What order should retrofit measures be done in?

That sequence is not a preference. It is written into the technical standards that govern funded retrofit. Under PAS 2035, fabric measures are assessed and prioritised before heating upgrades are finalised³, and the Social Housing Decarbonisation Fund describes its approach as fabric first, with heat loss prevention prioritised before other energy efficiency measures⁴. The same logic runs through the Warm Homes: Local Grant, where improvements are offered in the order recommended by the Retrofit Assessment⁵.

What follows is the order in practice, what drives it, where ventilation sits, and who is qualified to set the sequence for a particular home.

Why the order of retrofit measures matters

Retrofit means adding a component to something that did not have it when it was first made¹⁰. That definition matters because a house is a system, not a list of products. Insulate a loft without dealing with the ventilation it changes, and the building behaves differently. Fit a heat pump sized for a leaky house and then draught-proof the house, and the system is oversized. The order is what stops one improvement working against the next¹.

The evidence for sequencing comes from programme design rather than from any single trial. Deeper retrofitting of homes leads to greater carbon and cost savings¹¹, and the whole house approach was adopted in technical standards for retrofit installers precisely to ensure that all risks of retrofit measures were always considered¹². PAS 2035 is that standard, and it exists because piecemeal work kept producing unintended consequences.

There is a practical cost to getting the sequence wrong. Independent guidance recommends starting with lower risk measures, such as draught-proofing, before considering higher risk measures, such as solid wall insulation⁶. Risk here means the chance of trapping moisture, reducing ventilation or damaging the building. A household that starts with the low-risk items gets immediate comfort and can stop there if the budget runs out, without having created a problem that needs fixing.

Advice also has to be personal. Each home is different and personal circumstances are unique, meaning a one size fits all approach will fall short¹³, which is why retrofit advice is argued to be needed at both a local and national level¹³. The order below is the general shape. The detail belongs to the individual property.

Fabric first: insulation and airtightness before heat

Fabric first is the principle that the building itself is treated before the equipment inside it. The measures in scope are insulating roofs, walls and floors, replacing windows, improving ventilation, draught-proofing, installing an efficient heating and hot water system, and renewable energy such as solar panels¹⁴. Note the position of the heating system in that list: after the fabric items.

The Affordable Warmth Scheme in Northern Ireland shows how a funder translates this into priority order. Its Priority 1 measures are insulation, ventilation and draught-proofing: installation or topping up of loft insulation to 300mm, roof, loft and eaves ventilation, a hot water cylinder jacket, cavity wall insulation where the property is suitable, draught proofing of doors and windows, and removal and replacement of ineffective cavity wall insulation¹⁵. Heating and renewables come later in the scheme's structure.

Where two fabric measures compete, there is a specific ruling. If external wall insulation and replacement windows are both planned, the windows should be done first⁶. The reason is that windows are set into the wall, so fitting them after the wall has been insulated means cutting into the new insulation.

Heritage and traditional buildings add a prior step. Working with heritage and traditional buildings often reinforces the need to address repairs, moisture management and ventilation before considering more invasive energy efficiency measures like insulation or low-carbon heating systems¹⁶. For an older solid-walled property, the sequence starts with making the building sound, not with adding material to it.

Ventilation: where it sits in the sequence

Ventilation is not a stage after insulation. It is part of the fabric work, and in funded schemes it sits in the same priority group. The Affordable Warmth Scheme places roof, loft and eaves ventilation inside Priority 1 alongside the insulation itself¹⁵.

The regulatory position is explicit. Building regulations need adequate ventilation when installing loft insulation, and the guidance advises checking with local building control¹⁷. That applies across the UK, though building control is a devolved matter and the technical handbooks differ: Scotland publishes its own Building Standards Technical Handbook for domestic work¹⁸, and Wales has consulted on Approved Document L Volume 1¹⁹.

Airtightness is the other half of the same subject. PAS 2035:2023 requires that the retrofit design include a requirement for the main contractor or retrofit installer to demonstrate compliance with the airtightness standard by means of an approved test for every dwelling²⁰. For an existing property that has been retrofitted with energy efficiency measures, the expected reading is around 10m3 (h-m2) @ 50Pa⁷. That is a measurable target, not an aspiration, and it is why testing appears in the sequence rather than at the end.

Skills policy reflects the same priority. Airtightness and ventilation appears as a priority training area in official consultation work, alongside low-carbon heat²¹. Ventilation also appears as a named option in the Future Homes and Buildings Standards consultation²².

The household consequence is that a retrofit which tightens a building without addressing ventilation has not been sequenced correctly. Moisture, indoor air quality and, in the worst cases, the building fabric itself are what suffer.

Heating and controls: after the fabric work

Once the fabric is settled, the heating system can be sized for the building it will actually serve. The ordering rule is direct: if a new heating system and significant new insulation measures are both being considered, do the latter first⁶. Doing it the other way round means specifying equipment for a heat load that is about to change.

Controls are part of this stage, not an afterthought. Heating controls appear as a distinct measure category in the Energy Company Obligation, with 514,780 installed as of 11 August 2026⁸. Hot water system controls are treated as energy-saving materials in the relevant legislation²³, which matters for the VAT treatment of their supply and installation²⁴.

Where a heat pump is installed alongside an existing gas boiler under the Warm Homes: Social Housing Fund Wave 3 rules, the gas boiler must have an energy-related products rating of A or above, and the retrofit coordinator must be satisfied it is in good working order²⁵. The same scheme requires that the controls are capable of prioritising heat pump utilisation, and that the product is listed on the MCS product directory and fulfils MIS 3005-D requirements²⁵.

In Scotland, some heating-related work is treated differently for building warrant purposes. Work associated with pipework, radiators, convector heaters and thermostatic controls for, or associated with, type 6 work does not require a warrant¹⁸. That is a Scottish specific, and it affects the paperwork rather than the physical sequence.

The independence point is straightforward. A heating system chosen after the fabric work runs at lower flow temperatures, cycles less and is more likely to perform as designed. A system chosen before it is a guess.

Renewables and storage: last in the queue

Solar panels, batteries and other generation sit at the end of the sequence for two reasons: they are the most expensive per unit of benefit, and their output is only as useful as the building's demand for it. Reducing demand first makes every subsequent kilowatt go further.

There are also physical preconditions. For solar, the roof structure, walls and existing coverings must be in sound repair prior to installation²⁶. Panels fitted to a roof that then needs recovering have to come off again. Planning rules add their own gates: installing solar panels on a listed building requires listed building consent before commencement²⁷, and in a conservation area on a flat roof, prior approval is required before installation²⁸.

Storage is treated as an adjunct to generation rather than a standalone measure. The VAT rules cover the retrofitting of a battery for storing electricity generated by one or more qualifying energy-saving materials, solar being the obvious case²⁴. A battery without generation behind it is a different proposition.

Funded programmes place renewables within a wider strategic frame rather than at the front of it. The Warm Homes: Social Housing Fund Wave 3 sets out four strategic priorities: delivery at scale, preparing for the future scale-up of retrofit, low-carbon heating, and innovation²⁵. Its rationale for the scale-up priority is that meeting the government's legally binding fuel poverty and net zero targets will require all social landlords to be supported to retrofit their stock and the supply chain to be prepared for the scale of retrofit required²⁵. Innovation in retrofit is described as important both to overcome challenges facing the sector and to take advantage of opportunities²⁵.

For a household, the practical reading is that generation and storage are the last layer of a plan, not the first purchase. They reduce dependence on a supplier for the electricity a home still uses, but they do not reduce the heat a leaky building loses.

Who sequences the work: the Retrofit Assessor role

The first step in the process is assessment. A qualified retrofit professional will assess your home⁹, and a qualified retrofit assessor can help think through options and develop a whole-house plan²⁹. TrustMark is recommended for finding a qualified retrofit assessor¹⁰.

The roles around that assessment are distinct. Retrofit coordinators are qualified professionals who assess a property's whole energy output, with expertise on energy use, planning, compliance and quality assurance³⁰. They oversee projects from start to finish, bringing together information, risk understanding and coordinated decisions in line with PAS 2035³¹. Under the Green Homes Wales loan terms, a coordinator provides end-to-end support throughout the selection and installation of the eligible measures at the property³².

In Scotland, the review of retrofit assessment recognises four distinct but complementary roles in the design and delivery of retrofit projects: a retrofit project manager, a retrofit advocate, a retrofit consultant and a retrofit assessor³³. Wales points to the Retrofit Coordinator as an example of an appropriately trained person in risk assessment and management, specifically for assessing element suitability before insulation¹⁹.

Quality assurance sits above all of this. The quality assurance of retrofits to both technical and consumer protection standards is overseen by TrustMark³⁴. Retrofit assessments of this kind are largely aimed at government-funded retrofit schemes³⁵, so a household paying privately may not encounter the full structure, but the sequence it enforces is the same one that applies on the evidence.

One caution on capacity: the BS 40104 guidance will significantly increase the time it takes to carry out retrofit assessments, meaning a larger retrofit assessor workforce will be required just to maintain current levels of delivery³⁶. Households should expect assessment to take time.

Where the sequence bends

The order above is the default, and there are recognised exceptions.

Staging is permitted. Retrofit can be carried out in stages to suit budget, lifestyle or needs⁹. A household can do the loft and draught-proofing this year and the heating system later, provided the plan accounts for what comes next.

Solar before insulation is not forbidden, but it is constrained by the roof condition rule²⁶ and by the planning gates for listed buildings and conservation areas²⁷. If roof or loft work is planned, doing it first avoids a second visit from the scaffolding.

Heritage properties invert the usual starting point. Repairs, moisture management and ventilation come before insulation or low-carbon heating¹⁶, because a traditional building that is damp cannot safely be sealed.

The unresolved question is who pays for the assessment that sets the order. Retrofit assessments are largely aimed at government-funded schemes³⁵, while retrofit upgrades are usually the responsibility of the homeowner or landlord²⁹. For a household outside a scheme, the sequence is still the right one, but the professional structure around it may not be funded.