You’ve seen the diagram — a house with arrows and suspiciously precise percentages flying out of every surface. The truth is looser but more useful: every house leaks differently, and the right order of works follows payback, not percentages. This guide gives you the honest proportions and the sequence we recommend, drawn from 500+ installations and 590 real quotes across London and the South-East — including which measure we tell customers to do first even though it’s the cheapest one we sell.
The real proportions (and why they’re approximate)
For a typical uninsulated solid-wall house — the Victorian and Edwardian stock that dominates inner London — the rough picture looks like this:
| Escape route | Rough share of loss | Typical fix |
|---|---|---|
| Walls | Largest single share in solid-wall homes | EWI or IWI |
| Roof | Roughly a quarter, if uninsulated | Loft top-up to 270mm |
| Floor | Smaller, but felt strongly | Insulation between joists |
| Draughts | Meaningful and cheap to cut | Sealing and airtightness |
| Glazing | Modest — small total area | Usually last on payback |
Notice the hedging — “roughly”, “typically”. That is deliberate. The neat percentages on marketing diagrams imply a precision that doesn’t exist: the split depends on your wall construction, how much loft insulation is already up there, whether the ground floor is suspended timber or solid concrete, and how leaky the fabric is. What is robust is the ranking, and the ranking is what should drive your spending order.
The roof: a quarter of the loss, the smallest bill
Heat rises, the loft sits on top of every heated room, and an uninsulated roof lets roughly a quarter of your heat straight out. What makes the roof special isn’t the share — the walls usually lose more — it’s the price of fixing it. A cold-loft top-up to 270mm of mineral wool costs £400–£900 with us: no scaffolding, no redecoration, done in a day. No other measure touches that ratio of heat saved to money spent, which is why it pays back in a few years and why it is the first line of almost every whole-house plan we write.
An installer’s detail the diagrams never show: the top-up is only as good as its weakest square metre. An uninsulated loft hatch, downlighter gaps, and wool pulled back from the eaves and never re-laid all act like plugholes in a full bath. When we do a top-up we insulate the hatch, keep the eaves ventilation clear, and — where the loft is used for storage — raise the boarding above the wool rather than crushing it flat, which is why loft boarding appears on 45 of our real quotes. Recent example: a 40m² top-up with boarding in Wimbledon (SW19) at £2,448. Full details on the loft insulation service page.
The walls: the biggest share in solid-wall homes
Walls are most of your home’s external surface, so once the loft is sorted they dominate what’s left — and in a solid-wall house they were typically the largest share all along. A 9-inch solid brick wall simply conducts heat outdoors; there is no cavity to slow it down. This is why solid-wall houses feel cold at the same thermostat setting as a modern flat: the walls themselves are cold, and you feel that radiant chill regardless of air temperature.
The fixes are external wall insulation — wrap the outside, median project £17,300 across our 170 priced EWI quotes — or internal wall insulation, room by room from the inside, median £5,550 across 108 quotes and £1,800–£4,000 for a single room. Walls are the biggest investment on this page, but on solid-wall homes they are also the biggest single comfort and bill improvement available. If you’re not sure which wall type you have, start with our cavity wall vs solid wall guide.
Floors, draughts and glazing: the rest of the story
Floors. A suspended timber ground floor sits over a void that is deliberately ventilated to keep the joists dry — which means winter air is moving under your floorboards all season. The share of total heat loss is smaller than roof or walls, but the comfort effect is outsized: cold boards underfoot and draughts rising through every gap. Insulating between the joists (from below where there’s a crawl space, otherwise by lifting boards) runs £76–£110/m² on our quotes; a Camberwell (SE5) suspended floor job came to £1,250. See the floor insulation service page.
Draughts. Uncontrolled air leakage — gaps at floorboards, skirtings, the loft hatch, old vents and pipe penetrations — carries heat out as warm air rather than through the fabric. It is the cheapest loss to cut, and it is why proper insulation work is as much about airtightness as thickness: we include airtightness and vapour-membrane items on 73 of our quotes.
Glazing. Windows lose heat quickly per square metre, but there aren’t many square metres of them, so their share of total loss is modest — and replacement glazing is expensive relative to the heat it saves. If your windows are single-glazed and rotten, replace them for many good reasons; just don’t do it instead of the loft and walls and expect the bills to transform.
The order of works for fastest payback
- Loft first. £400–£900, fastest payback of anything here. If your loft has less than 270mm, this is your next phone call.
- Draught-proofing alongside. Cheap, quick, and best done with the loft work while trades are in.
- Floor next, if you have a suspended timber ground floor — median project £4,320 across our 59 priced floor quotes, and a disproportionate comfort win.
- Walls when budget allows. The biggest spend and the biggest prize on solid-wall homes. EWI if the outside is fair game, IWI where it isn’t — compared honestly in our EWI vs IWI guide.
- Glazing last on pure payback — or whenever the frames fail, which is a different decision.
Real prices for every step, from 454 priced quotes and 35 published installed jobs, are in our 2026 London price report and the insulation cost guide.
The honest caveat: when the diagram misleads
The order above assumes an uninsulated older house. If your loft already has 270mm, topping it further is near-pointless — the first inches of insulation do most of the work, and each additional layer saves less than the one before. If you have insulated cavity walls, wall insulation drops far down your list. And if your real problem is damp, mould or a failing roof, fix that first: insulation over an unresolved defect hides it, and we price damp and mould remediation on quotes precisely so it gets done before the boards go on, not after.
The point of a survey is to replace the generic diagram with your house’s actual picture. Get a free, fixed, itemised quote — if the answer is “your loft is fine, spend nothing”, that is what we’ll tell you.
Heat loss FAQs
Where does a house lose most of its heat?
In a solid-wall home with no insulation anywhere, the walls are typically the largest single escape route, with roughly a quarter of the heat going through an uninsulated roof, and the rest split between floors, draughts and glazing. The exact shares vary house to house — treat any precise-sounding percentage as approximate — but the ranking (walls, then roof, then everything else) holds for most older London housing.
What insulation should I do first?
Almost always the loft. A cold-loft top-up costs £400–£900, needs no scaffolding or redecoration, and pays back faster than any other measure because it is so cheap relative to the heat it saves. After that: the floor if you have a suspended timber ground floor, then the walls — which cost the most but, on solid-wall homes, save the most.
Is it worth insulating if I already have double glazing?
Yes — usually more so. Windows are a small fraction of a home’s external surface, so even good glazing addresses only a modest slice of total loss. Upgrading glazing before insulating an uninsulated roof or solid walls is doing the jobs in reverse payback order: glazing is among the most expensive heat saved per pound, a loft top-up among the cheapest.
How much heat goes through the floor?
A suspended timber ground floor over a ventilated void is a real but smaller loss route than roof or walls — and it punches above its weight on comfort, because cold air moving under the boards makes the floor feel cold and drives draughts up through the gaps. Insulating between the joists deals with both, at £76–£110/m² for suspended timber floors on our quotes.
Do draughts really matter compared to insulation?
Draughts are the cheapest thing on the whole list to fix and the fastest to feel. Air leakage through gaps — floorboards, skirtings, loft hatches, redundant vents — carries heated air straight outside. Sealing is low-cost, and good insulation work includes it: we carry airtightness and membrane items on 73 of our quotes because insulation without airtightness underperforms.