Key takeaways
- Loft insulation to 270–300mm is the cheapest energy upgrade you can make — £350–£900 with payback under 3 years.
- Cavity wall insulation is a no-brainer for most pre-1990 cavity walls — £600–£1,200 with payback under 4 years.
- Solid wall insulation (IWI £6–14K, EWI £12–25K) only makes sense alongside fabric-first sequencing and a moisture strategy.
- Floor insulation pays back in 6–10 years and is consistently underrated.
- Material choice matters less than U-value achieved and moisture management. Mineral wool is usually fine.
- Insulation without ventilation is a recipe for damp and condensation. Always pair them.
- ECO4 and GBIS grants can cover 60–100% of insulation cost for eligible households — check before paying.
Why insulation comes first in any retrofit sequence
Every retrofit advisor with their head screwed on says fabric first, and the reason is arithmetic. Heating a leaky building is paying premium rates to warm the outdoors. The cheapest kilowatt-hour is the one you don't need. Before you specify a heat pump, before you order solar panels, before you fit smart controls, the building's fabric should be doing as much of the work as it economically can.
The reason it's underrated: insulation is invisible. A solar array looks like progress; loft insulation looks like nothing. Heat pumps come with grants and salesforces; cavity wall insulation comes with a slightly grumpy installer in a high-vis vest and a powdery van. The aesthetics are wrong. The economics are right.
This guide walks through each location in the building (loft, cavity walls, internal/external solid walls, floors), what materials are appropriate, what U-value target is realistic, and where the moisture and ventilation traps are.
Loft insulation
The lowest-hanging fruit in any UK home. Building Regs require 270mm minimum for new builds; many older homes have 100mm or less and significant cost-effective room to improve.
| Material | Thickness for 0.16 W/m²K | Cost (typical 70m² loft) | Best for |
|---|---|---|---|
| Mineral wool (glass or rock) | 270–300mm | £350–£700 | Most lofts. Cheap, easy, fire-safe. |
| Sheep wool (e.g. Thermafleece) | 270–300mm | £900–£1,400 | Households with chemical sensitivity |
| Recycled cellulose (blown) | 250–270mm | £600–£1,000 | Awkward lofts, irregular joists |
| PIR/PUR rigid board | 120–140mm | £1,200–£1,800 | Loft conversions / room-in-roof |
| Wood fibre (e.g. Steico) | 240–280mm | £1,500–£2,500 | Breathable retrofit, period property |
For 90% of UK lofts, mineral wool to 300mm is the right answer. Lay 100mm between the joists, then 200mm across the joists at right angles. The cross-laid layer breaks thermal bridging through the joists. Don't compress the material; it loses thermal performance proportionally to compression.
Watch-outs: don't insulate underneath water tanks (you'll freeze them), maintain access paths if you store things in the loft (use loft-leg risers and boards laid over the insulation), and ensure ventilation paths at the eaves are not blocked. A blocked eaves vent + insulated loft is the classic recipe for condensation forming on the cold roof timbers.
Cavity wall insulation (CWI)
Most UK homes built between 1925 and 1990 have masonry cavity walls — two leaves of brick or block separated by a 50–75mm cavity. If that cavity is empty, filling it with insulation is one of the highest-ROI fabric improvements available. Typical U-value reduction: 1.5 W/m²K (uninsulated) to 0.5–0.6 W/m²K (insulated).
| Fill type | Cost (typical semi) | U-value achieved | Notes |
|---|---|---|---|
| Mineral wool fibre | £600–£900 | ~0.55 W/m²K | Cheap, breathable. Most common. |
| EPS bead | £700–£1,100 | ~0.50 W/m²K | Slightly higher performance, doesn't slump. |
| Polyurethane foam | £1,000–£1,500 | ~0.40 W/m²K | Best performance but harder to remove if problems arise. |
The big watch-out with CWI is moisture risk. Cavity walls were originally designed with the cavity acting as a drainage path — water that gets through the outer leaf drains down inside the cavity rather than reaching the inner leaf. Filling the cavity stops that drainage. In exposed locations (West Wales, west Scotland, North Devon, anywhere with persistent driving rain), wet CWI installations can cause damp problems on internal walls.
The mitigation: a CIGA-registered installer assesses exposure rating and refuses installs in unsuitable conditions. Always check your installer is CIGA-registered (Cavity Insulation Guarantee Agency); the 25-year guarantee is meaningful.
Red flag: if a previous CWI install has failed (damp on internal walls, mould growth that wasn't there before), removal is possible but expensive (£1,500–£3,500). Check before buying a property whether CWI was installed and whether there are any related issues.
Solid wall insulation (IWI vs EWI)
Solid wall homes — most pre-1925 properties, plus some interwar — have no cavity to fill. To insulate, you either add insulation to the inside (Internal Wall Insulation, IWI) or the outside (External Wall Insulation, EWI). Both are major projects with significant cost.
| Approach | Cost (3-bed semi exterior) | U-value achieved | Pros | Cons |
|---|---|---|---|---|
| EWI (mineral wool render) | £12,000–£20,000 | ~0.30 W/m²K | Wraps the building; no internal disruption; reduces thermal bridging. | Visual change; planning consent in conservation areas; not for ornate facades. |
| EWI (woodfibre render) | £15,000–£25,000 | ~0.30 W/m²K | Breathable; better moisture performance for older buildings. | More expensive; specialist installers only. |
| EWI (rigid board + brick slip) | £18,000–£30,000 | ~0.25 W/m²K | Maintains brick aesthetic; high performance. | Most expensive; complex detailing. |
| IWI (mineral wool) | £6,000–£10,000 | ~0.40 W/m²K | No external change; one room at a time possible. | Loses internal floor area; complex around windows/sockets; moisture risk if wrong. |
| IWI (woodfibre) | £8,000–£14,000 | ~0.40 W/m²K | Breathable; better moisture performance. | Premium price. |
| IWI (PIR rigid) | £6,000–£11,000 | ~0.30 W/m²K | Slim profile; high performance. | Vapour-impermeable — high moisture risk if installed without VCL. |
IWI is the budget option but carries the highest moisture risk. The classic failure: PIR rigid board fitted directly to internal solid wall without a vapour control layer. Warm moist air from inside hits the cold side of the insulation, condenses, soaks the wall, mould blooms behind the board, plaster fails after 2–3 winters. The mitigation: woodfibre or vapour-open mineral wool with a properly sealed vapour control membrane on the warm side, or fully filled cavity-style detailing. This is specialist work — get a PAS 2035-trained retrofit assessor involved.
EWI is more expensive but architecturally and physically more forgiving. It moves the dew point outside the wall, allowing the masonry to stay warm and dry. The biggest constraints are aesthetic (you change the look of the building) and contextual (planning consent in conservation areas, listed buildings).
Floor insulation
Floors lose 8–15% of typical home heat loss but get insulated last because the work is intrusive (lift floorboards, install between joists). For suspended timber floors, the benefit is significant.
| Floor type | Approach | Cost (50m² floor) | U-value achieved |
|---|---|---|---|
| Suspended timber | Mineral wool batts between joists, breather membrane underside | £1,500–£3,500 | ~0.18 W/m²K |
| Suspended timber | Woodfibre + airtightness membrane | £3,000–£5,000 | ~0.18 W/m²K |
| Solid concrete (existing) | PIR overlay on top of slab + new screed | £3,000–£6,000 | ~0.22 W/m²K |
| Solid concrete (new build/extension) | EPS/PIR under screed | £20–35/m² | ~0.15 W/m²K |
For suspended timber floors, the access route is critical. If you have a void deep enough to crawl into, the work can be done from below — much less disruptive than lifting floorboards. Some installers offer this; ask.
Common pitfall: ignoring airtightness. A draughty insulated floor is barely better than a draughty uninsulated one. Tape and seal the membrane junctions. Don't forget the perimeter — the worst air leakage is usually at the wall-floor junction.
Ventilation: the half of insulation that gets ignored
Insulation slows heat loss but it also slows moisture loss. A pre-improvement leaky home loses water vapour out through the gaps; an insulated airtight home traps it inside. Without a ventilation strategy, you'll get condensation, mould, and air quality problems.
The right pairing depends on how airtight the building has become:
| Airtightness (m³/h/m² @ 50Pa) | Ventilation strategy | Cost |
|---|---|---|
| >10 (typical pre-improvement) | Trickle vents + extract fans | £200–£600 |
| 5–10 (typical post-CWI/loft) | Trickle vents + extract fans + possibly PIV | £500–£1,500 |
| 3–5 (deep retrofit) | MEV (mechanical extract ventilation) or MVHR | £2,500–£8,000 |
| <3 (Passivhaus territory) | MVHR (mechanical ventilation with heat recovery) | £5,000–£12,000 |
For households doing loft + CWI insulation only (the most common upgrade combination), the existing trickle vents and extract fans are usually adequate provided they're working. Check the bathroom and kitchen fans actually pull air; replace with units rated for the room volume; add humidistat-controlled fans if condensation is appearing on windows.
U-value targets by location
| Building element | Building Regs minimum | Realistic upgrade target | Deep retrofit target |
|---|---|---|---|
| Roof (pitched, insulation at ceiling) | 0.16 W/m²K | 0.15 W/m²K | 0.10 W/m²K |
| Cavity wall (filled) | 0.55 W/m²K | 0.50 W/m²K | 0.20 W/m²K (with EWI) |
| Solid wall (IWI) | 0.30 W/m²K | 0.40 W/m²K | 0.20 W/m²K |
| Floor (suspended) | 0.25 W/m²K | 0.20 W/m²K | 0.15 W/m²K |
| Windows | 1.6 W/m²K | 1.4 W/m²K | 0.85 W/m²K |
ECO4 and GBIS grants
The Energy Company Obligation 4 (ECO4) and Great British Insulation Scheme (GBIS) cover insulation costs for eligible households:
- ECO4 — fully or substantially funds fabric and heating measures for households on means-tested benefits, council tax band A–D in England/Wales (A–E in Scotland), and EPC D or below. Local Authority Flex (LA Flex) extends eligibility based on local criteria.
- GBIS — insulation-only, broader eligibility. Council tax band A–D (E in Scotland), EPC D–G. Doesn't fund heating systems.
The eligibility check is worth running even if you think you don't qualify. LA Flex in particular captures households that the standard scheme misses — fuel poverty, vulnerable household members, certain medical conditions. The check is free and takes 5 minutes online.
When NOT to insulate
Most of this guide assumes insulation is a good idea. But there are situations where slow down is the right call:
- The building is wet. Fix the roof, gutters, downpipes, and any rising damp first. Insulating a wet building locks moisture in and accelerates decay.
- You're insulating one element in isolation in a building with no ventilation strategy. Loft insulation alone in a damp Victorian terrace can push the dew point into the wall.
- IWI on a north-facing solid wall in a cold humid climate without a vapour control plan. The interstitial condensation risk is high. Get a PAS 2035 assessor.
- EWI on a listed building or in a conservation area without consent. Don't start; get the consent first.
- Floor insulation when there's an active damp issue underneath. Investigate the void first.
- If you're moving in 2 years. The deeper measures (EWI, MVHR) won't pay back. Stick to the cheap wins (loft, CWI).
Red flags from installers
- Door-to-door sales pitches for cavity wall or external wall insulation. Reputable installers don't cold-call.
- "Free insulation, government scheme" — no scheme is fully free without proper eligibility check. Demand the eligibility paperwork.
- No CIGA registration on a CWI quote. The CIGA guarantee is the basic consumer protection.
- No PAS 2035 assessor involved on solid wall work. PAS 2035 is the proper QA framework for retrofit.
- Promises of dramatic EPC jumps. Realistic uplift from loft + CWI is one EPC band; deep retrofit with EWI is two bands. Anything more aggressive is optimistic.
- Insulation without any ventilation discussion. The two are inseparable.
FAQ
Q: How long does insulation last?
A: Mineral wool, sheep wool, and woodfibre have indefinite lifespans if kept dry. EPS and PIR boards last 50+ years. Cavity wall fill is typically guaranteed 25 years (CIGA) and lasts longer. The failure modes are usually moisture-related, not material breakdown — so keep the building dry.
Q: Will insulation make my house too airtight?
A: Loft and cavity wall insulation alone won't push you below the airtightness threshold where mechanical ventilation becomes essential. Most UK homes after standard insulation upgrades sit at 5–8 m³/h/m², which works fine with trickle vents and extract fans. Only deep retrofits with EWI + new windows + careful sealing get below 3 m³/h/m² where MVHR becomes necessary.
Q: Does insulation reduce condensation?
A: It changes where condensation forms. Warmer interior surfaces (because the insulation reduces heat loss) means less surface condensation on internal walls and windows. But if the moisture has nowhere to escape, it'll condense somewhere else — often inside the wall structure itself (interstitial condensation). The fix is ventilation, not less insulation.
Q: Can I install loft insulation myself?
A: Yes — it's the easiest DIY job in the building. Wear PPE (the fibres irritate skin and lungs), lay it correctly (don't compress, don't block eaves vents), and leave gaps around recessed lights and electrical fittings. A weekend's work for a typical loft, materials about £200–£400 from B&Q or similar.
Q: What about underfloor heating with insulation?
A: They work very well together. UFH runs at low flow temperatures (35–40°C) and is ideal with heat pumps. The floor itself needs proper insulation underneath — typically 100mm PIR for new builds or solid floor retrofit. UFH on top of an uninsulated slab is a heating-the-ground exercise.
Q: Will insulation affect my home's ventilation requirements legally?
A: Building Regulations Part F sets minimum ventilation requirements for new and refurbished homes. Major insulation work that significantly improves airtightness should be accompanied by a ventilation review — extract fan ratings, trickle vent area, etc. Most retrofits don't trigger formal Part F compliance but a competent installer will check.
Q: Do I need a vapour control layer (VCL)?
A: For IWI on solid walls, almost always yes. The VCL goes on the warm (room) side of the insulation and prevents moisture from migrating into the wall. For loft insulation laid above ceilings, no VCL is needed (the existing plasterboard ceiling acts as the air control layer). For floors, an airtightness membrane is helpful but not always essential.
If you'd like to identify the right insulation upgrades for your home and check eligibility for ECO4 / GBIS grants, the retrofit assessor lookup at healthyhomesnetwork.co.uk/find-assessor/ finds vetted local assessors who can produce a sequenced upgrade plan with grant routing. For straightforward loft and cavity wall jobs, the insulation calculator at ecosavinghub.co.uk/insulation-breakeven/ models the savings before you commit.
Want to model your specific home? Use our free ROI calculators →