Thermal Mass in the Home Energy Model — HEM-TP-07 Methodology

The Home Energy Model (HEM) models thermal mass dynamically at every half-hourly timestep using the methodology defined in BS EN ISO 52016-1:2017 and documented in technical paper HEM-TP-07. Rather than assigning a simplified category to the whole dwelling — as SAP does with its low, medium, or high thermal mass parameter — HEM calculates the effective heat capacity of each individual building element by discretising it into nodes and solving the heat balance at each node for every 30-minute interval. This approach captures how dense materials such as concrete, brick, and stone absorb solar gains during the day, moderate peak temperatures, and release stored heat gradually overnight — physical behaviours that are invisible to monthly or steady-state calculation methods.

What Is Thermal Mass in Building Physics?

Thermal mass refers to the ability of a material to absorb thermal energy, store it, and release it later. In the context of a dwelling, thermal mass acts as a thermal buffer: it dampens temperature swings by absorbing excess heat when internal temperatures rise and releasing that heat when temperatures fall. The relevant physical property is heat capacity, measured in kJ/(m²K) for building elements, which depends on three characteristics of each material layer:

• Density (ρ, kg/m³) — denser materials store more heat per unit volume
• Specific heat capacity (c, J/(kgK)) — the energy required to raise one kilogram of the material by one degree
• Thickness (d, m) — the physical depth of each layer that participates in heat storage

For a single homogeneous layer, the areal heat capacity is simply ρ × c × d. In practice, building elements comprise multiple layers — for instance, a masonry wall might consist of an external brick leaf, a cavity filled with insulation, a concrete block inner leaf, and a plaster finish. Not all of these layers contribute equally to thermal mass as experienced by the occupied space: insulation layers act as thermal barriers, and only the material on the room side of the insulation actively participates in buffering internal temperature.

SAP vs HEM — Thermal Mass Treatment Compared

The difference in how SAP and HEM treat thermal mass is one of the clearest examples of the step change in modelling fidelity. The table below summarises the key distinctions:

For a broader comparison of SAP and HEM across all modules, see our SAP vs HEM overview.

The BS EN ISO 52016-1 Approach to Thermal Capacity

HEM's thermal mass calculation follows the framework set out in BS EN ISO 52016-1:2017, Energy performance of buildings — Energy needs for heating and cooling, internal temperatures and sensible and latent heat loads — Part 1: Calculation procedures. This standard defines an hourly (or sub-hourly) heat balance method that explicitly models the thermal storage behaviour of the building fabric.

Nodal Discretisation of Elements

Each building element — external wall, party wall, internal partition, floor, roof, or door — is discretised into a series of nodes. Each node represents a material layer or a subdivision of a thick layer. The standard specifies rules for node placement: thin layers may be represented by a single node, while thick layers (particularly dense masonry or concrete) are subdivided so that the temperature gradient through the layer is adequately resolved.

At each node, HEM tracks two quantities: the temperature and the heat stored. The heat balance at each node considers conduction from adjacent nodes, convective and radiative heat exchange at the internal surface, and (for external surfaces) heat loss to the outside environment. The result is a set of simultaneous equations solved at every timestep to determine how heat flows into and out of each layer.

Effective Heat Capacity of Building Elements

The effective heat capacity of a building element is not a fixed property — it depends on the thermal conditions and the timescale over which heat is exchanged. For a wall with insulation on the outside (internally insulated or solid masonry with external insulation), the dense inner layers are thermally coupled to the room and contribute substantially to internal heat buffering. For a wall with insulation on the inside, the dense outer layers are thermally decoupled from the room by the insulating layer and contribute very little to the effective thermal mass experienced internally.

ISO 52016-1 handles this automatically through its nodal approach: the insulation layer has low thermal conductivity, which means the temperature at nodes beyond the insulation responds only slowly to changes in internal temperature. The model does not need to make assumptions about “effective depth” or apply correction factors — the physics emerges naturally from the calculation.

Surface Heat Transfer Coefficients

The rate at which heat moves between the room air and the internal surface of a building element depends on the internal surface heat transfer coefficient (h_si), which combines convective and radiative components. ISO 52016-1 provides standard values, but HEM also accounts for the effects of furniture and floor coverings that reduce the thermal coupling between the room and the mass beneath. A concrete floor covered with thick carpet and underlay, for example, has a significantly reduced effective thermal mass compared to the same floor with a thin tile finish — HEM captures this distinction.

Thermal Mass and Half-Hourly Timesteps

The interaction between thermal mass and HEM's half-hourly calculation interval is fundamental to the accuracy improvement over SAP. Thermal mass effects are inherently dynamic — they involve the absorption and release of heat over timescales ranging from minutes to many hours. A monthly calculation method cannot resolve these dynamics; it can only approximate them through “utilisation factors” that adjust monthly gains and losses to account for thermal storage in an averaged way.

At each half-hourly timestep, HEM's heat balance captures the following thermal mass interactions:

• Absorption of solar gains — when sunlight enters through glazing and strikes an internal surface, the fabric absorbs a portion of the energy. Dense surfaces absorb more heat before their surface temperature rises significantly. HEM tracks this absorption and the subsequent re-emission as long-wave radiation and convection to the room air.
• Absorption of internal gains — heat from occupants, lighting, appliances, and cooking is partly absorbed by the surrounding fabric. In a heavyweight building, this prevents internal temperatures from spiking during high-gain periods.
• Delayed heat release — stored heat is released gradually as the room cools, for instance after sunset or when the heating system turns off. The rate of release depends on the thermal diffusivity and thickness of the mass.
• Heating system interaction — when the heating system activates, part of the energy output goes to warming the fabric rather than the air. In a heavyweight building, pre-heating takes longer, but the stored energy sustains comfortable temperatures for longer after the heating switches off.

Thermal Mass by Construction Type

Different construction methods produce markedly different thermal mass profiles. HEM calculates thermal mass from first principles for every element, but understanding the typical characteristics of common construction types is essential for designers working with the model.

Masonry Construction

Traditional masonry — whether brick-and-block cavity walls or solid brick — provides the highest thermal mass of common UK construction types. A typical cavity wall with a 100mm dense concrete block inner leaf, 100mm cavity insulation, and 100mm brick outer leaf delivers an areal heat capacity on the internal side of approximately 150–200 kJ/(m²K), depending on block density and plaster finish. The insulation in the cavity thermally decouples the outer brick leaf, so only the inner leaf and plaster contribute to internal thermal mass.

In HEM, masonry homes benefit from reduced peak heating demand, improved heat pump efficiency through more stable demand profiles, and lower overheating risk — particularly when combined with night ventilation strategies.

Timber Frame Construction

Timber frame walls typically have insulation between the studs, with plasterboard on the internal face and a sheathing board externally. The areal heat capacity on the internal side is significantly lower than masonry — typically 15–30 kJ/(m²K) for the wall element alone, depending on plasterboard specification and whether an additional service void is present.

However, a timber frame dwelling's overall thermal mass is not determined solely by its external walls. Internal partitions, intermediate floors, and ground floors all contribute. A timber frame house with a concrete ground-floor slab, plasterboard-on-masonry party walls, and dense plasterboard internal partitions can achieve a respectable overall thermal mass. HEM rewards these design choices by calculating the contribution of every element individually.

SIPs and Other Lightweight Systems

Structural Insulated Panels (SIPs) consist of a rigid insulation core bonded between two structural facings, typically oriented strand board (OSB). This construction offers excellent thermal performance (low U-values) and airtightness, but very low thermal mass — typically 8–15 kJ/(m²K) for the wall element itself. The same applies to insulated concrete formwork (ICF) with thin concrete skins, prefabricated steel-framed panels, and other modern methods of construction (MMC) that prioritise insulation over mass.

For lightweight systems, HEM will show faster thermal response times but greater susceptibility to temperature swings. Designers working with SIPs should pay particular attention to overheating risk and consider strategies to introduce thermal mass internally — for example, a concrete ground-floor slab, phase-change materials in plasterboard, or dense internal partition walls.

Concrete Frame and Crosswall

Reinforced concrete frame and crosswall construction, common in flatted developments, provides very high thermal mass. Exposed concrete soffits, in particular, offer large areas of dense material in direct thermal contact with the occupied space. In HEM, these elements contribute significantly to peak temperature reduction and overnight heat release. Concealing the soffit behind a suspended ceiling with an air gap reduces its effective contribution — a detail that HEM models explicitly but SAP does not distinguish.

Impact on Heating Demand, Overheating, and System Sizing

Heating Demand

Thermal mass affects annual heating demand in two main ways. First, it improves the use of free heat gains: solar gains and internal gains that would otherwise cause the room to overshoot the setpoint temperature are instead absorbed into the fabric, stored, and released later when the room would otherwise require heating. In HEM's half-hourly simulation, this interaction is modelled explicitly at every timestep. In SAP, it is approximated through monthly utilisation factors that inevitably smooth out the true dynamics.

Second, thermal mass influences the pattern of heating demand over the day. A heavyweight building heated by a well-controlled system may draw heat more steadily over a longer period, avoiding the sharp demand spikes associated with rapid warm-up in lightweight buildings. This steadier demand profile is particularly beneficial for heat pumps, which operate most efficiently at part load with low flow temperatures.

Overheating Risk

Thermal mass plays a critical role in managing overheating, especially in a warming climate. High thermal mass absorbs excess gains during the hottest part of the day, preventing internal temperatures from rising as quickly. Provided the stored heat can be purged overnight (through ventilation or radiant cooling), the building starts the next day at a lower temperature.

HEM models this cycle explicitly at every timestep. It can determine whether a night ventilation strategy is sufficient to discharge the stored heat, and it can identify when heavyweight construction alone is not enough to prevent overheating — for instance, in a south-facing flat with limited cross-ventilation. SAP's simplified overheating check, by contrast, cannot represent these dynamics and may either underestimate or overestimate overheating risk depending on the circumstances.

System Sizing

The thermal response of the building affects heating system sizing. A heavyweight building requires more energy during the initial warm-up period (for example, after a weekend setback in an office or overnight setback in a dwelling), but it sustains temperature for longer once warm. HEM's half-hourly modelling captures this dynamic: it can show the actual peak demand during morning warm-up and the steady-state demand once the building reaches temperature.

For heat pump installations, this distinction is important. Heat pumps are most efficient when sized to meet the steady-state demand and operated continuously or near-continuously, rather than being oversized to achieve rapid warm-up. HEM allows designers to demonstrate that a smaller heat pump can maintain comfort in a heavyweight building — a design approach that SAP's steady-state sizing methodology does not fully support.

Why SAP's Simplified Approach Undervalues Thermal Mass

SAP's treatment of thermal mass has long been recognised as a weakness. The fundamental problem is that a monthly steady-state calculation cannot represent the dynamic interplay between heat storage and release that occurs over timescales of hours to days. SAP addresses this through utilisation factors — empirical correction factors applied to monthly solar and internal gains to account for the fraction that is “usefully” absorbed and later released during heating periods. The utilisation factor depends on the ratio of heat gains to heat losses and on the building's time constant (a function of the TMP and the total heat loss coefficient).

While this approach captures the broad direction of the thermal mass effect, it has several limitations:

• No element-level resolution — SAP's TMP is a whole-dwelling average. Two buildings with the same average TMP but different distributions of mass (one with heavy external walls and lightweight internal partitions, another with lightweight walls and a massive concrete floor) will produce identical results in SAP but different performance in reality.
• No time-of-day dynamics — the monthly utilisation factor cannot distinguish between solar gains arriving at midday (when they can be usefully absorbed) and gains arriving at 4pm (when the building may already be warm). HEM resolves this at every half-hourly step.
• Insulation position ignored — SAP's simplified approach does not account for the position of insulation within an element. HEM's nodal method naturally captures the effect of insulation position on accessible thermal mass.
• Overheating modelling limited — the dynamic interaction between thermal mass and overheating cannot be represented by monthly averages, leading to unreliable results in SAP's simplified overheating check.
• No reward for exposed mass — SAP does not distinguish between exposed concrete and concrete behind a suspended ceiling. HEM accounts for the thermal resistance of any intervening layers.

Interaction with Solar Gains and Internal Gains

Thermal mass and solar gains are deeply interconnected in HEM's calculation. When solar radiation passes through glazing and strikes an internal surface, the energy is absorbed according to the surface's solar absorptance. For a dense surface such as a concrete floor or masonry wall, the absorbed energy raises the surface temperature only slightly because the heat is conducted into the body of the element. For a lightweight surface, the same energy causes a larger surface temperature rise, which in turn drives greater convective heat transfer to the air and a faster increase in air temperature.

HEM models this at every timestep by distributing incoming solar gains across the internal surfaces of the zone according to area and absorption characteristics, then solving the nodal heat balance to determine how much energy is stored and how much is immediately re-emitted. This means that a room with a large south-facing window and an exposed concrete floor will behave very differently from the same room with an exposed timber floor — a distinction that has real implications for both winter heating performance and summer overheating.

Internal gains from occupants, lighting, and appliances follow the same logic. HEM distributes these gains between air and surfaces (using a convective/radiative split) and tracks their absorption into the fabric. In a heavyweight building, the fabric acts as a “thermal battery,” absorbing excess gains during occupied hours and releasing them during the evening and night. This reduces peak temperatures and shifts a portion of the useful heating contribution to later hours — a phenomenon that SAP's monthly method cannot represent.

Design Implications for Architects

HEM's treatment of thermal mass creates several design opportunities — and traps — that architects should be aware of when developing schemes for Future Homes Standard compliance.

Expose Internal Mass Where Possible

Exposed concrete soffits, fair-faced blockwork, and tile-finished concrete floors all provide thermal mass in direct contact with the occupied space. Concealing these surfaces behind suspended ceilings, dry-lined partitions, or thick carpet reduces their effective contribution. Where aesthetics or acoustic requirements demand concealment, consider designs that maintain at least partial exposure — for example, perforated ceiling tiles that allow some air circulation to the soffit above.

Consider Insulation Position

External insulation preserves the full thermal mass of the structural wall on the room side, while internal insulation effectively removes it. For masonry buildings, external wall insulation is the most effective strategy for maintaining both thermal performance and thermal mass. For new-build masonry, full-fill or partial-fill cavity insulation with a dense block inner leaf provides a good balance of insulation and accessible mass.

Compensate for Lightweight External Walls

If the construction system is lightweight (timber frame, SIPs, steel frame), consider introducing thermal mass through other elements: concrete ground-floor slabs, dense block internal partitions, or specialist products such as phase-change material plasterboard. HEM evaluates the contribution of every element individually, so these additions are fully credited.

Thermal Mass and Heat Pump Synergy

Buildings with higher thermal mass tend to produce smoother, more stable heating demand profiles. This is advantageous for heat pumps, which achieve higher seasonal efficiency when operating continuously at low output rather than cycling between high output and standby. Architects designing for heat pump heating should consider how thermal mass can flatten the demand profile, potentially allowing a smaller heat pump to be specified. HEM's half-hourly simulation demonstrates this benefit directly in the compliance calculation.

Thermal Mass for Overheating Mitigation

In dwellings at risk of overheating — particularly south-facing upper-floor flats — thermal mass can reduce peak temperatures by absorbing excess gains. However, mass alone is insufficient: a night purge ventilation strategy is essential to discharge the stored heat before the next day's gains arrive. HEM models this interaction explicitly, allowing designers to test combinations of mass, glazing area, shading, and ventilation strategy in a single integrated assessment.

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