Skip to main content
Delivery / post-retrofit

2 Detailed design


Once you have carried out the whole dwelling assessment and established the appropriate retrofit measures for the property, then detailed design can take place.

You will need to use a specialist project team to produce drawings and information which can then be used for contractors to price against and for instructions on how to carry out the installation.


    Why it matters

    Detailed design is crucial for a successful retrofit project. It allows for all foreseeable issues to be solved prior to commencing on site, giving you greater cost and quality control and reducing the risk of unintended consequences and the ‘performance gap’. Using standard details will not be sufficient in most cases. Each building will need an individual assessment and a bespoke detailed design package.

    Key steps to take

    1. Review the preliminary design: Look back at the information that was compiled during the preliminary design phase and ensure this is passed onto the design team. This will include any site constraints, known defects, intended outcomes and any concept design proposals.

    2. Review the retrofit assessment: Ensure that the retrofit measures and package of works has been established prior to briefing the design team.

    3. Check Statutory Consents: You will need to check whether planning permission is needed and any other statutory consents such as Listed Building Consent; these can add both time and cost to the project. Most retrofit measures also require Building Regulations Approval and need to comply with the Construction Design and Management (CDM) Regulations (2015).

    4. Consider the procurement route and any contractor’s design portion: There may be elements of the scheme which can be designed by a contractor (for example, specialist external wall details could be designed by the external wall insulation contractor). However, it is still important that these elements are incorporated into the overall design and coordinated by the Principal Designer. Make sure that your procurement strategy takes account of who is doing what.

    5. Assemble the right project team: You should already have a retrofit coordinator on board, and a designer may have been involved in the preliminary design. These would be your first port of call for moving forward with the detailed design. The project team for this phase will likely consist of an architect to produce the drawn information along with a quantity surveyor, structural engineer, mechanical and electrical designer and project manager as appropriate.

    6. Set up a project plan and communication route: All information that has been produced so far should be passed on to the design team, and a clear project brief should be provided. Regular meetings with the team should be set up so that key issues can be discussed, and the budget and timescales can be reviewed.

    Success factors

    Ensure the building is “retrofit ready”: If existing defects such as structural issues, asbestos, drainage issues and damp haven’t already been rectified then they will need to be incorporated into the detailed design package and specification. Sufficient budget and time should be allocated for this.

    Use “fabric first” principles: The fabric first principles should be carried through to the detailed design. All junctions should be carefully considered to avoid thermal bridges and a continuous layer of insulation and continuous air barrier should be applied at each detail.

    Consider the whole house: The preliminary design and retrofit assessment should have already established whole house principles which should be carried through to the detailed design. The designers should be aware of any future-proofing that needs to take place, e.g. positioning of windows and oversizing window sills to allow for external wall insulation to be installed at a later date.

    Consider the medium-term plan: The designer should be aware of the medium-term plan that should have already been established for the property (especially if working to PAS 2035). This will ensure they can make provision for any future proofing within the design and consider the whole house.

    Plan for maintenance: You will need to work with the designers and your maintenance team to consider your ongoing maintenance approach to the building so that elements can be incorporated into the design, e.g. location of MVHR filters for cleaning. Without a good plan for maintenance and ongoing repair, the retrofit measures will likely fail or not work at their full capacity.

    Appoint specialists: The key to a good retrofit is having the right project team in place with the specialist skills required. For example, if the building is historic, then a historic building specialist will be required. Read more in our guidance on creating a team.

    Statutory consents and Health and Safety: You must carry out your statutory duties for building regulations approval (including fire safety) and under Construction Design and Management (CDM) Regulations (2015). These are both legal requirements so it is worth spending time to understand your role in them. You may wish to appoint additional consultants, such as a fire safety consultant and a CDM consultant.

    Resident engagement: All the retrofit measures installed (especially heating and ventilation systems) will need to be used in a certain way to achieve maximum efficiency and avoid unintended consequences. Resident engagement, detailed handover procedures and continuing support are all essential and should be incorporated into the design stage.

    Deep dive

    Detailed design of retrofit measures

    In this section, we will go step-by-step through a range of retrofit measures, covering broad considerations that will need to be part of your detailed design. This is not a comprehensive list of measures or considerations, so you should work with your retrofit coordinator to develop your own project more fully.

    Step 1: Measures to improve the building fabric:

    When you think about building fabric, you need to consider all of the building elements. For a fabric first approach, this means thinking about a continuous line of insulation and a continuous air-barrier around the whole building envelope. The junctions of the different elements - between the wall and the floor, for example, or between the wall and the roof - should be carefully considered to avoid thermal bridges, which can lead to condensation and mould.

    The other key principle when improving the building fabric is to ensure that there is a sufficient ventilation strategy in place following installation of the measures.

    Examples of how the building fabric might be improved have been listed below:

    • Draughtproofing: This is a very simple measure that can easily be integrated into maintenance/repair activities. Examples of draught-proofing might include applying draught excluder strips to doors or applying rubber sealing around windows and doors.

    • External wall insulation (EWI): The existing external walls should be defect-free and structurally capable of supporting the insulation. You should also consider your fire safety strategy (if necessary, appoint a fire consultant, especially if working with high rise buildings). Pay attention to all junctions to ensure continuous insulation and think about whether you will need to relocate external services such as downpipes.

    • Internal wall insulation (IWI): For solid walls where external wall insulation is not possible, internal wall insulation may be used. The detailed design of this should consider internal decorations or features, electrics and services and loss of floor space. As before, pay attention to the junctions between floors, walls, windows and roof to avoid thermal bridging. The type of insulation used will depend upon the moisture balance of the existing building and whether a vapour-open or vapour-closed approach is being taken. Examples of internal wall insulation include insulated plasterboard, Aerogel backed plasterboard and timber studs with insulation in between.

    • Cavity wall insulation (CWI): This is a relatively straightforward measure consisting of injecting insulating material into the cavity. However, the cavity should be of a suitable width and the brickwork should be in good repair for this to take place. If the cavity has been previously insulated, then the old insulation may need to be removed; this can be expensive. It may be a good idea to do some post-installation testing using thermal imaging to check all areas have a good spread of insulation.

    • Loft insulation: This is relatively straightforward, consisting of adding mineral wool/blown cellulose within the roof space, generally between and above the ceiling joists. The junction with the external wall needs to be considered to avoid thermal bridging. If residents use the loft for storage, then you may need to provide boarding over the insulation to maintain access. Ventilation to the roof space should also be maintained.

    • Room-in-roof insulation: Where there is a room in the roof, insulation can be added along the slope of the roof. This can be above, between or below the roof joists. The design will need to consider the eaves junction to ensure there is no thermal bridge. A ventilation path should be maintained above the insulation (or a specialist breathable roofing membrane specified). If the insulation goes above the roof joists due to space issues, then the roof will need a new finish.

    • Flat roof insulation: If there is an existing flat roof, insulation can be added either above the existing roof (warm roof construction) or below/between the joists (cold roof construction). Warm roofs are considered best practice as they have better thermal properties (due to less thermal bridging caused by the roof joists), less risk of condensation and are easier to treat at the eaves junction particularly if EWI is also being specified.

    • Double/triple glazing and high-performance doors: If the windows are being replaced, it is important to consider the positioning of the frame within the wall to ensure there isn’t a thermal bridge (the frame should overlap the insulation). You will need to think about this if you are planning to add wall insulation later. Trickle vents should also be maintained (unless MVHR ventilation is being used). Overheating may be an issue if there are large openings particularly on the south, east, or west facades; therefore a low-e glass should be specified or solar shading incorporated in the design. Frame options include timber, UPVC or aluminium; it is important to consider maintenance and required lifecycle when selecting the frame. There may also be planning requirements if the building is historic or within a conservation area.

    • Floor insulation: The type of floor insulation used will depend upon the existing construction of the floor. If the floor is timber then either cellulose, mineral wool or PIR insulation can be applied between or under the joists provided a ventilation path is maintained; the main risks with this are thermal bridging and timber rot particularly at the joist ends. If the floor is solid, insulation will need to be added above the existing floor. This will impact floor-ceiling heights, door thresholds, internal fixtures/fittings and stairs; this is quite disruptive work and needs careful consideration of the detailed design.

    Step 2: Ventilation

    Getting the ventilation right is the key to avoiding condensation and mould, especially if the fabric of the building is being improved (i.e. the building is becoming more airtight).

    The type of ventilation used will depend upon a number of factors including:

    • External air quality

    • Occupant behaviour

    • The air tightness that can be achieved

    • Available space

    • Moisture control

    • Potential for summer overheating potential

    There are lots of ventilation options available including both passive and mechanical, and with or without heat recovery. Key factors to take into consideration are the maintenance approach (if filters need to be cleaned how they will be accessed) and providing residents with sufficient training and support to effectively use the systems.

    Step 3: Low carbon heating and hot water systems

    • Heat pumps: These consist of either air source heat pumps or ground source heat pumps. The appropriate system will depend upon space available, project budget and the heating demand of the property (following fabric improvement measures). Ground source heat pumps involve some external digging to lay out a ground loop or dig a borehole; air source heat pumps do not require this. Heat pumps operate at lower temperatures than gas boilers, so you will need to think about how heat is distributed through the property; the main options are radiators that appear over-sized compared to current systems or underfloor heating. Radiators are easier and less disruptive to install.

    • Other options: Where heat pumps are not feasible, there are other options available such as connecting to a heat network if there is one in the area, solar thermal heating, or high heat retention storage heaters. A mechanical and electrical engineer will be able to assess the best option for your project.

    Step 4: Decarbonising the electricity supply to the property

    • Solar PV: For maximum efficiency, solar panels should be installed on south or south-west facing sloping roofs. Installations are possible on flat roofs but the panels should be tilted at least 15o. The installation is relatively straightforward; however the roof must be capable of taking the extra weight and the roof finish should be defect-free.

    • Solar battery storage: Excess energy from solar PV can be stored in batteries to be used in evenings or overnight. The battery will need to be located in an accessible space that is out of reach of small children. Some batteries can only be situated on the ground floor and require ventilation.

    Further resources

    What are my obligations under Building Regulations and the Construction Design and Management (CDM) Regulations?

    Your retrofit coordinator, designer and contractor can help you with this. You can also read more at the following links.

    CDM Regulations, Health and Safety Executive:

    Visit Website

    Building Regulations (England and Wales) can be downloaded from the Planning Portal:

    Visit Website

    Scottish Building Standards can be viewed at:

    Visit Website

    How can I find out more about specific retrofit technologies?

    Guidance on insulation is available from the National Insulation Association:

    Visit Website

    The Retrofit Academy offers a range of guidance including:

    PAS 2035 compliance:

    Visit website


    Visit Website

    The National Insulation Association (NIA) provide guidance on insulation:

    Visit Website

    The Retrofit Academy have lots of guidance available on PAS2035 compliance, and have a good ventilation guide which is free to download:

    Retrofit Academy - Ventilation guide

    The BRE have produced a guide for PV battery storage which can be found here:


    How can I find installers for different technologies?

    The Solid Wall Insulation Guarantee Agency provide guarantees for solid wall insulation and hold a database of approved installers:

    Visit Website

    The Microgeneration Certification Scheme provide a quality mark for renewable and low carbon technologies such as solar PV and heat pumps and hold a database of certified installers:

    Visit Website

    Get in touch

    If you would like to discuss monitoring and evaluation strategies for your retrofit project, please contact the SHRA Support team.

    We would love to hear about your experiences. What has worked for your housing association? What lessons have you learned? What documents, reports or tools have you found most helpful? Please contact us if you would like to share your experiences.

    Related Documents

    Back to top