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 shading). Unlike SAP, which applies monthly tabulated solar flux values to windows alone, HEM takes the direct beam and diffuse components from the weather file and applies BS EN ISO 52010-1:2017 to convert them into the direct and diffuse irradiance on each surface of the dwelling, 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 converting the direct beam and diffuse radiation supplied by the weather file into the irradiance striking each surface.

Hourly Sun Position Calculation

HEM calculates the sun's position for every timestep using the solar geometry procedures in BS EN ISO 52010-1:2017, applied to the site latitude and longitude carried in the weather file (HEM-TP-03). 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 accepts two weather file formats, both carrying hourly data: EnergyPlus Weather (EPW) and CIBSE Test Reference Year (TRY).

Direct Beam and Diffuse Components

Solar radiation reaching a site has two components: direct beam radiation arriving in a straight line from the sun, and diffuse radiation scattered by clouds, aerosols, and the atmosphere. HEM does not estimate one from the other with a statistical correlation. Both come from the weather file.

An EPW file carries direct beam normal radiation and diffuse horizontal irradiation as separate hourly fields, and HEM takes both directly. A CIBSE TRY file carries global horizontal and diffuse horizontal radiation, so the direct beam on the horizontal plane is obtained by subtracting one from the other, then divided by the sine of the solar altitude at that timestep to give the direct beam normal radiation. That conversion happens during data ingestion and is described in HEM-TP-03.

The split matters because the two components shade differently. On clear days the direct beam dominates and casts strong directional shadows; on overcast days almost all radiation is diffuse and arrives from across the sky dome, so 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 the anisotropic sky treatment in BS EN ISO 52010-1. Rather than assuming diffuse radiation arrives uniformly from the whole sky dome, the standard separates it into an isotropic background, a circumsolar component concentrated around the sun's position, and a horizon-brightening component, weighted by sky conditions at each timestep. How much of the dome a surface can see still depends on its tilt: a vertical wall sees half, a horizontal roof sees all of it, and the sky view factor (1 + cosβ) / 2 expresses this. Ground-reflected radiation is added, calculated from the global horizontal irradiance and the ground reflectance (albedo), for which 0.2 is the ISO 52010-1 default.

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 them for each transparent element at every half-hourly timestep, following the procedure in BS EN ISO 52016-1:2017. The ingredients are:

• the direct and diffuse irradiance on that element's surface, from BS EN ISO 52010-1
• the shading factors for distant and nearby objects, applied separately to the direct and the diffuse components
• the g-value of the glazing for perpendicular radiation, multiplied by the fixed correction factor F_w
• the frame area fraction, since the frame transmits nothing
• a transmission reduction factor for each curtain or blind on the element that is closed at that timestep

Gains are calculated element by element and then summed per zone. The zone total is apportioned between the air node and the internal surfaces using the convective and radiative fractions given in BS EN ISO 52016-1:2017 Table B.11, and it is that apportioned energy that enters the zone heat balance.

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 is defined for radiation arriving perpendicular to the glazing, and HEM-TP-08 specifies that it is calculated in accordance with ISO 9050. In practice sunlight rarely arrives perpendicular. As the angle of incidence increases, more radiation is reflected by the glass and less passes through.

HEM does not recompute that loss at each timestep from the sun position. It applies a single fixed correction factor, F_w, taken from BS EN ISO 52016-1:2017 Annex B Table B.22 and applied to the g-value per equation E.3 of the same standard. The angle dependence is therefore represented, but as one averaged factor rather than as a per-timestep calculation. Assessors moving from dynamic simulation tools that do compute incidence angle explicitly should expect this difference.

Frame Factor

Not all of a window opening transmits solar radiation. The frame, mullions, and transoms are opaque. HEM's input for this is the frame area fraction: the proportion of the element taken up by frame rather than glass. It is the complement of the glazed fraction quoted in most window literature, so a window described as 75% glazed is entered with a frame area fraction of 0.25. On that basis, typical values are 0.20–0.30 for standard casement and sash windows, falling to 0.10–0.15 for curtain walling with slim profiles. The figure scales the gain directly, so an error of ten percentage points carries straight through to the calculated solar gain for 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 assigns a solar absorption coefficient to each external element and follows the procedure in BS EN ISO 52016-1:2017 section 6.5.6. The absorbed flux is applied at the element's external surface node, inside the same nodal network used for fabric heat loss and thermal mass. There is no separate factor for the share that reaches the interior. It emerges from the node temperatures, the resistances between the nodes, and the heat capacity distributed across them, resolved at every timestep. A well insulated element passes on very little of what it absorbs; a poorly insulated one passes on more, and the absorbed heat also reaches the interior later than it arrives at the surface.

Solar absorptance depends on the external finish, ranging from roughly 0.3 for light-coloured render to 0.9 for dark slate or dark brick.

The direct irradiance on an opaque surface may first be reduced by a shading factor for distant objects such as neighbouring buildings. Nearby shading objects, including overhangs and fins, do not apply to opaque elements at all: HEM-TP-08 restricts them to transparent elements.

Shading Modelling

Accurate shading modelling is essential for realistic solar gains calculation, and it is where most of the design control over summer overheating sits. HEM splits shading into two families. Shading from distant objects, principally neighbouring buildings, applies to opaque elements, transparent elements, and photovoltaic panels. Shading from nearby objects, meaning overhangs, side fins, reveals, and ground-attached obstacles such as balustrades, applies exclusively to transparent elements. Both are evaluated at every half-hourly timestep, with separate factors for the direct and the diffuse components, and the nearby calculation accounts for distant shading already applied so that overlapping shadows are not counted twice.

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 by dividing the ground plane around the dwelling into segments and describing the objects in each segment by their height and their distance from the building. Segments need not be of equal size and their angles may be fractional. Two categories are handled: obstacles, which are ground-attached and defined by their height above ground, and distant overhangs, which are suspended structures with no sky view above them, defined by the height of their lower boundary. These are not the window overhangs described above; those are nearby objects and apply to glazing only. Distant objects are treated as infinite in width, with no depth.

The direct shading factor comes from the shadow projected onto the element, per BS EN ISO 52016-1:2017 section F.3. The diffuse component is handled by recalculating the sky view factor segment by segment and summing the result. HEM-TP-08 notes that the standards do not clearly define a method for diffuse shading by distant objects, so a new method was developed for HEM using general principles from the standards, including an allowance for radiation reflected off the obstructing objects themselves.

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 to provide adequate means of removing excess heat. It is a separate compliance check from the energy calculation, with its own routes.

Approved Document O gives two. Section 1 sets out the simplified method: maximum glazing areas and minimum free areas by category of residential building. Section 2 sets out the dynamic thermal modelling method, which requires CIBSE's TM59 methodology, subject to the limits on TM59 modelling choices set out in paragraphs 2.5 and 2.6 of the Approved Document. HEM is not the compliance vehicle for either route, and no overheating assessment appears in the Future Homes Standard calculation.

What HEM's solar modelling offers is a view of the same physics during design, before a TM59 model is commissioned:

• Glazing area and orientation: HEM quantifies the solar gains from excessive or poorly oriented glazing at every timestep, making it visible when east- or west-facing glass is driving up summer temperatures
• 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 ≈ 0.25–0.40) 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 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.

Passive solar design has a few durable rules of thumb. They are design guidance. No orientation split is prescribed by the FHS notional specification, by Approved Document L, or by HEM.

• South-facing glazing gives the most useful winter solar gain, and the high summer sun angle makes overhang shading effective, so it is the orientation where extra glazing is easiest to justify
• North-facing glazing receives minimal direct gain year-round. It reduces overheating risk but adds heat loss with nothing to offset it, so keep it to what daylight and amenity require
• East and west-facing glazing receives low-angle morning and evening sun that overhangs cannot shade. This is the orientation most likely to drive summer overheating; side fins or solar control glazing are the usual responses

Glazing Ratios

The Future Homes Standard notional dwelling takes the actual dwelling's glazing area up to 25% of total floor area (TFA), and resizes it to that cap above it. The 25% figure therefore constrains the notional benchmark, not the design. 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. The gains calculated for each window are summed per zone and split between the zone air node and the internal surfaces using the fixed convective and radiative fractions in Table B.11 of BS EN ISO 52016-1:2017. What happens next depends on the heat capacity of the elements receiving the radiative share.

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 solar radiation, dry bulb temperature, wind speed, and wind direction. Two formats are accepted: EnergyPlus Weather (EPW), chosen because it is freely available for a wide range of locations and is used by comparable models, and CIBSE Test Reference Year (TRY), which brings access to CIBSE's projected future climate scenarios. Design Summer Year files are not among them.

The choice of weather file location changes the calculated irradiance substantially, southern England receiving considerably more annual solar radiation than northern Scotland. For Future Homes Standard compliance the question does not arise: the FHS assessment fixes a single national weather file for every dwelling, so results do not vary with site location. Location-specific files matter for design work outside the compliance calculation.

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: should come from the manufacturer's data for the specific unit specified, determined in accordance with ISO 9050; generic or assumed values reduce accuracy
• Frame area fraction: should be calculated from actual window drawings or manufacturer data, not assumed as a default, and entered as the frame's share rather than the glazed share
• 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 conversion, 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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