HEM Ventilation & Infiltration Model — HEM-TP-06 Technical Reference

The Home Energy Model (HEM) models ventilation and infiltration using a pressure-driven methodology defined in technical paper HEM-TP-06, based on BS EN 16798-7:2017. At each half-hourly timestep, HEM calculates the pressure difference across every opening in the building envelope — accounting for wind speed, wind direction, indoor–outdoor temperature differences (stack effect), and mechanical system operation — then derives the resulting air flow rate through each opening using power-law relationships. This replaces SAP's simplified shelter factor approach with a physics-based simulation that captures the dynamic, time-varying nature of air movement through buildings.

Scope and Standards Basis

HEM-TP-06 covers the calculation of all air movement into, out of, and through the dwelling. This encompasses two distinct categories of air movement:

• Purpose-provided ventilation — controlled air paths designed into the building, including mechanical supply and extract systems (MVHR, MEV), trickle ventilators, and openable windows
• Infiltration — uncontrolled air leakage through gaps, cracks, and imperfections in the building envelope, quantified by the air permeability measured during a pressurisation test

The underlying calculation methodology is drawn from BS EN 16798-7:2017 (Energy performance of buildings — Ventilation for buildings — Part 7: Calculation methods for the determination of air flow rates in buildings including infiltration). This European standard defines a zonal air flow model that calculates pressure-driven air movement through a network of openings. HEM implements this standard within the broader dynamic simulation framework of BS EN ISO 52016-1:2017, coupling the ventilation heat loss calculation to the heat balance at each timestep.

For an accessible overview of how ventilation interacts with the Future Homes Standard, see our Ventilation & Part F guide. For architects and designers, see the architect guidance on airtightness strategy.

Air Paths: Purpose-Provided Ventilation vs Infiltration

HEM distinguishes rigorously between air paths that are intentionally designed into the building and those that result from imperfections in the building envelope. This distinction is fundamental to the pressure-driven model because each type of air path has different flow characteristics and responds differently to pressure differences.

Purpose-Provided Ventilation Paths

Purpose-provided ventilation paths include all openings that are deliberately incorporated into the building design to provide ventilation. In HEM, each purpose-provided opening is defined by:

• Equivalent area (A_eq) — the effective free area of the opening when in its ventilation position, expressed in m². For mechanical systems, this is replaced by the declared volume flow rate at a reference pressure
• Facade orientation — the compass direction the opening faces, which determines the wind pressure coefficient applied to it
• Height above ground — the vertical position of the opening, which affects the stack pressure acting across it
• Opening type and regime — whether the opening is always open (e.g. a continuously running MVHR extract), occupancy-controlled (e.g. a window opened by the occupant), or thermostatically controlled (e.g. a humidity-sensitive trickle vent)
• Flow exponent (n) — the exponent in the power-law flow equation, typically between 0.5 (fully turbulent flow) and 1.0 (fully laminar flow). For most ventilation openings, a value of 0.5–0.67 is appropriate

The volume flow rate through a purpose-provided opening is calculated using the power-law relationship:

Q = C × (ΔP)ⁿ

Where Q is the volume flow rate (m³/s), C is the flow coefficient derived from the equivalent area and reference conditions, ΔP is the pressure difference across the opening (Pa), and n is the flow exponent.

Infiltration Paths

Infiltration represents uncontrolled air leakage through the building envelope — through gaps around windows and doors, at junctions between building elements, through service penetrations, and through the fabric itself. Unlike purpose-provided ventilation, infiltration paths cannot be individually identified or measured in situ. Instead, HEM derives the total infiltration characteristic from the air permeability test result (blower door test).

The air permeability, expressed as q₅₀ in m³/(h·m²) at 50 Pa referenced to the total envelope area, is converted to an equivalent leakage area at a reference pressure. HEM then distributes this leakage across the building envelope proportionally to the area of each facade, creating a set of distributed infiltration paths that respond to local pressure conditions. This approach means that infiltration on the windward side of a building can differ significantly from infiltration on the leeward side at any given timestep — a level of detail entirely absent from SAP.

The Pressure-Driven Ventilation Model

At the heart of HEM-TP-06 is the pressure-driven calculation from BS EN 16798-7. At each half-hourly timestep, the model determines the net air flow through the dwelling by calculating the pressure difference across every opening in the building envelope and solving for the resulting flow rate. The total pressure difference across any opening has three components: wind pressure, stack pressure, and mechanical system pressure.

Wind Pressure Effects

Wind creates positive pressure on the windward facades of a building and negative pressure (suction) on the leeward facades and roof. The wind pressure at any point on the building envelope is calculated as:

P_w = 0.5 × ρ × C_p × v_z²

Where ρ is the air density (approximately 1.2 kg/m³), C_p is the wind pressure coefficient for the specific facade and wind direction, and v_z is the wind speed at the reference height of the building, corrected for terrain roughness and local shielding.

The wind pressure coefficient (C_p) is a dimensionless value that varies with facade orientation relative to the wind direction, building geometry, and surrounding obstructions. HEM uses standardised C_p values from BS EN 16798-7 based on the building's exposure category and shielding conditions. Typical values range from approximately +0.5 to +0.7 on the windward face to −0.2 to −0.5 on the leeward face.

Crucially, wind speed and direction vary at each timestep, drawn from the hourly weather data for the specific site location. HEM applies a terrain roughness correction to convert the meteorological wind speed (measured at 10 m height in open terrain) to the local wind speed at the building reference height, accounting for urban, suburban, or rural exposure.

Stack Pressure (Buoyancy) Effects

Stack pressure, also called the buoyancy effect, arises from the density difference between warm indoor air and cooler outdoor air. Warm air is less dense than cold air, creating a pressure distribution where the lower parts of the building experience inward pressure (outdoor air pushing in) and the upper parts experience outward pressure (indoor air pushing out). The neutral pressure level — the height at which internal and external pressures are equal — lies somewhere between the lowest and highest openings.

The stack pressure difference at height z relative to a reference height is calculated as:

ΔP_s = ρ_o × g × (z − z_ref) × (T_i − T_e) / T_e

Where ρ_o is the outdoor air density, g is gravitational acceleration (9.81 m/s²), T_i and T_e are the indoor and outdoor temperatures (K), and z − z_ref is the height difference from the reference level.

Stack effect is most significant during cold weather (when the indoor–outdoor temperature difference is greatest) and in taller buildings (where the height difference between lowest and highest openings is larger). In a typical two-storey dwelling during winter, stack-driven infiltration can contribute significantly to the total air change rate, particularly at low wind speeds when wind-driven flows are small.

Combined Pressure Calculation

At each timestep, HEM calculates the total pressure difference across every opening by combining the wind pressure, stack pressure, and any mechanical system pressure. For openings connected to a mechanical system (e.g. MVHR extract or supply terminals), the system-generated pressure is added to the natural pressure components. The net air flow through each opening is then calculated using the power-law equation, and the total ventilation rate for the dwelling is determined by summing the flows through all openings.

This combined approach means that HEM naturally captures interactions that SAP could not model:

• Wind-assisted and wind-opposed infiltration on different facades simultaneously
• The interplay between stack-driven flow and mechanical extract, particularly in tall buildings or at low wind speeds
• Cross-ventilation through openings on opposite facades under specific wind conditions
• The reduction in infiltration when a balanced MVHR system minimises the pressure difference across the envelope

MVHR Modelling in HEM

Mechanical ventilation with heat recovery is the dominant ventilation strategy for homes built to Future Homes Standard airtightness levels, and HEM models it in considerably more detail than SAP. The MVHR model in HEM accounts for four key parameters: supply and extract flow rates, heat recovery efficiency, fan power consumption, and the summer bypass function.

Heat Recovery Efficiency

The heat exchanger at the core of an MVHR unit transfers thermal energy from the warm extract air to the incoming fresh supply air. HEM models this using the declared heat recovery efficiency (η_HR) from the PCDB product data. At each timestep, the temperature of the supply air leaving the heat exchanger is calculated as:

T_supply = T_ext + η_HR × (T_extract − T_ext)

Where T_ext is the external air temperature, T_extract is the temperature of the air being extracted from the dwelling (taken as the zone air temperature), and η_HR is the heat recovery efficiency (typically 0.85–0.95 for modern units). The heat recovered at each timestep directly reduces the space heating demand by warming the incoming ventilation air.

High-efficiency heat exchangers — those with η_HR above 0.90 — are increasingly standard in FHS-targeted designs. The difference between a unit recovering 85% and one recovering 93% of heat is significant over a heating season and is properly credited in HEM's half-hourly calculation, where SAP could only apply a seasonal average.

Specific Fan Power

MVHR fans consume electrical energy to move air through the duct network. HEM models this using the specific fan power (SFP), expressed in W/(l/s), which represents the electrical power consumed per unit of air volume flow rate. The total fan energy at each timestep is:

P_fan = SFP × q_v

Where q_v is the total volume flow rate through the system (l/s). Fan energy is included as both an electrical demand (increasing the dwelling's electricity consumption) and as an internal heat gain (the fan motor heat is partially released into the conditioned space). Lower SFP values indicate more efficient fan and duct design.

Summer Bypass

During warm weather, recovering heat from the extract air is counter-productive — it would raise the supply air temperature above the outdoor temperature, increasing the risk of overheating. MVHR systems therefore include a summer bypass that routes the incoming air around the heat exchanger when heat recovery is not beneficial.

In HEM, the bypass is modelled as a control function that deactivates heat recovery when the external air temperature exceeds a threshold (typically when T_ext > T_{indoor,setpoint} or when T_ext is above a defined bypass temperature, often around 20–22°C). When the bypass is active, supply air enters at approximately the outdoor temperature (minus any duct heat gain), and the heat recovery efficiency is effectively set to zero. This interacts directly with Part O overheating assessments, where summer ventilation strategy plays a critical role.

Natural Ventilation Modelling

Although MVHR will be the dominant strategy for FHS homes, HEM must also model natural ventilation for dwellings that rely on passive air movement — whether as the primary strategy (in less airtight homes that do not target FHS compliance) or as a supplementary mechanism (e.g. purge ventilation through opening windows).

Natural ventilation in HEM is modelled through purpose-provided openings such as:

• Trickle ventilators — small, controllable openings typically located in window frames, providing continuous background ventilation. Modelled using their declared equivalent area and flow exponent
• Openable windows — large openings that can provide high flow rates when open. HEM requires the equivalent area in the open position, the opening regime (occupant-controlled or thermostatically triggered), and the facade orientation
• Passive stack ventilators — vertical ducts that draw air from wet rooms using stack effect. Modelled using the duct equivalent area, length, and terminal height

For each natural ventilation opening, HEM applies the same pressure-driven calculation as for all other air paths: the pressure difference from wind and stack effects drives air flow through the opening according to its flow characteristics. The direction of flow (inward or outward) can reverse at different timesteps depending on wind conditions and temperature differences.

This approach is particularly valuable for modelling single-sided ventilation (openings on only one facade) versus cross-ventilation (openings on opposite facades). Cross-ventilation benefits from wind-driven pressure differences across the building, while single-sided ventilation relies more heavily on stack effect and turbulent wind fluctuations — resulting in significantly lower flow rates. HEM captures this distinction naturally through the pressure-driven model, whereas SAP treated all natural ventilation in a simplified manner.

Infiltration Calculation

Infiltration — uncontrolled air leakage through the building envelope — is one of the most significant ventilation heat loss pathways, particularly in older or poorly sealed buildings. HEM's treatment of infiltration is fundamentally more sophisticated than SAP's.

Airtightness Test Data Input

The primary input for infiltration in HEM is the air permeability measured by a pressurisation (blower door) test at 50 Pa pressure difference, expressed as q₅₀ in m³/(h·m²) referenced to the total building envelope area. Current Building Regulations set the maximum permitted air permeability at 8 m³/(h·m²) at 50 Pa. The Future Homes Standard notional dwelling assumes a target of 3 m³/(h·m²) at 50 Pa — this represents a very tight building envelope where uncontrolled infiltration is minimal and controlled mechanical ventilation is essential.

Conversion to Infiltration Paths

HEM converts the 50 Pa test result into a set of distributed infiltration paths through the following process:

1. Convert to equivalent leakage area: The q₅₀ value is converted to an equivalent leakage area (ELA) at a reference pressure (typically 4 Pa) using a power-law relationship with an assumed flow exponent (typically n = 0.66 for infiltration through cracks and gaps)
2. Distribute across facades: The total ELA is distributed across the building envelope proportionally to the area of each facade and the roof/floor, creating individual infiltration paths at specific orientations and heights
3. Calculate timestep flows: At each half-hourly timestep, the pressure difference across each infiltration path is calculated from the combined wind and stack pressures, and the resulting flow rate is determined using the power-law equation

This distributed approach means that HEM correctly models directional infiltration — wind-driven infiltration on the windward facade and exfiltration on the leeward facade occurring simultaneously. SAP, by contrast, applied a single correction factor to derive an average infiltration rate for the whole building.

SAP vs HEM Ventilation Modelling

The shift from SAP to HEM represents a fundamental change in how ventilation and infiltration are modelled. The following table summarises the key differences:

For a broader comparison of SAP and HEM across all calculation modules, see SAP vs HEM — What's Changed. For the regulatory context of these changes, see FHS Compliance Pathways.

Interaction with Part F Requirements

Part F (Means of Ventilation) of the Building Regulations for England sets minimum ventilation rates for dwellings to protect indoor air quality. HEM's ventilation model must be understood in the context of these regulatory requirements, because Part F defines the minimum performance standard that any ventilation system must achieve, while HEM calculates the energy consequences of that system's operation.

Part F requires:

• Whole-dwelling ventilation rate — a minimum continuous fresh air supply calculated from the number of bedrooms and total floor area (e.g. 13 l/s for a one-bedroom dwelling, rising to 25 l/s for five bedrooms)
• Extract ventilation — minimum continuous and boost extract rates from kitchens (13 l/s continuous, 30 l/s boost), bathrooms (8 l/s continuous, 15 l/s boost), and other wet rooms
• Purge ventilation — the ability to rapidly ventilate any habitable room (typically through openable windows) to clear accidental pollutant releases

In HEM, the ventilation system must be defined with flow rates that at least meet these Part F minima. The model then calculates the energy impact of operating the system at these rates — including the heat loss through ventilation air (offset by any heat recovery) and the electrical energy consumed by fans. If the declared flow rates fall below the Part F minimum, the system would not comply with Building Regulations regardless of the HEM energy calculation outcome.

Practical Implications for Assessors and Designers

The pressure-driven ventilation model in HEM has significant practical implications for energy assessors, building services engineers, and architects. Understanding these implications is essential for producing accurate assessments and optimising designs.

Increased Data Requirements

HEM requires substantially more detailed ventilation input data than SAP. For each purpose-provided ventilation opening, the assessor must specify the type, equivalent area, facade orientation, and height. For mechanical systems, the make and model from the PCDB must be specified, including heat recovery efficiency, specific fan power, and volume flow rates. Missing data triggers punitive default values that will produce significantly worse results than accurate product-specific data.

Design Sensitivity

Because HEM models ventilation at each timestep with site-specific weather data, design decisions that had minimal impact in SAP now have measurable consequences:

• Airtightness improvements are properly credited — reducing air permeability from 5 to 3 m³/(h·m²) produces a quantifiable reduction in infiltration heat loss that varies with weather conditions throughout the year
• MVHR unit selection matters more — the difference between an 85% and a 93% heat recovery efficiency translates directly to reduced heating demand at every timestep
• Site wind exposure is properly represented — a sheltered urban site will show genuinely different infiltration characteristics from an exposed rural site, based on real terrain roughness corrections rather than SAP's crude shelter factor categories
• Building orientation relative to prevailing winds affects the distribution of infiltration across facades and the performance of natural ventilation openings

Commissioning and As-Built Data

For as-built assessments, HEM requires measured data from the completed dwelling. The airtightness test result is mandatory, and the MVHR commissioning data (measured flow rates at each terminal) should inform the assessment inputs. Poorly commissioned systems — those delivering significantly different flow rates from design intent — will produce different energy performance results than the design-stage assessment predicted.

Advanced Topics

Multi-Zone Ventilation

Unlike SAP, which modelled only two zones (living area and rest of dwelling), HEM supports user-defined zones. The ventilation model calculates air flows into and out of each zone separately, meaning that zones with more openings on windward facades may experience higher ventilation rates than sheltered zones at any given timestep. Inter-zone air movement — for example, transfer air from habitable rooms to wet rooms in an MVHR system — is also accounted for, ensuring that the heat balance in each zone reflects the actual ventilation air flow it experiences.

Wind Speed and Terrain Correction

HEM applies a terrain roughness correction to the meteorological wind speed to derive the local wind speed at the building reference height. This correction accounts for the reduced wind speed in urban and suburban environments compared with open rural terrain. The correction follows a logarithmic wind profile model with terrain roughness parameters defined for several exposure categories:

• Open terrain (rural, few obstructions) — highest wind speeds at building height; greatest wind-driven infiltration
• Country with scattered wind breaks — moderate reduction from open terrain
• Suburban — significant reduction due to surrounding buildings and vegetation
• Urban centre — greatest reduction; lowest wind-driven infiltration

This terrain correction replaces SAP's simplified shelter factor, which assigned a single numerical value (0 to 1) based on a subjective assessment of the site's exposure. The terrain roughness approach in HEM is more physically rigorous and produces results that vary with wind speed and building height — tall buildings in urban areas may still experience significant wind exposure at upper levels despite being sheltered at ground level.

Interaction with the Heating Calculation

Ventilation heat loss is one of the primary heat loss pathways in the overall heat balance calculated at each timestep. The ventilation heat loss rate (in watts) is calculated as:

Q_vent = ρ × c_p × q_v,net × (T_i − T_supply)

Where ρ × c_p is the volumetric heat capacity of air (approximately 1,200 J/(m³·K)), q_v,net is the net ventilation air flow rate (m³/s), T_i is the zone internal temperature, and T_supply is the temperature of the incoming air (which may be pre-warmed by an MVHR heat exchanger). This ventilation heat loss feeds directly into the space heating demand calculation (HEM-TP-04), which determines the energy the heating system must supply at each timestep.

The coupling between the ventilation model and the heating model means that any change to the ventilation system — a different MVHR unit, a change in airtightness target, or a different duct configuration affecting SFP — propagates directly through to the heating demand and ultimately to the dwelling's energy consumption and carbon emissions. This tight coupling is a key advantage of HEM's integrated simulation approach over SAP's sequential calculation.

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