Understanding your EPC — My Warm Home

EPC / MY WARM HOME

Understanding your EPC.

Understand the data. Reduce heat loss. Choose improvements that suit your home.

Understand

Read the certificate and check the evidence behind it.

Identify

Know how the walls, floors and roof are built.

Reduce

Cut unnecessary heat loss and plan ventilation.

Improve

Compare suitable upgrades before spending money.

An Energy Performance Certificate brings together information about your home’s construction, insulation, heating, hot water, lighting and renewable energy systems. Understanding that information helps you choose improvements that suit the property and reduce the energy needed to keep it comfortable.

EPC EXPLAINED

What does EPC data tell you?

A domestic EPC shows a current energy rating from A to G, a potential rating and recommended improvements. The rating is calculated using standard assumptions about occupancy and use.
It does not come from your actual energy bills, and it cannot predict exactly what your household will spend.

The familiar energy efficiency rating is based on modelled energy costs. Carbon emissions are a separate result. A measure can reduce emissions or improve comfort without producing the largest increase in EPC points.

An assessor records the home’s dimensions, age and construction, together with details of insulation, windows, heating, controls, hot water and other relevant systems. Different extensions and building parts can have different construction and insulation levels.

“Assumed” usually means the calculation uses a prescribed default where direct inspection or acceptable documentary evidence is unavailable. It does not prove that insulation is absent. “N/A” against a floor or roof does not automatically mean it has been left out of the calculation.

Check the assessment date, address, floor area and descriptions before relying on an old certificate. Work completed since the assessment will not appear automatically. Keep installation records and photographs of concealed insulation so an assessor can check whether they meet the evidence requirements.

Source: Government guide to domestic EPCs.

RDSAP 10

What changed with RdSAP 10?

RdSAP means Reduced Data Standard Assessment Procedure. It is used to assess existing homes from a site survey, with prescribed assumptions where a full construction specification is unavailable.

RdSAP 10 was introduced on 15 June 2025. It brought more detailed window measurements and glazing inputs, finer insulation thickness options for walls, roofs and floors, more detail for rooms in roofs, expanded ventilation inputs, additional renewable technology options and the ability to use suitably evidenced air pressure test results.

These changes give assessors more ways to describe the property. They do not remove every assumption. A newer assessment can produce a different result because of changes in the method, evidence or recorded details, even when the home itself has not changed. Compare the underlying information as well as the letter rating.

Sources: Government explanation of SAP and RdSAP, Elmhurst’s overview of RdSAP 10 changes.

WALL CONSTRUCTION

Walls: construction determines the options

Cavity walls

Cavity walls have two separate layers, usually an outer layer of brick or stone and an inner layer of brick or concrete blocks. Wall ties connect the layers, with a gap (the cavity) between them.

For domestic EPC assessments, cavity walls are recorded as a cavity wall construction, with the insulation status identified separately.

Solid walls

Solid walls have no cavity to fill. They can be built from brick, stone, concrete blocks or cob (an earth-based material). Solid brick walls often use a bonded pattern, where bricks laid across the wall’s thickness to tie the brickwork together.

Timber-frame walls

Timber frame walls contain a structural timber frame, often with insulation between its timber studs. The outside may be finished with brick, rendered boards, timber cladding or other materials, so appearance alone does not reliably identify the construction.

System-built and other non-traditional walls

System-built walls use a construction system, often involving prefabricated concrete panels, a steel frame or other manufactured components. Their outer finish may conceal the underlying structure, so appearance alone does not reliably identify the construction.

FLOOR CONSTRUCTION

Floors: look below the surface, if you can?

Suspended timber floors

Have boards and joists over a ventilated void. Insulation can be fitted between the joists, with attention to draughts and support. Underfloor ventilation must remain effective to protect the timber. Look for vents outside below the damp course.

Solid floors

Often contain a concrete slab. Insulation may be added above it or incorporated during replacement. Floor height, doors, thresholds and moisture protection affect the design and disruption. Do vents exist outside at floor level? do we have a small step into the property?

Suspended concrete floors

Including beam-and-block construction, require a suitable system for their particular build-up. Floors over garages, outside air or unheated spaces also need attention. A carpet or decorative floor finish is not equivalent to a designed floor insulation layer.

ROOF CONSTRUCTION

Roofs: where is the insulated boundary?

Pitched roofs with an unheated loft

are commonly insulated at ceiling level. Coverage and continuity matter: gaps, compressed material and an untreated loft hatch reduce performance. Keep eaves ventilation clear and provide raised boarding where storage is needed.

Rooms in roofs

have sloping ceilings, dwarf walls and other surfaces around the heated space. Insulating only the remaining loft floor leaves much of this boundary untreated.

Flat roofs

need a designed insulation and moisture-control arrangement. Roof renewal can provide an opportunity to improve insulation, but the position of insulation and the condition of the structure must be considered together.

Source for floor and roof construction principles: Energy Saving Trust insulation toolkit.

EPC EXPLAINED

Insulation: material, thickness and installation

Mineral wool, rigid foam boards, blown materials and wood-fibre products have different properties and applications. Thickness alone does not establish thermal performance: thermal conductivity, continuity, fixings, gaps and the complete construction all matter.

A U-value describes heat transfer through a building element in W/m²K. A lower U-value means less heat passes through each square metre for the same temperature difference. Thermal bridges at junctions can still lose heat and create cold surfaces even where the main wall or roof is insulated.

EPC EXPLAINED

Heating and controls

Boilers burn fuel to heat water for radiators or underfloor heating. The appliance, fuel, efficiency, distribution and controls affect the assessment. A replacement should be considered alongside the condition and performance of the existing system.

Heat pumps transfer heat from air, ground or water. Their performance depends on design, flow temperature, emitters and operation. A room-by-room heat loss calculation is needed for system sizing; the EPC alone is insufficient.

Direct electric heaters turn electricity into heat at the point of use. Storage heaters store heat for later use and depend on suitable charging controls and tariffs. Neither should be treated as having the same performance as a heat pump.

Communal heating supplies heat from a shared system. Its assessment depends on the system information and applicable methodology.

A programmer sets heating times. A room thermostat regulates temperature where it senses it. Thermostatic radiator valves regulate individual radiators; zoning can provide separate control for different areas. Weather and load compensation can adjust heating operation to demand. Smart controls are useful where they add appropriate functions, but an app alone does not guarantee extra EPC points.

Hot water also matters: the source, cylinder insulation and controls can affect energy use. Record what is actually installed and provide product details where available.

Source for heating design: Energy Saving Trust heat loss guidance.

HEAT LOSS

Why heat loss matters

Heat leaves a home through its fabric, thermal bridges and air exchange. The heating system must replace it to maintain the indoor temperature. Reducing unnecessary losses lowers heating demand and can improve comfort.

For an illustrative wall calculation, heat loss equals U-value × area × temperature difference. A 20 m² wall with a U-value of 1.5 W/m²K loses 600 W when the temperature difference is 20°C. Reducing its U-value to 0.3 reduces that figure to 120 W. This example covers the wall alone; it is not a whole-house calculation or a predicted bill saving.

Heat loss is a rate, measured in watts or kilowatts. Energy used over time is measured in kilowatt-hours. The EPC’s annual model and a heating designer’s peak room heat losses answer different questions.

Draught reduction must be planned with ventilation. Fresh air and moisture removal remain necessary. Do not block purpose-provided vents to chase an EPC improvement.

EPC EXPLAINED

Which improvements should come first?

There is no universal package that gives every home the best result. A practical sequence is:

Check the evidence and baseline. Resolve inaccurate descriptions and identify improvements already present. An evidence correction may improve the assessment without changing physical performance.
Address building defects. Repair leaks, damp causes and failed components before covering them with insulation.
Consider modest improvements. Where missing or inadequate, accessible loft insulation, suitable heating controls, cylinder insulation and LED lighting may offer worthwhile improvements. Their EPC impact varies.
Assess larger fabric upgrades. Suitable cavity insulation, solid-wall insulation, floor insulation and roof upgrades can reduce heating demand. Consider cost, disruption, ventilation and moisture together.
Plan the heating system around the home. Model the effect of replacement heating and controls. Use detailed heat loss calculations for design and sizing.
Consider renewable generation. Solar PV can improve modelled performance where suitable. Roof condition, shading and available area matter. Batteries are a separate decision whose benefit depends on the system and household.
Model the complete package before committing. Ask an accredited assessor to compare suitable scenarios, including the target rating. Measures interact, so individual EPC point gains cannot simply be added together.

Replacing all windows is not automatically the best first investment. Existing windows, draughts, condition and other opportunities determine the value. Similarly, servicing and adjusting heating can improve operation without necessarily changing the EPC’s recorded rating.

Choose improvements using both the EPC and a property-specific assessment. The aim is a home that needs less energy, remains healthy and comfortable, and has an accurate certificate supported by evidence.

Sources: Energy Saving Trust guide to EPC improvements, Energy Saving Trust heat loss advice.

PLAN YOUR NEXT STEP

Start with the home. Then choose the improvements.

Get practical advice on EPC information, heat loss and suitable retrofit improvements.

Contact My Warm Home