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

Thermal mass is the ability of a building's materials to absorb, store, and release heat over time. Heavy materials like concrete, brick, and stone have high thermal mass, meaning they warm up slowly but hold heat for longer, smoothing out temperature swings between day and night. 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 takes an areal heat capacity for each building element, covering the entire thickness of the construction, together with one of five mass distribution classes describing where that mass sits relative to the thermal resistance. Each opaque element is modelled as five heat balance nodes, the capacity is distributed among them according to the class, and the equations for every node in the zone are solved simultaneously at each 30-minute interval. This approach captures how these materials 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 the areal heat capacity of an element, which HEM takes in J/(m²K) (SAP quotes the equivalent kappa value in kJ/(m²K)). It 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 the layer

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.

HEM's input for that wall is a single areal heat capacity covering the entire thickness of the element, outer brick leaf included. Where the mass sits relative to the insulation is carried separately, by the mass distribution class. A monthly method such as SAP takes the opposite route: it uses a kappa value, which counts only the thickness of the construction active in thermal storage at the internal surface. HEM-TP-07 draws this distinction explicitly in its first footnote.

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 opaque building element (external wall, party wall, internal partition, floor, roof, or door) is represented by exactly five heat balance nodes, as specified in section 6.5.6.3.1 of the standard. The number is fixed. Nodes are not placed layer by layer, and there are no rules about subdividing thick layers, because the layer build-up is not an input to HEM at all. Transparent elements are handled differently again: two nodes, for the internal and external surfaces, and their thermal mass is ignored.

What HEM is given for each opaque element is a single areal heat capacity and one of five mass distribution classes. The class decides how that capacity is spread across the five nodes, following the procedure in section 6.5.7 of the standard. The mass is either assigned to one node as a whole unit or divided into halves, quarters, or eighths. The five classes are:

• Mass concentrated on the internal side (I): construction with external thermal insulation, or equivalent
• Mass concentrated on the external side (E): construction with internal thermal insulation, or equivalent
• Mass divided over internal and external side (IE): insulation between two main mass components, such as an insulated cavity wall
• Mass equally distributed (D): uninsulated construction such as solid or hollow brick, or lightweight construction with negligible mass
• Mass concentrated inside (M): construction with both internal and external insulation, the mass sitting near the centre

The five nodes for each element then join the heat flow network for the zone. Each node has a heat capacity, each connection between nodes has a heat transfer coefficient, and the heat balance equations for every node in the zone are solved simultaneously at each timestep using a linear algebra solver.

Effective Heat Capacity of Building Elements

How much of an element's heat capacity the room actually experiences depends on where that mass sits relative to the insulation. A solid wall insulated externally is class I, so the capacity is placed at the internal nodes, close to the room, and responds quickly to changes in internal temperature. The same wall insulated internally is class E, placing the capacity at the external nodes behind the insulating resistance, where it responds slowly and contributes little to internal buffering. The areal heat capacity input is identical in both cases; only the class changes.

The class is an assessor input, not something the model derives from the construction, and HEM-TP-07 is direct about what that costs. The classes are an approximation. They suit simple layer arrangements such as solid brick, cavity wall, and solid wall insulated internally or externally. Other constructions are less well served. The paper's own example is a solid brick wall lined with insulated plasterboard: assign it to class E and the thermal mass of the plasterboard is not represented at the internal node at all. TP-07 also notes that for some construction types the appropriate class is not obvious, which can lead to incorrect choices, and lists a class-selection tool as a candidate for future development.

Alongside the main heat balance, HEM reports a single Heat Capacity Parameter (HCP) for the dwelling: the summed heat capacity of all building elements in all zones, divided by total floor area. It is a comparison figure only and plays no part in 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. HEM takes this from BS EN ISO 13789:2017 section 9.5, which gives the convective and radiative components separately: the convective coefficient governs transfer between the internal air and each surface, the radiative coefficient governs transfer directly between surfaces. The convective value also depends on the direction of heat flow, and HEM selects the appropriate one (upwards, downwards or horizontal) at each timestep.

The air and furniture in a zone carry thermal mass of their own. HEM does not model furniture as objects. It applies a fixed allowance of 10,000 J/K per square metre of zone floor area to the zone air node, the suggested default in BS EN ISO 52016-1:2017 Table B.17, which assumes the presence of furniture. Floor coverings have no separate treatment: a concrete floor is represented by its areal heat capacity and mass distribution class whether it is tiled or carpeted.

Heat emitters and underfloor heating are handled apart from the fabric. Their thermal mass sits in the emitter model (HEM-TP-16) rather than in a heat balance node, but it remains coupled to the zone's operative temperature, including when the heating is off, so the overall effect is similar to treating it as a fabric element. Where part of a floor forms an underfloor heating emitter, that proportion of the floor's thermal mass is excluded from the floor element to avoid double counting.

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 takes an areal heat capacity and a distribution class for every element, so construction choices feed into the model directly.

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 has a kappa value of approximately 150–200 kJ/(m²K), depending on block density and plaster finish. In HEM the areal heat capacity entered for that wall would include the outer brick leaf as well, and the insulated cavity between two mass components makes it a class IE construction, so the capacity is divided between the internal and external nodes.

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 kappa value is much 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. Each of those elements carries its own capacity and class in HEM, so each is accounted for separately.

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) on the same kappa basis. 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 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. HEM has no explicit model of suspended ceilings or air gaps: the difference can be reflected only through the areal heat capacity and mass distribution class chosen for the element.

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 where the insulation sits within an element. HEM carries this through the mass distribution class, which places the element's capacity nearer the internal or the external nodes.
• 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.

Interaction with Solar Gains and Internal Gains

Thermal mass and solar gains are deeply interconnected in HEM's calculation. Gains are calculated for each transparent element, then summed per zone and apportioned between the zone air node and the internal surfaces using the convective and radiative fractions given in BS EN ISO 52016-1:2017 Table B.11. The fractions are fixed. What varies between dwellings is what the fabric does with the radiative share once it arrives.

For an internal surface backed by high heat capacity, such as a concrete floor or a 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 produces a larger surface temperature rise, which drives greater convective heat transfer to the air and a faster increase in air temperature. The nodal heat balance resolves this at every timestep, so a room with a large south-facing window and a heavyweight floor behaves differently from the same room with a lightweight one, with consequences 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 in the building, though HEM will only show the difference if the element's heat capacity and mass distribution class are chosen to reflect it. 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 puts the structural mass on the room side of the resistance, class I, where it responds quickly to internal temperature. Internal insulation puts the same mass behind the insulation, class E, where it responds slowly and contributes little to internal buffering. The areal heat capacity entered is the same in both cases; the class is what changes. 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 or dense block internal partitions. Each element carries its own areal heat capacity, so these additions register in the calculation.

Phase-change materials are a different case. HEM represents sensible heat storage only, through a constant areal heat capacity per element. The latent storage that gives phase-change plasterboard its effect has no representation in the model, so specifying it will not show up in the result.

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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