The UK’s Warm Homes Plan and the Boiler Upgrade Scheme have put serious money behind home energy upgrades. But grant funding and good intentions don’t prevent the most common and costly retrofit mistake: doing things in the wrong order.
A heat pump installed in a draughty, poorly insulated house will be expensive to run, may underperform against its rated output, and could fail to pass performance testing. Conversely, a household that does insulation first will find the same heat pump works more efficiently, costs less to run, and may be eligible for a higher grant tier. The sequence is not just a technical recommendation — it affects how much you spend, what you qualify for, and whether your upgrades actually work.
The Core Principle: Reduce Demand Before Changing Supply
The logic behind retrofit sequencing is straightforward. Your heating system is sized to meet your home’s heat demand. If you improve the fabric of the building first — insulation, draughtproofing, glazing — the heat demand drops. A smaller, less expensive heat pump can then meet that reduced demand. If you install the heat pump before the fabric work, you’ll oversize it (because it’s sized for the old, leaky building), and you’ll still have the draughty rooms after the heat pump is in.
The golden rule, codified in PAS 2035 (the British standard for domestic retrofit), is: fabric first, then heating system, then renewables.
Step 1: Get an Assessment
Before any physical work, commission a retrofit assessment by a PAS 2035-qualified assessor. This produces a medium-term improvement plan (MTIP) tailored to your property — specifying which measures to install, in what order, and flagging any compatibility issues. This assessment is required by some grant schemes (including some local authority Warm Homes Local Grant applications) and is strongly recommended before any significant spend.
The assessment will also identify your current EPC rating and what each measure is likely to contribute toward improvement. This matters both for understanding your starting point and for planning grant applications — some schemes require a minimum EPC rating before or after works.
Step 2: Loft Insulation
If your loft is accessible and currently uninsulated (or inadequately insulated), loft insulation is the single most cost-effective energy upgrade available for most UK homes.
- Cost: £300–£900 for a typical semi-detached house, often free under ECO4 or the Warm Homes Local Grant for eligible households
- Savings: 15–25% reduction in heat loss for an uninsulated loft, translating to annual bill savings of £150–£300 in many homes
- Payback: Often 2–4 years at current energy prices even if self-funded
Loft insulation typically causes minimal disruption and can be done in a day. It’s the obvious starting point.
Step 3: Cavity Wall or External Wall Insulation
For homes built after around 1920, cavity walls are the norm, and cavity wall insulation (CWI) is the second priority after loft.
- Cavity wall insulation: £500–£1,500, commonly funded under ECO4 for eligible households. Drilled and injected in a day, minimal disruption.
- External wall insulation (EWI): Required for solid-wall properties (pre-1920 construction, or homes where CWI was not possible). Costs £8,000–£15,000 for a typical house — expensive but the only option for solid walls. The Warm Homes Local Grant can fund EWI for eligible households.
- Internal wall insulation (IWI): An alternative to EWI for solid walls where external changes aren’t permitted (conservation areas, flats). Reduces interior floor area slightly; requires careful attention to vapour barriers.
For homes with partial or poor-quality existing cavity wall insulation, a survey may reveal whether CWI is still the right approach or whether the cavity needs clearing and re-filling.
Note on sequencing: The PAS 2035 standard requires wall insulation to be completed before heat pump installation in most cases. Failing to do so can mean the heat pump is incorrectly sized and that the installer — and your grant claim — faces problems at assessment.
Step 4: Draught-Proofing
Draught-proofing is low-cost, high-impact, and often overlooked because it’s not glamorous. Sealing gaps around:
- External door frames and letterboxes
- Window frames and sills
- Floorboards (especially in older properties)
- Loft hatches
- Pipework penetrations through walls
…costs £100–£400 as a DIY or professional job and typically reduces heat loss by a further 5–10%. It also makes the building more comfortable and improves the effectiveness of any heat pump you install afterward.
Step 5: Glazing (Where Appropriate)
If your home still has single glazing, upgrading to double or triple glazing is worth doing before the heat pump — but it’s a lower priority than insulation for most homes, and more expensive relative to the heat loss reduction it achieves.
For homes with double glazing that’s less than 20 years old, replacing windows is generally not the priority. The condition and fit of existing windows matters more than the glazing specification.
Step 6: Heat Pump Installation
Once the fabric work is done, the home’s heat demand has dropped and is more stable. At this point, a heat pump installer can correctly size the system. The Boiler Upgrade Scheme provides a £7,500 grant for air source heat pumps and £7,500 for ground source heat pumps in England and Wales (2026 figures), regardless of income. The Warm Homes Local Grant can provide additional support for eligible households.
The PAS 2035 / PAS 2030 framework requires that heat pump installations be properly designed and sized using heat loss calculations rather than rule-of-thumb estimates. After the fabric work is done, this calculation produces a smaller, more efficient system specification.
High-temperature heat pumps (operating up to 70°C flow temperature) are now available as an option for homes with older radiator systems that aren’t well-matched to standard heat pump flow temperatures — removing the need to upsize radiators in some cases. However, they run less efficiently than standard heat pumps and typically suit homes where radiator replacement would be disruptive.
Step 7: Solar PV and Battery Storage
Solar panels come after the heating upgrade for two reasons. First, if you install solar before a heat pump, you’re sizing the system to your current electricity demand. Heat pumps increase electricity consumption significantly (replacing gas consumption) — potentially changing your solar system sizing calculation. Second, many heat pump owners use time-of-use tariffs and load shifting to charge when electricity is cheap; solar plus a battery fits into this system and is best planned holistically.
The smart export guarantee (SEG) remains available for surplus solar export, and SEG rates vary significantly between energy suppliers in 2026 — worth checking before installation.
Battery storage makes sense once solar is installed if:
- Your day/night electricity price differential is £0.10/kWh or more (likely on Octopus Agile or similar)
- Your solar panels generate noticeably more than your home uses during daylight hours
- You have or plan a heat pump, EV, or other large controllable load
Grant Stacking in 2026
The Warm Homes Local Grant, administered by local authorities using government funding, is the main route for households below an income threshold. It can fund insulation, heat pumps, solar, and associated measures. The Boiler Upgrade Scheme covers heat pump grants for any household regardless of income.
Grant schemes often have rules about sequencing — some require insulation to be at or above a certain standard before heat pump funding is approved. Your retrofit assessor will know the current requirements for your local authority’s scheme.
A Common Mistake to Avoid
Installing insulation after a heat pump can create moisture and ventilation problems. When a building is tightened significantly (by adding insulation, draughtproofing, and new glazing), background ventilation needs to be considered. A heat-recovery ventilation unit (MVHR) or demand-controlled ventilation may be needed in highly insulated homes to maintain air quality. Your PAS 2035 assessor should flag this if it applies.
The Simplified Sequence
For most UK semi-detached or terraced homes:
- PAS 2035 assessment — establish your baseline and get the MTIP
- Loft insulation — fast, cheap, high impact
- Cavity or external wall insulation — depends on construction type
- Draught-proofing — cheap, often DIY
- Heat pump — sized correctly for the post-insulation building
- Solar PV + battery — sized for the post-heat-pump electricity demand
Doing it in this order means each upgrade works better, each system is sized correctly, and the total cost of the retrofit package is lower than it would be if you reversed the sequence.