Who we help · Insulation
Insulation for semi-detached homes — three exposed walls and a roof.
A semi loses heat through three exposed walls and its roof. Most were built from the 1920s to the 1970s with cavity walls, so loft and cavity insulation usually come first — with extraction and refill where an old fill has failed, floor insulation, and EWI on the side and rear walls.
Typical prices
Semi / end-terrace
- Loft insulation (new or top-up)£600–£1,300
- Cavity wall insulation£1,200–£2,500
- Cavity extraction and refill£3,500–£7,500
- Suspended floor insulation£1,500–£4,500
- External wall insulation + render£14,000–£24,000
- EWI with brick slip finish£18,000–£32,000
- Room-in-roof£4,000–£9,000
- Draught-proofing£300–£800
Before grants · updated 11 October 2026 · a survey confirms the final figure.
Property types covered
Construction, heat loss and watch-outs
Semi-detached house
Full guide- Typical construction
- Mostly 1920s–1970s cavity walls; some Victorian solid
- Where heat is lost
- Three exposed walls and roof
- Usual measures
- Loft, cavity (or extraction and refill), floor, EWI on side and rear
- Watch-outs
- Match the neighbour's look on the front
End-of-terrace
Full guide- Typical construction
- Victorian solid walls or post-war cavity
- Where heat is lost
- Gable wall, roof, rear extension
- Usual measures
- EWI or IWI on gable, loft, floor, room-in-roof
- Watch-outs
- Gable wall is the main loss
1930s home
Full guide- Typical construction
- Early cavity walls, bay windows, suspended floors
- Where heat is lost
- Bays, floor, unfilled cavities
- Usual measures
- Cavity fill, bay insulation, floor, loft
- Watch-outs
- Check cavity ties and width
Good to know
What makes these homes different
Match the neighbour on the front
Half the front elevation belongs to next door. If the front is insulated or re-rendered, we match the look of the pair — or keep EWI to the side and rear.
1930s semis: check the ties and the bays
Early cavity walls need the cavity width and wall ties checked before filling, and bay windows are a common cold spot worth insulating at the same time.
End-of-terrace: the gable wall
On an end-terrace, the gable wall is usually the main heat loss — EWI or IWI on the gable is often the first step.
Recommended measures
Where to start
Loft insulation
Mineral wool laid between and across the joists to 270–300 mm so heat stays in the rooms below.
Cavity wall insulation
A borescope survey, then blown mineral wool or bonded bead injected into empty cavity walls.
Cavity wall extraction and refill
Failed, wet or slumped cavity insulation removed, wall ties checked, then refilled.
Suspended timber floor insulation
Mineral wool or rigid board between the joists with an airtight membrane.
External wall insulation (EWI)
A complete certified build-up of insulation board, mesh and render from one system holder.
EWI with brick slip finish
External wall insulation finished in brick slips so it looks like real brick.
Room-in-roof insulation
Insulating the sloping ceilings, stud walls, flat ceiling, gables and dormer cheeks of an attic room.
Draught-proofing
Sealing doors, windows, letterboxes, hatches, floorboards and unused chimneys — every home's quick win.
Prices
Insulation prices for semi-detached homes
| Measure | Semi / end-terrace |
|---|---|
| Loft insulation (new or top-up) | £600–£1,300 |
| Cavity wall insulation | £1,200–£2,500 |
| Cavity extraction and refill | £3,500–£7,500 |
| Suspended floor insulation | £1,500–£4,500 |
| External wall insulation + render | £14,000–£24,000 |
| EWI with brick slip finish | £18,000–£32,000 |
| Room-in-roof | £4,000–£9,000 |
| Draught-proofing | £300–£800 |
Indicative ranges for London and the South East, supplied and installed, including VAT, before grants. Updated 11 October 2026. A survey confirms your final price.
Grants and savings that may help
- ECO4Closes in 73 days
Up to fully funded; paid by energy suppliers per measure
Grant via local councils (£960m nationally in 2026/27)
- Social Housing Fund
Landlord-led bids
Saves 20% VAT
- Local schemes
Varies
Eligibility depends on your circumstances and is confirmed by evidence and the funder or council.
0% VAT on energy-saving materials
Insulation, solar panels, batteries and heat pumps installed in homes in Great Britain are zero-rated for VAT until 31 March 2027. Windows and doors are not included.Questions
Common questions
What's the best insulation for a semi-detached house?
For most semis: loft insulation to 270 mm or more, then cavity wall insulation if the cavities are empty, then the floor. Where walls are solid or the cavity can't be filled, EWI on the side and rear walls is the next step.
Can I have external wall insulation on just my half of a semi?
Yes. Side and rear walls are straightforward; on the front, the look of the pair matters, so we match the neighbour or keep EWI to the side and rear. Planning may be needed where the appearance changes.
Is cavity wall insulation worth it on a 1930s semi?
Usually, if the cavities are empty and the walls are in good condition. 1930s cavities can be narrow and the ties older, so we check both with a borescope survey first.
What grants are available for insulating a semi?
Depending on eligibility, ECO4 and Warm Homes: Local Grant may fund loft, cavity and other insulation for qualifying households. Our 60-second checker shows what's likely.
Begin your project
Price insulation for your home in two minutes
Tell us your property type and see a price range, with grants and 0% VAT applied where they fit. A survey always confirms the final figure.