You are not required to insulate before a heat pump: the Boiler Upgrade Scheme dropped its loft and cavity insulation condition in 2024. You should do it anyway. A heat pump runs at 35 to 55°C, so the house has to hold heat, and a well insulated home needs a smaller unit, fewer new radiators and less electricity. Loft top-up and draught proofing cost a few hundred pounds and are done in a day; solid walls are the big decision on older homes. A free heat loss survey tells you which measures matter for your house.
Key takeaways
- The £7,500 Boiler Upgrade Scheme no longer requires insulation first; that rule went in 2024.
- Insulation still matters because a heat pump runs at a low flow temperature. A leaky house forces it to run hot, and the SCOP falls.
- A well insulated home needs a smaller unit and smaller radiators, so insulating first lowers the heat pump price as well as the running cost.
- Do the cheap measures first: loft top-up £400 to £900, draught proofing £250 to £700, both 0% VAT and done in a day.
- Solid wall homes: EWI £12,000 to £22,000 or IWI £8,000 to £15,000 for a 3 bed is the measure that moves a Victorian or 1930s house into heat pump territory.
- A free heat loss survey gives you a written, prioritised plan before you commit to either the insulation or the heat pump.
Is insulation required for a heat pump grant?
No. When the Boiler Upgrade Scheme launched, a home needed loft and cavity wall insulation (where possible) before it could claim the grant. That condition was removed in 2024. Today the MCS certified installer applies for the £7,500 (£9,000 for oil or LPG homes with no mains gas from 21 July 2026 to 31 March 2027), deducts it from the quote, and you sign. No EPC recommendation has to be cleared first.
That is the rule. The advice is different, and every good heat pump designer will give it: insulate first where you can. The rest of this guide explains why, and which measures are worth doing before the heat pump goes in.
Why flow temperature makes insulation matter
A gas boiler heats the radiator water to 60 to 75°C. A heat pump is most efficient at 35 to 45°C and can reach about 55°C at a cost. The lower the flow temperature, the higher the SCOP: around 4 at the bottom of that range, nearer 3 at the top. The flow temperature the system needs is set by how much heat the house loses on the coldest day and how much radiator area there is to give that heat out.
- Leaky house. High heat loss, so a bigger unit, more radiator changes, and a higher flow temperature to keep up on cold days. SCOP nearer 3, higher bills.
- Insulated house. Lower heat loss, so a smaller unit, most radiators kept, flow temperature 35 to 45°C. SCOP nearer 4, lower bills.
The difference between a SCOP of 3 and 4 is a third off the electricity used for heating, every winter, for the life of the system. The heat pump vs gas boiler guide shows how that decides whether the heat pump beats gas on running cost.
Which insulation should I do first?
In order of cost per unit of heat saved. Prices are from our own service hubs and quote book; all are 0% VAT until 31 March 2027 and installed by our own PAS 2030:2023 certified team with a 10-year manufacturer warranty and 25-year insurance backed guarantee.
| Measure | Typical cost | Time | Do it before a heat pump? |
|---|---|---|---|
| Loft top-up to 270 mm | £400 to £900 | Half a day | Always, if not already done |
| Draught proofing | £250 to £700 whole house | A day | Always; cheapest measure, fastest payback |
| Floor insulation (suspended timber) | Median £4,320, £76 to £110/m² | 1 to 3 days | Yes where the floor is accessible; floors matter more at low flow temperatures |
| Cavity wall fill | £1,500 to £3,500 for a 3 bed semi | A day | Yes, if the cavity is suitable and dry |
| External wall insulation (solid walls) | £110 to £140/m², 3 bed semi £12,000 to £22,000 | 2 to 4 weeks | Yes on solid wall homes; the measure that changes the heat pump size |
| Internal wall insulation (solid walls) | £100 to £140/m², full 3 bed £8,000 to £15,000 | 1 to 3 weeks | Yes where EWI is not possible; can be done room by room |
| Windows | £5,000 to £12,000 for 8 to 10 windows | A few days | Later; windows are usually 10 to 20% of heat loss and 20% VAT |
The first two rows are non negotiable and cost less than one radiator change. Floors and cavity walls are next. Solid walls are the big decision and the one where a survey earns its keep: on a mid terrace only two walls face outside, so the cost is lower than the table suggests. See where homes lose heat and the insulation cost guide.
Do not forget ventilation
Insulating and draught proofing make a house more airtight, which is the point, but moisture from cooking, washing and breathing still needs a way out. Trickle vents, working extractor fans in the kitchen and bathroom (£200 fitted from our quote book) and, in some flats and terraces, a PIV unit stop condensation showing up after the work. We check ventilation on every survey. See ventilation after insulation.
What the free heat loss survey gives you
Before you commit to insulation or a heat pump, a surveyor visits and checks the walls, loft, floors, windows, ventilation and heating. In the heating season (October to April, when there is roughly a 10°C difference between inside and out) we use thermal imaging to find missing insulation, cold bridges and air leaks. You get a written, prioritised plan with indicative costs for each measure and a note of what it does to the heat loss the heat pump would be sized for.
- Book the free heat loss survey.
- Insulate in the order the plan sets out, cheap and fast measures first.
- The heat pump is designed against the insulated heat loss. Arranged and project-managed by Stay Warm, installed by an accredited, MCS certified installer who deducts the grant from the quote.
Costs for the heat pump itself are in the air source heat pump cost guide. For a Victorian terrace or 1930s semi, see heat pumps in old houses.
Frequently asked questions
Do I have to insulate to get the Boiler Upgrade Scheme grant?
No. The requirement for loft and cavity insulation was removed in 2024. The MCS certified installer applies for the £7,500 and takes it off the quote regardless of the insulation in the house.
Can a heat pump work in an uninsulated house?
It can, but it will be a larger unit running at a higher flow temperature, most radiators will change, and the SCOP will sit nearer 3 than 4. The running cost may not beat gas. Insulating first fixes all of that.
What is the cheapest insulation to do before a heat pump?
A loft top-up at £400 to £900 and draught proofing at £250 to £700. Both are 0% VAT, done in a day, and cut the heat loss the heat pump is sized for. Do these before anything else.
Do I need to insulate solid walls before a heat pump?
On a Victorian or Edwardian house, solid walls are the largest heat loss, so insulating them is what brings the home into heat pump territory. EWI or IWI, or a mix, depending on the elevation. A survey tells you which and what it costs for your house.
Should I replace windows before a heat pump?
Usually not first. Windows are typically 10 to 20% of heat loss and cost £5,000 to £12,000 at 20% VAT. Walls, loft, floor and draughts give more heat saved per pound. Replace windows when they fail, not as a heat pump preparation.
How long should I leave between insulating and fitting the heat pump?
No gap is needed. The heat pump can be designed once the insulation plan is fixed and installed as soon as the insulation is complete. Doing the insulation first only matters for sizing the system.