservation and heritage, building services, ventilation and design.

It remains to be seen whether aspects of BS 40104 could be used as part of the PAS 2035 retrofit process – for example, to help with harmonisation of assessment – even though the former has been developed as stand-alone guidance for anyone carrying out domestic retrofitting or pre-retrofit assessments.

1. Desktop research: before the on-site work commences, the assessor should gather data to determine their competence and understand site conditions. This involves reviewing both the regulatory and geographical contexts. Among the former considerations are planning and statutory restrictions such as conservation areas and protected buildings, and historical documentation such as evidence of previous retrofits, airtightness tests and energy performance certificate ratings. This should be complemented by understanding of ground conditions, such as flood risk and site access for construction, local environmental factors including overshading, orientation and proximity to other buildings, and potential hazards such as radon and asbestos.

2. On-site condition assessment: assessors should next inspect each building element, focusing on issues that might affect retrofit decisions including external elements such as roofs, chimneys, drainage, walls, windows and doors, as well as internal elements such as ceilings, floors, walls, fixtures and services. They should also note any concerns about damp, rot and mould, and any likely obstacles to retrofit, as well as any construction defects. BS 40104 contains a condition rating system to help risk assessment, where: 1 indicates no issues requiring action before retrofit; 2, issues that must be addressed before measures are installed; and 3, that further investigation is required before proceeding.

3. Ventilation assessment: proper ventilation is crucial to avoid issues such as condensation and mould after retrofitting. If stakeholders are not informed of a building's poor ventilation before works are carried out, those interventions could make the issue worse. Assessing ventilation involves checking the functionality of existing systems, measuring airflow rates using a vane anemometer, ensuring adequate undercuts on internal doors for air circulation between rooms, and verifying that the whole house is being ventilated appropriately.

4. Occupancy assessment: the standard is designed to ensure building occupants are involved in the retrofit process from the beginning. Because understanding occupant living patterns and needs is critical to effective retrofitting, assessors must collect data on energy usage, heating fuel types, meters and patterns, the number of residents, any vulnerable individuals, occupant satisfaction, well-being and comfort, and any planned home improvements and historical modifications.

5. Energy performance assessment: the home's current energy performance will be evaluated using standardised models such as RdSAP, SAP, PHPP or dynamic simulation software. The goal is to establish a baseline and suggest appropriate energy-efficiency measures. BS 40104 suggests using the RdSAP occupancy assessment software, which allows for non-standard occupancy or non-typical energy usage to model a more representative version of the home, giving more accurate predictions of potential energy savings.

6. Site-specific context and heritage considerations: a significant component of the assessment is identifying obstacles to retrofit on site, which may include overhangs that affect external insulation, satellite dishes and alarm boxes, loft boarding that could limit insulation options, and pipe diameters that may influence heating system choices. Properties with heritage value require additional assessments using BS 7913 Guide to the conservation of historic buildings, to ensure retrofits are sympathetic to original materials and design.

7. Data collection requirements: BS 40104 standardises collection with aim of creating a national dataset to benefit research and policy-making. The required data includes drawings such as site plans, floor-plans, elevations and material details, photographs with geotagging, time-stamps and annotations, and condition reports that note defects, material incompatibilities and moisture issues. BS 40104 stipulates that the final assessment report must be structured with assessor details, photographs, drawings and condition reports, completed within six months, and digitally formatted and stored in a centralised portal.

Information gathered during the assessment is not only useful to a range of stakeholders, including funders, project managers, designers and installers, but also critical to the project's success.

Standards such as PAS 2035, RICS' Residential retrofit standard and BS 40104 will ensure that retrofit assessments in the UK are delivered at scale and to a high standard.