How draughts and ventilation affect heat loss

Heat does not only escape through walls and windows. It also leaves with the air. Every time warm air leaks out through gaps in the building fabric, cold air replaces it and the heating system has to warm that replacement air from scratch. This is ventilation heat loss.

In a draughty older house, ventilation heat loss can account for a third or more of the total. In a modern, well-sealed building it may be much less. Either way, it is the second of the two mechanisms — alongside fabric heat loss — that together make up every room's design heat loss.

What ventilation heat loss is

Ventilation heat loss is the heat carried away by air movement through and out of a building. Warm indoor air escapes and is replaced by cold outside air. The heating system then has to raise the temperature of that incoming air to the room's target — and that takes energy.

The amount of energy depends on how much air is exchanged and how large the temperature difference is. More air movement and a bigger temperature gap both mean more heat loss.

Infiltration vs intentional ventilation

Air enters and leaves a building through two distinct routes, and it matters which one dominates:

Infiltration

Infiltration is unintentional air leakage through gaps and cracks in the building fabric. It happens through poorly sealed window frames, gaps around doors, cracks at wall-floor junctions, service penetrations, letterboxes and loft hatches. Wind pressure and the stack effect (warm air rising) drive air through these paths continuously.

Infiltration is uncontrolled, unmeasured and often invisible. You can feel a draught from a badly sealed window, but much of the leakage happens through paths you cannot see or easily fix.

Intentional ventilation

Intentional ventilation is air movement through designed openings: trickle vents in window frames, background ventilators in walls, extract fans in kitchens and bathrooms, and mechanical ventilation systems. These are deliberate — they provide fresh air and remove moisture.

In a well-sealed modern home, intentional ventilation may be the main source of air movement. In a leaky older building, infiltration usually dominates and the intentional vents add relatively little on top.

Why draughts matter

Draughts are the symptom of infiltration. A draughty room is one where cold air enters fast enough to feel it — around windows, under doors, up through suspended timber floors, around pipe runs. Beyond the discomfort, every cubic metre of cold air that enters must be heated, and that costs energy.

Draughts also create localised cold spots. Even if the average air temperature in a room reaches the target, a draught path near a seating area can make the room feel cold. This is one reason draughty rooms sometimes end up with oversized radiators — the heating output is there, but the comfort is not.

Simple draught-proofing — sealing around windows and doors, blocking unused chimneys, fitting draught excluders — is one of the most cost-effective ways to reduce ventilation heat loss.

Air change rate and heat loss

The air change rate (ACH) describes how many times the entire volume of air in a room is replaced per hour. An ACH of 1.0 means the room's full volume of air is exchanged once every hour. An ACH of 0.5 means half the volume is exchanged per hour.

To calculate ventilation heat loss, you first need the ventilation rate in cubic metres per hour:

The ventilation heat loss formula then uses a factor of 0.34, which represents the volumetric heat capacity of air — the energy required to raise one cubic metre of air by one degree:

Typical air change rates in domestic buildings vary widely. A 1980s house with original windows and no draught-proofing might have an effective ACH around 1.5. A post-2022 new build might achieve 0.5 or less. A Passivhaus-standard dwelling targets around 0.6 ACH at 50 Pa pressure — which translates to roughly 0.03 ACH under normal conditions.

Why airtightness assumptions are uncertain

Air permeability can be measured with a blower door test — a fan pressurises the building and the resulting airflow is recorded. But in practice, most existing homes have never been tested. The air change rate used in a heat loss calculation is usually estimated from the building's age, type, observable evidence of draughts and the presence or absence of draught-proofing.

This makes ventilation heat loss one of the most uncertain parts of the calculation. Two apparently similar 1970s semi-detached houses could have very different airtightness depending on whether windows have been replaced, floors have been sealed, or chimneys have been capped.

The uncertainty is real, but it does not make the calculation useless. It means the ventilation assumptions deserve attention. If a house feels noticeably draughty, the air change rate is probably higher than a default assumption. If extensive draught-proofing has been done, it may be lower. An honest estimate that acknowledges this uncertainty is more useful than a false precision that hides it.

Worked example

How this appears in Heatworx

In Heatworx, ventilation heat loss is not a single hidden number. It is built from survey evidence that you can see and edit: draughty windows and doors, suspended floors, background vents, extract fans and the overall airtightness assessment for the building.

These inputs feed the ventilation rate for each room. The app shows the ventilation heat loss contribution separately from fabric heat loss, so you can see how much of a room's heat loss comes from air movement versus conduction through surfaces.

If you know the house is particularly draughty — or particularly well sealed — you can adjust the evidence inputs and see the effect on the result. The aim is a ventilation estimate that reflects what you actually observe, not a generic assumption that may be wrong by a factor of two.

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