When assessing a home’s energy performance, one of the most important questions is simple: how much heat escapes through the building itself? The answer has a huge impact on heating demand, running costs and overall efficiency.
Within the Home Energy Model fabric heat loss approach, this isn’t treated as a simple U-value calculation. Instead, the model uses a dynamic approach that simulates how heat moves through the parts of a building over time. This gives energy assessors a more comprehensive picture of how a dwelling performs throughout changing weather conditions.
In this article, we’ll explain how fabric heat loss is modelled within the Home Energy Model (HEM) and why it represents a significant step forward from simpler steady-state calculations.
What is fabric heat loss?
Fabric heat loss is the heat that escapes through the physical elements of a building, including:
- External walls
- Roofs
- Floors
- Windows and glazed doors
- Party walls and adjoining spaces
The amount of heat lost depends on several factors, including the thermal resistance of the construction materials, the thickness of those materials, the size of each building element, and the temperature difference between inside and outside. Rather than treating these as static values, the Home Energy Model incorporates them into its wider dynamic heat balance calculation.
Fabric heat loss is part of a dynamic model
Unlike traditional assessment methods that calculate heat loss under fixed conditions, the Home Energy Model Fabric Heat Loss methodology forms part of HEM’s dynamic hourly heat balance.
Each building element is represented within a heat flow network made up of multiple nodes. These nodes represent the internal surface, external surface and the layers between them. During every timestep, HEM calculates how heat moves through these layers based on:
- The thermal resistance between materials
- The heat capacity of each layer
- Internal heat gains
- Solar gains
- External weather conditions
This allows the model to simulate how the building fabric stores and releases heat, rather than assuming heat passes straight through the construction.
How different building elements are treated
Most opaque building elements — such as walls, roofs and floors — are modelled using five connected nodes, each with its own heat capacity and thermal resistance.
Transparent elements such as windows are treated slightly differently. Rather than multiple internal layers, they are represented using a single thermal resistance between the internal and external surfaces.
The Home Energy Model also considers the effect of curtains and blinds. When closed, these provide additional thermal resistance and reduce heat loss. When open, they have no effect. Window coverings can either follow a user-defined schedule or automatically respond to solar radiation levels, allowing the model to represent their operation dynamically rather than assuming they are always open or always closed.
External conditions also affect heat loss
Heat transfer doesn’t stop at the outer surface of a wall.
For building elements exposed to the outside, HEM models both convective and radiative heat transfer between the building and the external environment.
It also accounts for solar absorption on opaque surfaces. When sunlight warms an external wall or roof, the temperature difference between the internal and external surfaces reduces, meaning less heat flows from inside to outside.
This means that weather conditions influence fabric heat loss throughout the year, rather than relying on a single fixed design temperature.
Party walls and unheated spaces receive special treatment
One interesting feature of the Home Energy Model is its handling of party walls.
Although party walls sit between two heated dwellings, research has shown that cavity party walls can still lose heat through air movement inside the cavity — known as thermal bypass. Because this heat loss isn’t captured through standard thermal bridging or infiltration calculations, HEM models it separately using evidence-based U-values for different party wall constructions.
The model also includes a practical method for dealing with thermally unconditioned spaces, such as integral garages, access corridors and certain roof spaces.
Instead of requiring detailed information about these adjoining spaces, HEM applies an additional thermal resistance to represent their insulating effect. The technical documentation even provides recommended values for common situations, providing a practical approach where detailed construction data is unavailable.
Heat loss to the ground
Ground floors behave differently from walls and roofs because heat flows into the surrounding ground in three dimensions.
Rather than applying the same method used for above-ground elements, the Home Energy Model follows dedicated procedures from BS EN ISO 13370 to calculate ground temperatures and heat transfer. Different calculation methods are used depending on whether the dwelling has:
- A slab-on-ground floor
- Edge insulation
- A suspended floor
- A heated basement
- An unheated basement
This provides a more realistic representation of ground heat transfer throughout the year.
Heat Transfer Coefficient and Heat Loss Parameter
Although the main Home Energy Model calculation uses its dynamic heat balance rather than steady-state values, it still calculates the Heat Transfer Coefficient (HTC) and Heat Loss Parameter (HLP).
These figures are included primarily to maintain compatibility with SAP 10.2 and other existing tools, making it easier to compare results across different methodologies.
The HTC calculation also includes ventilation heat loss and assumes average weather conditions together with a standard allowance for window coverings over the course of a year.
A more realistic approach to building performance
The Home Energy Model fabric heat loss methodology moves beyond simple U-value calculations by modelling how heat actually moves through a building over time.
By considering thermal mass, changing weather conditions, solar absorption, party wall thermal bypass, unheated adjacent spaces and dynamic window coverings, HEM provides a much richer representation of building performance.
For energy assessors, this means fabric heat loss becomes part of a wider, physics-based simulation that better reflects how homes behave in the real world. As the Home Energy Model continues to replace SAP, understanding how fabric heat loss is calculated will become increasingly valuable for producing accurate energy assessments and understanding the performance of modern buildings.
