Solar Gains in HEM — Technical Guide

The Home Energy Model (HEM) calculates solar gains at half-hourly resolution using the methodology set out in HEM-TP-08 (Solar Gains and Solar Absorption). Unlike SAP, which applies monthly tabulated solar flux values to windows alone, HEM splits global solar irradiance into direct beam and diffuse components following BS EN ISO 52010-1:2017, then calculates gains through both glazed and opaque building elements at every timestep. This approach captures the hour-by-hour interplay between sun position, building orientation, shading devices, glazing properties, and fabric absorption — producing far more accurate energy demand and overheating risk predictions than SAP's simplified monthly method.

Solar Irradiance Calculation

Before HEM can calculate solar gains through any building element, it must determine the solar irradiance incident on each surface at every half-hourly timestep. This is handled through a two-stage process: first calculating the sun position, then decomposing global horizontal irradiance into the direct beam and diffuse components that strike each surface.

Hourly Sun Position Calculation

HEM calculates the sun's position for every timestep using standard solar geometry equations defined in HEM-TP-03 (External Conditions). The two key angles are:

• Solar altitude (α) — the angle of the sun above the horizon, ranging from 0° at sunrise/sunset to a maximum that depends on latitude, day of year, and time of day
• Solar azimuth (ψ) — the compass bearing of the sun measured clockwise from north, determining which elevations of a building are illuminated at any given moment

These are derived from the solar declination (which varies through the year as the Earth orbits the sun), the hour angle (which tracks the sun's apparent east-to-west movement through the day), and the site latitude. HEM uses weather files containing hourly measured global horizontal irradiance in CIBSE Test Reference Year (TRY) or EPW format, which provide the total solar radiation falling on a horizontal surface at the site location.

Direct Beam and Diffuse Decomposition

Global horizontal irradiance consists of two components: direct beam radiation arriving in a straight line from the sun, and diffuse radiation scattered by clouds, aerosols, and the atmosphere. HEM decomposes the measured global horizontal irradiance into these components following the procedures in BS EN ISO 52010-1:2017, the international standard for solar irradiance calculations used in energy performance assessment.

The decomposition uses a clearness index (the ratio of measured global irradiance to extraterrestrial irradiance) to estimate the diffuse fraction. On clear days, the direct beam component dominates and creates strong directional shadows; on overcast days, almost all radiation is diffuse and arrives from all parts of the sky dome, meaning shading devices have less effect.

Irradiance on Tilted Surfaces

Once the direct and diffuse components on the horizontal plane are known, HEM converts these to irradiance on each building surface using the surface orientation (azimuth) and tilt angle. The direct beam component is converted geometrically using the angle of incidence — the angle between the sun's rays and the normal to the surface. When the sun is behind a surface (angle of incidence greater than 90°), the direct beam contribution is zero for that surface.

The diffuse component on a tilted surface is calculated using an isotropic sky model as the baseline approach in BS EN ISO 52010-1. This assumes diffuse radiation arrives uniformly from all visible parts of the sky dome, with the visible fraction determined by the surface tilt. A vertical wall sees half the sky dome; a horizontal roof sees the entire dome. Ground-reflected radiation is also included, calculated from the global horizontal irradiance and the ground reflectance (albedo), typically assumed as 0.2 for the UK.

Solar Gains Through Glazing

Solar gains through windows and glazed doors are typically the largest solar contribution to a dwelling's energy balance. HEM calculates these gains for each glazed element at every half-hourly timestep using the following relationship:

Q_sol,glazing = I_sol × A_gl × g_value × F_frame × F_shading

Where:

• I_sol is the total solar irradiance on the window surface (direct beam plus diffuse plus ground-reflected), in W/m²
• A_gl is the total area of the glazing opening, in m²
• g_value is the total solar energy transmittance of the glazing unit, determined from EN 410 test data and corrected for angle of incidence
• F_frame is the frame factor — the fraction of the opening area that is glazed rather than frame (typically 0.7–0.8 for standard windows)
• F_shading is the combined shading factor accounting for overhangs, side fins, and external obstructions (ranging from 0 for fully shaded to 1 for no shading)

The g-value and Angle Correction

The g-value (also known as the solar heat gain coefficient or solar factor) represents the total fraction of incident solar energy that passes through the glazing and enters the building as heat. It includes both the directly transmitted solar radiation and the fraction absorbed by the glass that is subsequently re-radiated inward. Typical values for common glazing types include:

• Double glazing, clear glass: g-value ≈ 0.63–0.72
• Double glazing, low-e coated: g-value ≈ 0.50–0.63
• Triple glazing, low-e coated: g-value ≈ 0.40–0.55
• Solar control glass: g-value ≈ 0.25–0.40

The g-value provided by manufacturers is measured at normal incidence (light hitting the glass perpendicular to its surface). In practice, sunlight rarely arrives at normal incidence. As the angle of incidence increases (the sun moves to a more oblique angle relative to the window), more solar radiation is reflected by the glass and less passes through. HEM applies an angle-dependent correction factor at each timestep to account for this effect, based on the actual angle between the sun's rays and the window normal. This correction is particularly significant for east- and west-facing windows, which receive sunlight at steep angles during morning and evening.

Frame Factor

Not all of a window opening transmits solar radiation — the frame, mullions, and transoms are opaque. The frame factor (F_frame) is the ratio of the glazed area to the total opening area. HEM requires this as an input for each window element. Typical values are 0.70–0.80 for standard casement and sash windows, rising to 0.85–0.90 for curtain walling systems with minimal frame profiles. Using accurate frame factors is important because a 10% error in frame factor translates directly to a 10% error in calculated solar gains through that element.

Solar Absorption Through Opaque Fabric

A significant advancement in HEM over SAP is the modelling of solar absorption through opaque building elements — external walls, roofs, and opaque doors. SAP ignores this entirely, but in reality, dark-coloured external surfaces can absorb substantial solar radiation that raises the outer surface temperature and increases the temperature gradient driving heat flow into the building.

HEM calculates opaque solar absorption at each timestep using:

Q_sol,opaque = I_sol × A_el × α_sol × R_se × U_el

Where:

• I_sol is the total solar irradiance on the element surface, in W/m²
• A_el is the area of the opaque element, in m²
• α_sol is the solar absorptance of the external surface (ranging from approximately 0.3 for light-coloured render to 0.9 for dark slate or dark brick)
• R_se is the external surface resistance, in m²K/W (typically 0.04 m²K/W for exposed surfaces)
• U_el is the U-value of the element, in W/m²K

The product R_se × U_el represents the fraction of absorbed solar energy that flows inward through the element rather than being lost to the external air by convection and long-wave radiation. For highly insulated walls (low U-values), this fraction is small, meaning less absorbed solar heat reaches the interior. For poorly insulated elements, a greater proportion of absorbed solar energy contributes to internal gains.

Shading Modelling

Accurate shading modelling is essential for realistic solar gains calculation, particularly for Part O overheating assessment. HEM models three categories of shading, each calculated geometrically at every half-hourly timestep based on the sun position.

Overhangs

Horizontal overhangs above a window (such as a roof soffit, brise soleil, or balcony) cast a shadow that moves down the window as the sun rises higher. HEM calculates the shadow depth on the window based on the overhang projection, the vertical distance between the overhang and the top of the window, and the solar altitude angle projected onto the plane perpendicular to the window surface.

Overhangs are particularly effective for south-facing windows in the UK because the summer sun is high (solar altitude up to approximately 62° at latitude 51.5°N in late June), so a relatively modest overhang can shade much of the window. In winter, the low sun (altitude approximately 15° at midday in December) passes below the overhang, allowing useful solar gains to enter. This seasonal selectivity makes overhangs one of the most effective passive strategies for balancing heating season gains against summer overheating risk.

Side Fins

Vertical fins to the side of a window (such as projecting reveals, privacy screens, or architectural fins) cast shadows that sweep across the window as the sun moves through the sky. HEM calculates the shadow width based on the fin projection, the horizontal distance between the fin and the near edge of the window, and the solar azimuth angle relative to the surface normal.

Side fins are most effective for east- and west-facing windows, where low-angle morning or evening sun would otherwise cause significant solar gains and potential overheating. They are less effective for south-facing windows, where the sun's azimuth stays relatively close to the surface normal during peak irradiance hours.

Remote Obstructions

External obstructions such as neighbouring buildings, trees, and terrain features can significantly reduce solar irradiance reaching a building's surfaces. HEM models these using a simplified horizon profile that defines the angular elevation of obstructions at each compass bearing. When the sun's altitude is below the obstruction angle at the sun's azimuth, the direct beam component is fully blocked. The diffuse component is reduced proportionally based on the fraction of the sky dome obscured by the obstruction profile.

SAP vs HEM — Solar Gains Comparison

The difference between SAP and HEM solar gains modelling is one of the most significant methodological changes in the transition. The table below summarises the key differences:

For a non-technical comparison of SAP and HEM, see our SAP vs HEM overview. For the full list of HEM technical papers, see the HEM Technical Reference hub.

Interaction with Part O Overheating

Part O of the Building Regulations (Approved Document O) requires new homes to be designed to limit unwanted solar gains in summer and provide adequate means of removing excess heat. HEM's detailed solar gains modelling feeds directly into the overheating risk assessment for Future Homes Standard compliance.

Under Part O, designers can demonstrate compliance either through the simplified method (prescriptive glazing limits and cross-ventilation rules) or through dynamic thermal modelling. HEM's half-hourly simulation is well suited to the dynamic approach because it calculates internal temperatures at every timestep, accounting for solar gains, ventilation, internal gains, and thermal mass effects simultaneously.

The key areas where HEM solar gains modelling influences Part O assessment are:

• Glazing area and orientation — HEM quantifies the solar gains penalty for excessive or poorly oriented glazing at every timestep, making it clear when east- or west-facing glass is causing summer overheating
• Shading effectiveness — HEM can demonstrate the benefit of overhangs, fins, and external shading devices with half-hourly precision, supporting design decisions about shading geometry
• Glazing specification — the choice of g-value directly affects calculated solar gains; solar control glass (g-value ≈ 0.25–0.35) can significantly reduce overheating risk on vulnerable orientations
• Ventilation interaction — HEM models the balance between solar gains and heat removal through ventilation (see ventilation modelling), allowing designers to test whether purge ventilation or MVHR summer bypass adequately manages peak solar loads

Design Implications for Solar Gains

HEM's detailed solar gains modelling has significant practical implications for building design. Because HEM evaluates solar contributions at half-hourly resolution, design decisions about orientation, glazing, and shading have a more granular and measurable impact than under SAP.

Orientation Strategy

Under SAP's monthly calculation, orientation had a limited effect on compliance because monthly averaging smoothed out the benefits of good solar design. Under HEM, orientation matters considerably more. A south-facing living space with appropriate shading can capture significant passive solar gains during the heating season — reducing space heating demand — while an overhang prevents those same windows from causing summer overheating.

Industry guidance based on early HEM testing suggests the following orientation principles:

• South-facing glazing (≈55% of total): maximises useful winter solar gains; high summer sun angle makes overhang shading highly effective
• North-facing glazing (≈15% of total): minimal direct solar gains year-round; reduces overheating risk but increases net heat loss; keep to a minimum consistent with daylight and amenity requirements
• East/west-facing glazing (≈30% of total): receives low-angle morning and evening sun that is difficult to shade with overhangs; contributes to overheating risk under Part O; consider solar control glazing or external side fins

Glazing Ratios

The Future Homes Standard draft notional building specification caps total glazing area at approximately 25% of the total floor area (TFA). This cap reflects the balance between useful solar gains, daylight provision, and overheating risk. Under HEM, exceeding this ratio does not necessarily cause compliance failure, but it significantly increases the solar gains that the building must manage through a combination of shading, ventilation, thermal mass, and potentially solar control glazing.

Architects should note that HEM's half-hourly modelling means that two buildings with the same total glazing area but different orientation distributions will produce materially different energy performance results. A building with 25% glazing ratio concentrated on the south elevation will typically outperform one with the same area distributed equally across all four elevations.

Thermal Mass and Solar Gains Interaction

The interaction between solar gains and thermal mass is one of the most significant aspects of HEM's dynamic modelling. When solar radiation enters a room through glazing, it strikes internal surfaces — floors, walls, furniture — which absorb and store a portion of the energy. The rate and extent of absorption depend on the thermal capacitance of the exposed materials.

Heavyweight construction (concrete floors, masonry internal walls, plaster finishes on dense block) absorbs solar gains gradually, reducing the peak internal temperature during the sunniest hours. The stored heat is then released slowly over subsequent hours as the room cools, extending the period of comfortable temperatures and reducing the evening heating demand. HEM calculates this absorption and release process dynamically at every timestep using the methodology defined in BS EN ISO 52016-1:2017.

Lightweight construction (timber frame, plasterboard on battens, insulated linings) has lower thermal capacitance. Solar gains cause rapid temperature rises, which may trigger overheating during peak irradiance, followed by rapid cooling once the sun moves off. This means lightweight buildings are more sensitive to solar gains and may require lower glazing ratios, more aggressive shading, or solar control glass to avoid overheating, particularly in southern England.

Practical Modelling Considerations

When preparing HEM inputs for solar gains modelling, several practical considerations affect accuracy:

Weather Data Selection

HEM uses hourly weather data containing measured global horizontal irradiance, dry bulb temperature, wind speed, and wind direction. The choice of weather file location affects solar irradiance values significantly — a site in southern England receives approximately 15–20% more annual solar irradiance than one in northern Scotland. HEM supports CIBSE Test Reference Year (TRY) files for typical conditions and Design Summer Year (DSY) files for overheating assessment, as well as EPW format files used internationally.

Input Data Quality

The accuracy of HEM's solar gains calculation depends on the quality of the input data provided. Key inputs that require careful attention include:

• Window orientation and tilt: each window must be assigned the correct azimuth and tilt angle; errors here directly affect the calculated irradiance on the window surface
• Glazing g-value: must be obtained from the manufacturer's EN 410 test data for the specific glazing unit specified; generic or assumed values reduce accuracy
• Frame factor: should be calculated from actual window drawings or manufacturer data, not assumed as a default
• Shading geometry: overhang depths, fin projections, and offset distances must be measured from the architectural drawings to the relevant window edges
• External surface absorptance: for opaque elements, the solar absorptance depends on the external finish colour; this should be selected from published data rather than estimated
• Obstruction profiles: remote shading from adjacent buildings and terrain requires a site assessment to establish horizon angles at each relevant bearing

Software Implementation via ECaaS

All official HEM calculations are performed through the ECaaS (Energy Calculation as a Service) platform. Software providers submit building geometry, fabric specifications, glazing properties, and shading inputs through the ECaaS API, and the platform executes the solar gains calculation as part of the full HEM simulation. This centralised approach ensures consistency — every provider uses the same solar irradiance decomposition, the same shading algorithms, and the same interaction with thermal mass modelling — eliminating the discrepancies that arose under SAP when different software packages implemented the same specification differently.

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