Airtightness, vents and mechanical ventilation
A building's airtightness determines how much uncontrolled air exchange occurs between inside and outside. Leakier buildings lose more heat through air movement. But some ventilation is deliberate and necessary — for air quality, for combustion safety, and because building regulations require it.
This page explains the different paths air takes through a building and how each one affects ventilation heat loss.
Why airtightness matters
Airtightness describes how well a building's envelope resists uncontrolled air leakage. A tight building lets less air in and out through gaps and cracks; a leaky one lets more. Every cubic metre of warm air that escapes has to be replaced by cold outside air, and the heating system has to warm that replacement air from scratch.
Airtightness is measured formally using a blower door test. A fan pressurises the building to 50 Pa and measures how much air leaks out. The result is expressed as air permeability in m³/(h·m²) — the leakage rate per square metre of building envelope. A modern well-sealed house might achieve 3 m³/(h·m²). A draughty Victorian terrace might be 15 or higher.
Most existing homes have never had a blower door test. In that case, the air leakage rate has to be estimated from observable evidence: the age and type of building, the condition of windows and doors, whether floors are sealed, and whether there are obvious draught paths. This is where a SAP estimation method is commonly used — it builds up an air permeability figure from the building's characteristics rather than a direct measurement.
Background vents and trickle vents
Background ventilators — usually trickle vents built into window frames — provide a controlled path for fresh air to enter a building. They are required by building regulations in most new and refurbished homes to maintain indoor air quality.
From a heat loss perspective, trickle vents add to the ventilation rate. Each vent has a known equivalent area, and the total equivalent area of all background ventilators in a room contributes to the calculated air flow. More vents mean more air exchange, which means more ventilation heat loss.
This creates a tension that confuses many homeowners: trickle vents increase heat loss, but they exist for a good reason. Blocking them reduces ventilation heat loss on paper but can cause condensation, mould and poor indoor air quality. The heat loss calculation includes them because they are a real part of how the building exchanges air.
Intermittent extract fans
Kitchens and bathrooms typically have intermittent extract fans that run when the room is in use. These pull warm, moist air out of the building. While they do not run continuously, they contribute to ventilation heat loss during their operating periods and are accounted for in the ventilation calculation.
Mechanical ventilation with heat recovery (MVHR)
An MVHR system provides continuous mechanical ventilation through a central unit with a heat exchanger. Stale air is extracted from kitchens, bathrooms and utility rooms. Fresh air is supplied to living rooms and bedrooms. Inside the unit, the outgoing warm air passes through a heat exchanger that warms the incoming cold air before it enters the building.
A well-installed MVHR system can recover 50 to 90 percent of the heat from the extracted air. This dramatically reduces the ventilation heat loss compared to a naturally ventilated building with the same air change rate. The building still gets fresh air, but without throwing away most of the heat that the extracted air carried.
The catch is that MVHR only works well in an airtight building. If the building is leaky, uncontrolled infiltration overwhelms the controlled ventilation and the heat recovery unit cannot recover heat from air that bypasses it through cracks and gaps. This is why MVHR is typically paired with airtightness targets of around 3 m³/(h·m²) or better.
Suspended floors and unsealed junctions
Some of the biggest air leakage paths in older homes are not around windows and doors — they are under the floor and at structural junctions.
Suspended timber floors sit above a ventilated void. Air bricks in the external walls ventilate the sub-floor space to prevent rot, but the gaps between floorboards, around skirting boards and at the junction between floor and wall allow cold air from the void to enter the living space. In some older houses, this is the single largest infiltration path. The effect on ventilation heat loss can be significant — far more than a few draughty windows.
Disused flues are another common leakage path. An open chimney or an unsealed flue acts as a warm air chimney, pulling heated room air up and out of the building continuously. Even a capped but unsealed flue allows measurable air movement.
Unsealed junctions between walls and ceilings, loft hatches without draught strips, and service penetrations (pipes, cables, recessed lights into the loft space) all contribute to the total air leakage of the building. In a poorly sealed older house, these paths together can produce an air change rate that dominates the heat loss picture.
How survey evidence feeds the ventilation calculation
In Heatworx, the ventilation heat loss for each room is not a single fixed assumption. It is built up from observable survey evidence about the building.
The survey captures whether windows and doors are draughty or draught-stripped, whether the building has suspended timber floors, whether there are background ventilators (trickle vents), whether extract fans are present, and whether a mechanical ventilation system is installed. These inputs feed into the air permeability and ventilation rate calculation, which in turn determines how much heat each room loses to air movement.
Where no blower door test result is available, Heatworx uses a SAP-based estimation approach to derive an air permeability from these building characteristics. The inputs are editable — if you know the building is tighter or leakier than the estimate suggests, you can adjust the assumptions.
This matters because ventilation heat loss is one of the most uncertain parts of any heat loss calculation. Making the inputs visible and editable is more honest than hiding them behind a single fixed air change rate.
Frequently asked questions
Does MVHR reduce heat loss?
Yes, significantly. An MVHR system recovers 50 to 90 percent of the heat from extracted air, so the incoming fresh air arrives already partly warmed. This reduces the effective ventilation heat loss compared to natural ventilation with the same air change rate. The reduction is largest in airtight buildings where most air movement passes through the heat recovery unit rather than bypassing it through cracks and gaps.
Should I block up trickle vents to reduce heat loss?
No. Trickle vents are there for a reason — they provide the minimum controlled ventilation needed for indoor air quality and to reduce the risk of condensation and mould. Blocking them does reduce ventilation heat loss on paper, but it creates moisture problems that are harder and more expensive to fix than the heat loss they prevent. The right approach is to include them in the calculation and accept that some ventilation heat loss is the cost of a healthy indoor environment.
How does a suspended floor affect ventilation heat loss?
A suspended timber floor with a ventilated sub-floor void can add a substantial amount of infiltration. Cold air from the void enters the living space through gaps between floorboards, around skirting boards and at floor-wall junctions. In older houses with original timber floors that have not been sealed, this can be one of the largest single contributors to air leakage. The effect is included in the ventilation calculation because it is a real and often dominant path for uncontrolled air movement.