Fabric-first retrofit before a heat pump: what to do, in what order, in UK 2026

The principle: heat pumps are sized to heat-loss

Every MCS heat-pump install starts with a heat-loss calculation under BS EN 12831 — a room-by-room model that tells the installer how many kilowatts your house needs at design temperature (typically −3°C for most of England). That kW number directly determines:

• The size of the outdoor unit (and therefore its price).
• The flow temperature the system runs at day-to-day.
• Whether your existing radiators are large enough.
• The annual electricity bill, via the coefficient of performance.

Insulating first lowers the heat-loss number. A lower number unlocks: smaller heat pump, lower flow temperature, better COP (often 3.5+ instead of 2.8), keeping more of your existing radiators. The bill saving from a better COP compounds over 15+ years — the lifetime of the heat pump — so fabric work pays back across the whole life of the system, not just the first winter.

The order: ROI ranking for heat-pump readiness

The standard EPC recommendation order isn’t the same as the right order when your goal is heat-pump readiness. EPCs prioritise pure kWh saved; we’re prioritising heat-loss reduction per pound spent. Here is the ranking for a typical UK pre-1990 home:

1. Loft insulation top-up to 270mm

Cost: £400–£800 for a typical semi.
Heat-loss reduction: 20–25% on pre-1990 stock.
Disruption: One day, no decorating.
Required by BUS? Yes, if your EPC flagged it.

Loft insulation is the highest-ROI fabric measure by miles. Going from 100mm to 270mm of mineral wool is cheap, fast, and dramatic. If your current EPC says “loft insulation recommended”, this is non-negotiable for BUS eligibility anyway. Worth doing whether or not you eventually install a heat pump.

2. Draughtproofing

Cost: £200–£800, mostly DIY-able.
Heat-loss reduction: 10–15%.
Disruption: Hours, not days.
Required by BUS? No.

Draughtproofing is the most underrated fabric measure because it’s cheap, fast, and surprisingly effective. Target: door + window seals, loft hatch, unused chimneys, floorboard gaps in older houses, letterboxes, service penetrations. A weekend with a tube of acrylic sealant and a few door brushes can shave more off your heat-loss than people expect. Many UK houses are leaky enough that draughtproofing matters more than glazing upgrades.

3. Cavity wall insulation

Cost: £1,500–£3,500 for a typical semi.
Heat-loss reduction: 25–30%.
Disruption: One day, minor external drill holes that get re-pointed.
Required by BUS? Yes, if your EPC flagged it.

If your house was built between roughly 1930 and 1990 with an unfilled cavity, this is huge. The work is non-destructive — bonded beads or mineral fibre blown into the cavity through small holes drilled in the external mortar joints. Most UK homes built post-1995 already have insulated cavities. Check your EPC or get a borescope survey if unsure.

4. Floor insulation

Cost: £1,500–£4,000 typical.
Heat-loss reduction: 8–10%.
Disruption: Moderate — lifting boards or working from a cellar.
Required by BUS? No.

Suspended timber floors over a ventilated underfloor void lose a surprising amount of heat. Insulating between the joists with mineral wool or PIR boards is moderately disruptive — boards have to come up, or access is from below if you have a cellar. Solid concrete floors are much harder to retrofit and usually not worth it.

5. Solid-wall insulation

Cost: £8,000–£15,000 internal, £12,000–£25,000 external.
Heat-loss reduction: 30–40%.
Disruption: Major.
Required by BUS? No.

If your house is pre-1930 with solid brick or stone walls, this is where the heat is going. The impact is large but the cost is too — and it’s only worth it if you can integrate it with other work (replastering, extension, re-rendering). A heat pump can absolutely work in a solid-wall house without this; you’ll just have a bigger unit and higher running cost than an insulated equivalent.

6. Double or triple glazing

Cost: £6,000–£15,000 whole house.
Heat-loss reduction: 10–15%.
Disruption: Moderate — installer days.
Required by BUS? No.

Glazing surprises people by being LAST on this list. It’s the highest cost per percentage point of heat-loss reduction of any fabric measure. Replace glazing if the windows are failing (broken seals, rotted frames, single-glazing in cold rooms), not as a heat-pump pre-step. If your house already has decent double glazing, leave it alone.

Grants that can pay for the fabric work

Two main UK schemes in 2026 fund insulation work:

• ECO4 (Energy Company Obligation, 4th iteration). For low-income households, those on means-tested benefits, or households where a member has a qualifying health condition. Can cover loft, cavity, solid-wall insulation, and sometimes heating systems. Administered by your energy supplier — apply via the supplier or via gov.uk/energy-company-obligation.
• Great British Insulation Scheme (GBIS). For households in EPC bands D–G AND council tax bands A–D (England) or A–E (Scotland). Covers loft and cavity primarily; some suppliers also cover solid-wall. Apply at gov.uk/apply-great-british-insulation-scheme.
• Nest (Wales) and Home Energy Scotland are the devolved-nation equivalents with similar but distinct eligibility rules.

None of these stack with BUS on the same property — you can’t double-claim a single piece of work — but the sequence is straightforward: do insulation under ECO4/GBIS first, then apply for BUS for the heat pump. The insulation work also clears any blocking EPC recommendations.

Common mistakes — and how to avoid them

• Doing glazing first. Glazing is the worst £/heat-loss-reduction of the fabric measures. Do it last unless the windows are failing.
• Skipping draughtproofing.It’s cheap enough to be invisible in the budget but contributes 10–15% on most pre-1990 houses. Worth a weekend.
• Solid-wall insulation without ventilation planning. Adding insulation to a solid wall changes the moisture profile. Vapour barriers, breathable membranes, and PIV or MVHR ventilation may be needed. Use a Retrofit Coordinator under PAS 2035 for any solid-wall work; this is non-negotiable.
• Not getting a fresh EPC after the work. If you insulated to clear a BUS-blocking recommendation, you need a NEW EPC reflecting the completed work — not the old one. Fresh EPC: £60–£120, 1–2 weeks.
• Doing fabric work AFTER the heat pump. If you insulate after the pump is installed, the pump is oversized for the new heat-loss. It’ll short-cycle in mild weather, costing more and wearing faster. Always do fabric BEFORE sizing.

Realistic timeline if you’re starting now

For a typical UK semi where loft + cavity are flagged on the EPC and the homeowner wants a heat pump:

• Week 0: Check ECO4/GBIS eligibility. Order loft + cavity quotes (or apply via energy supplier if eligible).
• Weeks 1–4: Loft + cavity installed. Draughtproofing done in parallel.
• Week 4: Fresh EPC commissioned.
• Weeks 5–6: Fresh EPC issued, showing the work completed and recommendations cleared.
• Weeks 6–8: Request MCS heat-pump quotes. Installers will size to the NEW heat-loss number (lower than before).
• Weeks 8–14: Install + commissioning + BUS claim, per the BUS application walkthrough.

End-to-end: 3–4 months from starting fabric work to a commissioned heat pump. Compress to 6–8 weeks if you have no insulation work to do.

Where Propertoasty fits

The free pre-survey at propertoasty.com/check pulls your EPC, flags any uncleared recommendations, and indicates heat-loss for your property based on EPC + floor area. Run it BEFORE doing any fabric work to see the starting point; run it again after work to see the new heat-loss figure that installers will quote against.

The summary

Fabric-first works because heat pumps are sized to heat-loss, and heat-loss is what insulation reduces. The right order in UK 2026 is loft top-up, then draughtproofing, then cavity wall, then floor, with solid-wall and glazing last. Loft + cavity are also the only two BUS-blocking EPC items, so they pull double duty. Use ECO4 or GBIS if eligible to fund the work, get a fresh EPC after the work is done, then quote the heat pump against the new (lower) heat-loss number. Don’t do the heat pump first and the fabric work after — you’ll end up with an oversized system that cycles inefficiently.