Where a UK Home Loses Heat and Why It Matters First

Every home leaks heat, but not evenly, and the pattern is remarkably consistent across UK housing. In a typical uninsulated home, around a third of all the heat you pay for escapes through the walls, about a quarter goes through the roof, 10 to 15% is lost through the ground floor, and the remainder leaves through windows, doors and draughts¹. Those proportions are what make the "fabric first" idea concrete: the biggest losses sit in the biggest surfaces, so the order in which you insulate should follow the size of the leak, not the appeal of the gadget.

The stakes are higher in the UK than almost anywhere nearby. The UK has the highest rates of building heat loss in Western Europe, in part due to the age of its building stock³, and heating our homes accounts for 15% of the UK's total greenhouse gas emissions⁴. Every kilowatt-hour that leaks out through an uninsulated cavity or a draughty floorboard is energy a household has bought and never felt.

The breakdown by element: where the heat goes

The figures below come from several independent UK bodies, and they agree closely. Uswitch, citing the Energy Saving Trust, gives a typical uninsulated home's split as 33% through the walls, with 22 to 32% through other areas: windows, doors and areas of ventilation or through draughts¹. The Centre for Sustainable Energy puts the walls at about 35%, the roof at about 25% and the windows at only about 10%². Energy Saving Trust's own figures put the roof at around 26% and windows at around 18% of an uninsulated home's heat loss⁵.

Two things shape how these numbers apply to an individual house. The first is house type: large, detached houses and bungalows lose a high proportion of their heat through the roof, because they have more roof area relative to their volume than a mid-terrace⁶. The second is what has already been done. These percentages describe an uninsulated home; once the loft is done, the walls' share of what remains grows, which is why the order of work matters as much as the work itself. The page on what order home energy improvements should be done in works through that sequencing in detail.

Walls: the biggest single route out

Across every source, the walls come first. Uninsulated homes lose more than a third of their heat through the external walls⁸, and the Energy Saving Trust estimates that an un-insulated dwelling loses a third of all its heat through the walls and a further quarter through the roof⁹. The Federation of Master Builders puts the figure at approximately 35%⁷, and the Centre for Sustainable Energy's advice is the same: you actually lose about 35% through the walls².

Why so much? Because walls are the largest external surface most homes have, and because the UK's wall types vary enormously in how well they resist heat. A cavity wall built after the 1920s can be filled with insulation relatively cheaply; a solid brick or stone wall, common in pre-1919 terraces across England, Wales and Scotland, cannot, and needs either internal or external insulation, both of which are bigger, costlier jobs. Internal solid wall insulation should be fitted by a professional installer¹⁰.

For a household thinking about energy independence, the walls are the first place the money goes because no heating system, heat pump or otherwise, can cheaply compensate for a fabric that loses a third of its heat before it reaches you. The fabric first approach exists for exactly this reason.

Roofs: a quarter or more escapes through the top

Heat rises, and the roof is where it leaves. Around 26% of an uninsulated home's heat will escape through the roof⁵; the Centre for Sustainable Energy says about 25%²; and the Planning Portal's guidance is starker still: without loft insulation you could be losing as much as a third of your heating costs through your roof¹¹. The Energy Saving Trust's estimate, quoted in the Scottish House Condition Survey, is a quarter⁹.

The variation between a quarter and a third reflects house type and roof construction. Large, detached houses and bungalows lose a high proportion of their heat through the roof⁶, because their roof area is large relative to the enclosed volume, while a flat in a mid-floor apartment loses almost nothing through a roof it does not have. Loft insulation is also the cheapest of the big fabric measures per unit of heat saved, which is why it almost always comes first in any sensible order of work, ahead of the walls and well ahead of the floor.

Floors: 10 to 15%, and how to tell yours is leaking

Floors are the least-discussed element, but the figures are consistent. With 10% of your home's heat escaping through the floors, underfloor insulation can make a big difference to your bills⁷. Which? puts it higher: up to 15% of lost heat goes through the ground floor of your home, so it should be insulated if possible⁶, and as much as 15% of the heat in a room can be lost through uninsulated ground floors¹². Scottish Government analysis of retrofit possibilities puts floor insulation, for the ground floor only, at a 15% saving on heat demand¹³.

The signs of a leaking floor are physical as well as thermal. A suspended timber ground floor with air bricks or vents in the outside wall is drawing cold outside air through the underfloor void, and households often report cold ankles, draughts at floor level and floorboards that feel cold underfoot in winter. A solid concrete slab behaves differently: it loses heat more slowly but steadily into the ground below, and feels cold to the touch even without draughts.

One limit is worth stating plainly: a floor cannot be treated if there is a room below it that contains a heat emitter such as a radiator¹⁴. Upper floors above heated rooms are not a heat loss route in the same way, which is why floor insulation is always a ground-floor measure.

Windows, doors and draughts: air leakage

The window figures differ more than any other element, and the difference is instructive. The Centre for Sustainable Energy says only about 10% of heat is lost through the windows²; Energy Saving Trust says around 18% of an uninsulated home's heat is lost through windows⁵. Uswitch's figure, citing the Energy Saving Trust, is 22 to 32% through other areas: windows, doors and areas of ventilation or through draughts¹.

The gap between 10% and 18% is the glazing itself; the gap between 18% and 22 to 32% is air leakage. Draughts through gaps around windows, doors, letterboxes, floorboards and loft hatches carry warm air out and cold air in, and that loss is separate from the heat that conducts through the glass. It is also, usually, the cheapest loss to fix: draught-proofing is a low-cost measure that pays back quickly, and the page on how much draught-proofing saves covers it in detail. The related page on how much heat is lost through windows and doors takes the glazing side.

For a household weighing replacement windows against insulation, the numbers say insulation first: the fabric losses through walls and roof are two to three times the window losses, and no window upgrade recovers heat that is leaving elsewhere.

Measuring heat escape through your home's materials

A U-value states how much heat passes through a square metre of a building element for every degree of temperature difference, in watts per square metre kelvin (W/m²K). The lower the number, the better the element resists heat loss. Building regulations use U-values to set the minimum standard for each element when work is done: the installation of insulation in your floor must meet the minimum energy efficiency values set out in the Approved Documents¹⁵, and the same requirement applies in Wales¹⁶. For wet heating systems, guidance has been that ground floors and those in contact with the outside should be insulated to limit heat losses to not more than 10 W/m²¹⁷.

U-values also underpin the energy ratings a household actually sees. A fabric metric, the Fabric Energy Efficiency Standard or FEES, is currently used to assess compliance with Part L of the Building Regulations for domestic properties¹⁸, and a home's floor construction is recorded in its energy assessment: the RdSAP10 specification lists the lowest floor options as unknown, solid, basement floor, suspended timber, or suspended not timber¹⁹.

The crucial caveat is that design values are not measured values. Independent field testing of UK dwellings found that fabric heat losses can be between 50% and 60% more than design predictions²⁰. In a co-heating study of 27 new build non-Passivhaus UK dwellings, the average difference between designed and measured performance was +50 W/K, that is 50% greater than predicted, with two buildings losing twice as much heat as predicted²⁰.

Floor insulation: methods for the two floor types

Almost all UK homes will have one of two types of floor: a suspended timber floor, with boards laid over wooden joists, or a solid concrete floor²¹. The method follows the floor.

Suspended timber floors are insulated either by lifting the boards and laying insulation between the joists, or, where there is an accessible underfloor void or basement, by fitting insulation from below without disturbing the floor above. Mineral wool, rigid boards or suspended quilt each work between joists. This is the variant a competent DIYer can take on, and doing it yourself changes the economics: insulating your floor and skirting boards can save as much as £100 a year and, if you can install it yourself, pay for itself in around two years¹². The DIY or professional page sets out which measures sit on each side of that line.

Solid concrete floors are insulated by breaking out the slab, laying insulation beneath a new screed or slab, and relaying the floor. In almost all cases this is a job for a professional builder²¹. You have to insulate under a replacement concrete floor to follow building regulations¹⁰, so a worn-out slab is also an opportunity. Under the ECO4 scheme, separate scores exist for both variants, underfloor insulation (UFI) and solid floor insulation (SFI)¹⁴.

Floor insulation: price and payback

The cost side is straightforward and installer-quoted: around £1,400 to £2,500 for suspended floor insulation with professional installation¹, with floor insulation of 150 mm thickness typical where the depth allows⁷. Solid floor insulation costs more because of the slab work involved, and no published range for it exists in the sources here, so treat it as a job for individual quotes.

The savings differ by house type and by nation, and the honest answer is a range:

The carbon saving runs alongside the bill saving: floor insulation improves your carbon footprint by about 270 kg a year in GB and 430 kg in NI¹⁰. At the lower end of the savings range and the upper end of the cost range, professional floor insulation is a slow payer, which is why it usually follows walls and loft in the order of work; the page on payback periods compared puts it alongside the other measures. Grants can change the arithmetic, and the page on checking what grant help you can get covers where to look.

Rules on consent and compliance for insulation work

Floor insulation is not paperwork-free. The installation of insulation in your floor must meet the minimum energy efficiency values set out in the Approved Documents¹⁵, and the same requirement is stated for Wales¹⁶. The trigger matters: renovation of more than 25 per cent of a solid or suspended floor involving the replacement of screed or a timber floor deck brings the full requirement into play¹⁵. However, if such an upgrade is not technically or functionally feasible, the element should be upgraded to the best standard which can be achieved within a simple payback of no greater than 15 years¹⁵.

Loft work has its own rule: installing insulation to your loft area requires an application for approval under the Building Regulations²². Approved Document L Volume 1 also carries requirements for new dwellings, including that for wet heating systems in new dwellings with a floor area of 150 m² or greater, a minimum of two independently controlled heating circuits should be provided²³.

Planning permission is usually not required for internal insulation, although both planning and listed building consent may be required in some situations²⁴. For listed buildings the position is firmer: more disruptive work like internal or external insulation will usually need Listed Building Consent²⁵. Householders in conservation areas or listed properties should also read the page on doing energy work as part of a renovation, because bundling measures can spread the consent and compliance cost.

Fabric first: the route to better EPC scores and self-sufficiency

The fabric numbers are not just about bills; they are the foundation of everything else a household might do. Installing loft and cavity wall insulation not only lowers the amount of heat escaping, but could also cut your fuel bills²⁶, and a home that loses less heat needs a smaller heating system, less electricity, and less storage to run through a cold snap. That is the independence argument in one line: reduce demand before you try to supply it. The page on whether you need insulation before a heat pump makes the same point for low-carbon heating.

The ratings regime is catching up with that logic. EPC ratings are currently based on the costs of home heating, rather than its carbon footprint, which means that cheaper heating systems can score well despite higher emissions²⁷. Reform is under way across the nations. The Climate Change Committee has recommended that domestic EPCs should include four primary metrics, using real-world units and clear simple names: Energy (total energy use intensity, kWh/m²/yr), Fabric (space heating demand intensity, kWh/m²/yr), Heating (heating system type, ranked 1 to 6) and Cost (energy cost intensity, £/m²/yr)²⁸. Scotland has announced a new EPC rating system for domestic buildings, which will give clearer information on the fabric energy efficiency of a property, the emissions, efficiency and running costs of its heating system, and the cost of energy to run the home to standardised conditions²⁹, and under the proposed Heat in Buildings Bill a good efficiency rating means EPC C, achieved when a home requires less than 120 kWh/m²/year of energy for space heating only³⁰.

The enforcement dates give this teeth. Landlords will be required to ensure their properties meet a minimum score of EPC C against the fabric performance metric and either the heating system or smart readiness metric by 1 October 2030³¹. Scotland's Heat in Buildings Strategy states that by 2045 all of our homes and buildings will need to significantly reduce their energy use and use a zero emissions heating system³², and the Climate Change Committee's pathway for Scotland has 92% of existing homes with low-carbon heat by 2045, reaching all homes by 2050³³.

The data shows the gap between where homes are and where this is heading. Around two-thirds of Council or Housing Association houses have EPC ratings A to C, compared to around 40% of houses in other tenure types³⁴, and around a third of domestic properties in England and Wales do not have an EPC rating at all³⁴. More energy efficient houses, as measured by the EPC rating, tend to have lower gas consumption, even after adjusting for floor area³⁴, which is the fabric-performance link made real in billing data.

For a household planning a route to lower bills and less dependence on supplied energy, the sequence the numbers support is: find out where your home loses heat, fix the biggest leaks first, and use the EPC and its recommendation report or a whole house plan to put the measures in order. The getting started guide covers that route from the beginning.