How walls, windows and floors lose heat

Heat moves from warm to cold. Every surface that separates a heated room from the outside — or from an unheated space — conducts heat outwards. This is fabric heat loss, and it is usually the largest component of a room's total heat loss.

Understanding which surfaces lose the most heat, and why, is the starting point for any useful heat loss estimate. It also explains why two rooms of the same size can have very different heating requirements.

What fabric heat loss means

Fabric heat loss is the heat conducted through the solid parts of a building's envelope. It happens continuously whenever the inside is warmer than the outside. The rate depends on the materials — a thin sheet of glass conducts heat much faster than an insulated cavity wall.

This is distinct from ventilation heat loss, which is heat carried away by air movement. Both contribute to every room's total, but fabric losses are driven by the physical construction of walls, windows, floors, ceilings and doors.

Surfaces that lose heat

Any surface separating a heated room from a colder space contributes to fabric heat loss. The main ones are:

External walls — typically the largest surface area and a major source of heat loss, especially if uninsulated
Windows — glass is a poor insulator, so even a modest window area can lose significant heat
Ground floors — heat lost downwards into the ground, influenced by floor type and insulation
Ceilings and roofs — heat rises, and an uninsulated loft or flat roof can lose heat quickly
External doors — often overlooked, but a solid timber door or glazed door adds to fabric losses

Internal walls between two heated rooms at the same temperature do not contribute, because there is no temperature difference to drive heat flow. A wall between a heated room and an unheated hallway or garage does contribute — just with a smaller temperature difference.

The fabric heat loss formula

The heat loss through any single surface is calculated from three things: its area, its thermal transmittance (the U-value), and the temperature difference across it.

Formula

Q = A × U × ΔT

where Q is heat loss in watts, A is the surface area in m², U is the U-value in W/m²K, and ΔT is the temperature difference in °C (or K)

A room's total fabric heat loss is the sum of Q for every external surface: each wall section, each window, each door, the floor and the ceiling. Each surface has its own area and U-value, so each contributes a different amount.

Why U-values matter

The U-value measures how easily heat passes through a material or construction, in watts per square metre per degree of temperature difference (W/m²K). A lower U-value means better insulation and less heat loss.

The difference between constructions is dramatic. Single glazing has a U-value around 4.8 W/m²K. Modern double glazing might be 1.6 W/m²K. That means the single-glazed window loses roughly three times as much heat per square metre. For walls, the range is equally wide: an uninsulated solid brick wall at around 2.0 versus a modern insulated cavity wall at 0.3 or less.

Because U-values appear directly in the formula, they have a proportional effect on the result. Halving the U-value halves the heat loss through that surface. This is why getting U-values right matters so much in a heat loss calculation.

How construction type affects U-value

U-values are not measured for each individual wall in practice. Instead, they are estimated from the construction type — the materials, the number of layers and whether insulation is present.

A pre-1920 solid brick wall has a very different thermal performance to a 1970s unfilled cavity wall, which again differs from a modern insulated cavity. The same applies to floors (solid concrete vs suspended timber, insulated vs not) and to glazing (single, double, triple, with or without low-emissivity coatings).

In practice, a surveyor identifies the construction type from observable evidence — the building's age, wall thickness, brick pattern, window frames — and assigns a U-value accordingly. This is why heat loss calculations depend heavily on getting the construction type right.

Why old buildings are harder to estimate

Older buildings present genuine uncertainty. A Victorian terrace might have solid brick walls with lime mortar, partial pointing repairs with cement, patches of internal dry lining, and windows replaced at different times. You cannot see inside the wall without invasive investigation.

Cavity walls from the 1950s-70s may or may not have been retrofitted with blown insulation — and even if they have, the insulation may have settled or degraded. Loft insulation varies from nothing to 300mm, sometimes in the same house.

This does not make a heat loss calculation pointless for older buildings. It means the estimate carries more uncertainty, and the person reviewing it should pay extra attention to the construction assumptions. A transparent calculation that says "I assumed this is an uninsulated cavity wall at U 1.5" is far more useful than a black-box number with no explanation.

Worked example

Example: One external wall, uninsulated vs insulated

A room has a 12 m² external wall (after subtracting the window area). The indoor target temperature is 21°C and the design outside temperature is −1°C, giving a temperature difference of 22°C.

Uninsulated cavity wall (U-value 1.5 W/m²K)

Q = 12 × 1.5 × 22 = 396 W

Same wall, insulated cavity (U-value 0.35 W/m²K)

Q = 12 × 0.35 × 22 = 92 W

Insulating the cavity reduces the heat loss through that wall from 396 W to 92 W — a reduction of over 75%. That single change could meaningfully affect the radiator size needed for the room.

Of course, the wall is only one surface. The room also loses heat through windows, the floor, the ceiling and via ventilation. But this comparison shows why construction assumptions have such a large effect on the result.

How this appears in Heatworx

In Heatworx, every room's fabric heat loss is calculated surface by surface. Each wall, window, door, floor and ceiling has its own area — captured from a room scan or entered manually — and its own construction type with a corresponding U-value.

The construction assumptions are not hidden. Each surface type is editable: if you know the wall has been insulated, you can change the construction type and the U-value updates accordingly. The app shows the heat loss contribution from each surface, so you can see which parts of the room envelope drive the result.

This transparency matters. If you disagree with an assumption — perhaps you know the cavity has been filled, or you suspect the floor is uninsulated — you can change it and see the effect immediately. The aim is a heat loss estimate you can understand and review, not a number you have to take on faith.

Limitations and assumptions

Fabric heat loss calculations rely on assumptions that are worth understanding:

U-values are estimates based on assumed construction types. Unless you have had a wall core-drilled or have original building specifications, the actual U-value is uncertain.
Areas depend on geometry that may have been measured, scanned, or estimated. Small errors in room dimensions propagate into every surface area.
Thermal bridging at junctions, lintels and corners adds extra heat loss beyond what the surface-by-surface calculation captures. This is usually handled with a simplified allowance rather than detailed modelling.
Mixed constructions are common in older buildings — one wall may have been partially insulated or dry-lined, while another has not. The calculation assumes uniform construction across each surface.

These limitations do not make the calculation unreliable. They mean the result is a design estimate, not a laboratory measurement, and that the construction assumptions deserve careful review.

Frequently asked questions

What counts as fabric heat loss?

Fabric heat loss is heat conducted through the solid parts of the building envelope — external walls, windows, doors, ground floors and roofs or ceilings above unheated spaces. It does not include heat carried away by air movement, which is ventilation heat loss.

How do windows affect heat loss?

Windows typically have much higher U-values than walls, so they lose more heat per square metre. Single glazing (U ~4.8) loses roughly three times as much heat as standard double glazing (U ~1.6). Even though window areas are usually smaller than wall areas, they can account for a significant share of a room's fabric heat loss — especially in rooms with large or bay windows.

Why is an old wall harder to estimate?

Because you often cannot see what is inside it. A Victorian solid brick wall might have been dry-lined internally, or partially patched. A 1960s cavity wall may or may not have been retrofitted with insulation. Without invasive investigation, the construction type — and therefore the U-value — has to be estimated from age, thickness and external evidence. This is the largest single source of uncertainty in most domestic heat loss calculations.

What is the difference between fabric and ventilation heat loss?

Fabric heat loss is heat conducted through solid surfaces — walls, windows, floors, roof and doors. Ventilation heat loss is heat carried away by air movement — draughts, open vents, extract fans and general air leakage through gaps in the building. Both contribute to the total heat loss of a room, and both are calculated separately in a room-by-room heat loss estimate.

Calculation note

The fabric heat loss formula (Q = A × U × ΔT) and U-value ranges referenced in this guide are informed by recognised UK domestic heating design principles, including CIBSE guidance and Building Regulations Approved Document L maximum U-values. Heatworx calculates fabric heat loss per surface using the construction type and U-value selected for each element.