The Standard Advice: Insulate First
The standard advice from government guidance, heat pump manufacturers, and retrofit specialists is: reduce your home's heat demand before sizing a heat pump. The reasoning is straightforward — a better-insulated home needs a smaller, cheaper heat pump, which operates more efficiently and has lower running costs. If you insulate after installing a heat pump, the system is oversized for the reduced heat demand, and oversized heat pumps cycle on and off rather than running continuously at high efficiency.
This advice holds, and we follow it in our surveys. A full MCS-compliant heat loss calculation is the first step — this tells us exactly what heat pump size your home needs now, and what it would need after any insulation improvements you are considering.
When You Can Install a Heat Pump Without Insulating First
Here is what surprises many homeowners: the majority of properties we survey can install a heat pump immediately, without any insulation work. Why? Because the heat pump is sized to the property as-is. A Victorian terraced house with a heat loss of 12 kW gets a 12 kW heat pump — it works perfectly well, it costs more to run than the same house after EWI, but it is not broken or suboptimal in an absolute sense.
The decision is actually financial. If EWI costs £12,000 and reduces the required heat pump size from 12 kW to 8 kW (saving roughly £3,000 on the heat pump capital cost) and reduces annual running costs by £350, the incremental payback on the EWI is around 25 years. For many households, getting the heat pump now — potentially while the BUS grant is available — is the better financial decision.
The Best Insulation ROI Measures First
If you do want to insulate before or alongside a heat pump, prioritise measures by ROI. Loft insulation to 270mm: low-hanging fruit, saves £180–£280/year, costs £300–£600 for a DIY-friendly top-up. Cavity wall insulation (if you have an unfilled cavity): typically free under ECO4 or £700–£1,500 self-funded, saves £200–£300/year. Draught-proofing: very cheap (DIY £100–£300), meaningful in draughty older homes.
External wall insulation for solid-wall properties: expensive (£8,000–£20,000 for a mid-size home), high impact, but long payback unless ECO4-funded. Do EWI first if you can get it funded; otherwise, the heat pump now plus EWI later when it fits your budget is perfectly rational.
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