WHOLE HOUSE RETROFIT APPROACH WHLG

Whole house retrofit approach for Warm Homes Local Grant are not intended to be isolated or piecemeal improvements. Instead, they are planned and delivered using a whole house retrofit approach, which recognises that a dwelling functions as an integrated system, rather than as a collection of independent elements.

This approach considers how the building fabric, heating system, ventilation, and occupant use interact to influence overall performance. The fundamental objective is to deliver measurable and sustained improvements in energy efficiency, thermal comfort, and household affordability, while minimising the risk of unintended consequences, such as condensation, damp, overheating, or reduced indoor air quality.

Retrofit interventions are therefore sequenced and combined, in line with PAS 2035 retrofit planning principles, to achieve the following core outcomes.

1. Reduce Heat Loss (Fabric Efficiency) with a Whole House Retrofit Approach

The priority for Whole House Retrofit Approach WHLG is to reduce the amount of heat required to maintain comfortable internal temperatures. This is achieved through targeted improvements to the building fabric, including wall, loft/roof insulation, alongside appropriate airtightness measures.

Reducing fabric heat loss:

Lowers overall space heating demand
Improves thermal comfort and temperature stability
Establishes a suitable baseline for efficient heating system operation

This step is essential to ensure that any subsequent heating system upgrades operate effectively and efficiently, and that systems are not oversized.

2. Improve Heating System Efficiency

Once heat demand has been reduced for Whole House Retrofit Approach WHLG, the focus shifts to how efficiently heat is generated, controlled, and distributed within the home.

This may involve:

Upgrading to low carbon heating systems, such as heat pumps, where the property is suitable; or
Optimising existing heating systems through improved controls, zoning, system balancing, and heat distribution, where full replacement is not appropriate.

Heating systems perform best in homes with reduced heat loss, enabling lower flow temperatures, improved comfort, and enhanced overall system efficiency.

3. Lower Energy Bills and Carbon Emissions Over the Long Term

By addressing fabric performance and heating efficiency together, households benefit from sustained reductions in energy consumption, rather than short‑term gains that may diminish over time.

This integrated approach supports:

Long term reductions in energy bills
Improved SAP and EPC ratings
Lower operational carbon emissions

These benefits are resilient to future changes in energy prices and directly support national decarbonisation objectives.

SAP Focused Measures in Correct Installation Order

Warm Homes Local Grant upgrades are delivered using a planned and sequenced retrofit approach, ensuring that early measures reduce heat demand and prepare the dwelling for higher impact interventions installed later. Installing measures out of sequence can reduce SAP uplift, increase overall costs, or introduce building performance risks.

The following sequence represents best practice installation order, aligned with PAS 2035 whole house retrofit principles, while achieving the highest possible SAP rating improvement.

1. Loft and Roof Insulation with a Whole House Retrofit Approach

Priority: reduce heat demand

Fabric improvements form the starting point for most retrofit projects. Improving roof and loft insulation reduces heat loss, stabilises internal temperatures, and provides a strong foundation for all subsequent measures.

Typical works include:

Topping up loft insulation to 270- 300 mm
Insulating loft hatches and service penetrations
Insulating flat roofs where applicable

Why this comes first

Low disruption and low technical risk
Immediate reduction in space heating demand
Improves the performance and sizing of future heating systems

2. Wall Insulation (Cavity or Solid Wall)

Second priority Whole House Retrofit Approach WHLG : address the largest heat loss area

Once roof heat losses have been reduced, retrofit interventions focus on walls, which typically represent the largest source of fabric heat loss.

Measures may include:

Cavity wall insulation (where suitable)
External wall insulation (EWI)
Internal wall insulation (IWI)

Why this comes early

Significantly reduces space heating demand
Enables smaller, more efficient heating system design
Delivers substantial SAP improvements, commonly 8-15 points

All wall insulation measures are assessed, designed, and installed to manage moisture and condensation risk in line with PAS 2035 risk pathways.

3. Heating Controls and System Optimisation

(Where a heat pump is not yet being installed)

Before replacing heating systems, existing systems should be optimised to operate as efficiently as possible.

Typical upgrades include:

Programmable or smart thermostats
Thermostatic radiator valves (TRVs)
Zoned heating controls
System balancing and optimisation

Why controls come before system replacement

Reduces wasted heat and improves user control
Delivers SAP uplift at relatively low cost
Can reduce or defer the scale of later heating upgrades

This stage represents a high value intervention and helps avoid unnecessary heating system oversizing.

4. Low‑Carbon Air Source Heat pumps

Fourth priority: decarbonise heat once demand is reduced

After fabric and system optimisation measures have been completed for Whole House Retrofit Approach WHLG, homes are better suited to low carbon heating systems, such as air source heat pumps.

Why heat pumps are installed later

System performance depends on low heat demand
Smaller systems can be specified more cost effectively
SAP uplift is maximised only after fabric improvements

Air Source Heat pumps work particularly well with Solar PV and Battery Storage

Heat pumps are electrically powered systems and therefore integrate effectively with on site renewable electricity generation. When combined with solar PV, a proportion of the electricity required to operate the heat pump can be generated directly on site, reducing reliance on imported grid electricity.

The addition of battery storage further enhances system performance by:

Storing surplus solar electricity generated during the day
Allowing stored energy to be used to power the heat pump during evenings or periods of low solar generation
Increasing on‑site energy self consumption, which improves household energy affordability

From a SAP perspective, this combination:

Reduces calculated purchased electricity
Improves dwelling emission rates
Strengthens EPC outcomes, particularly when heat pumps and PV are assessed together

When correctly sequenced, heat pump installations can deliver SAP increases of 10- 25 points, often representing the single largest improvement under Warm Homes Local Grant funding.

5. Solar Photovoltaic (PV) Panels and Battery Storage with a Whole House Retrofit Approach

Final step Whole House Retrofit Approach WHLG: reduce energy demand and support electric heating

Solar PV systems in Whole House Retrofit Approach WHLG are typically installed toward the end of the retrofit process, once the dwelling’s energy demand profile and heating system configuration are better understood.

Measures may include

Roof‑mounted solar PV system (e.g. 3- 4 kWp)
Domestic battery storage system to retain excess generation

Why PV and battery storage come last

They complement electric and low carbon heating systems, such as heat pumps
They reduce calculated purchased energy within SAP assessments
Battery storage increases the proportion of generated energy used within the home

Technical benefits of pairing PV with battery storage

Surplus daytime generation can be stored rather than exported
Stored electricity can be used to support heating, hot water, and household loads
Reduces exposure to peak electricity tariffs
Improves resilience to future energy price volatility

Installed at this stage, solar PV combined with battery storage can deliver SAP improvements of 5-12 points, while acting as the final optimiser that supports progression to higher EPC bands, often enabling C → B outcomes.

Why This Sequencing Matters

Delivering measures in the correct order ensures that:

SAP improvements are maximised and sustained
Heating systems are correctly sized and operate efficiently
Risks related to moisture, ventilation, and overheating are managed
Public funding delivers robust, long term value

This fabric first, system second approach is central to Warm Homes Local Grant delivery and reflects both technical best practice and full PAS 2035 compliance.

Typical EPC Band Movement Whole House Retrofit Approach WHLG

One of the key performance indicators for Warm Homes Local Grant delivery is improvement in EPC band, which is directly linked to increases in the property’s SAP score. While outcomes vary depending on starting condition and property type, Whole House Retrofit Approach WHLG, sequenced approach is designed to enable step change improvements, rather than marginal gains.

The examples below illustrate typical EPC band movements seen when SAP focused measures are installed in the correct order.

An Example of Whole House Retrofit Approach WHLG:

Fabric → Heat Pump → Solar PV, Battery

Property Profile (Before Retrofit)

Property type: 1930s semi detached house
Construction: Solid walls, uninsulated loft
Heating: Gas boiler, basic controls
Starting EPC band: E (SAP ~45)
Key issues: High heat loss, high running costs, poor thermal comfort

Stage 1 Whole House Retrofit Approach WHLG : Loft and Wall Insulation

Measures installed

Loft insulation top‑up to 300 mm
External or internal wall insulation to solid walls
Associated airtightness improvements

Technical impact

Significant reduction in fabric heat loss
Lower space heating demand
Improved internal temperature stability

SAP / EPC outcome

Typical SAP increase: +15 to +20 points
EPC movement:
E (45) → D / low C (60- 65)

Why this stage matters This stage creates the thermal baseline needed for efficient low carbon heating. Without these fabric improvements, later measures would be less effective and more costly.

Stage 2 Whole House Retrofit Approach WHLG: Low Carbon Heating System (Heat Pump)

Measures installed

Air‑source heat pump
Upgraded radiators where required
Improved heating controls and zoning
(Where applicable) preparation for integration with on site renewable electricity and battery storage

Technical impact

Replacement of fossil‑fuel heating with low‑carbon technology
Much higher system efficiency (SCOP >3)
Significantly lower carbon intensity, strongly rewarded within SAP
Electrification of heat, enabling effective integration with solar PV and battery storage

SAP / EPC outcome

Typical SAP increase: +10 to +20 points
EPC movement:
D / C → solid C or low B (SAP 70- 80)

Why this stage follows insulation

With heat demand already reduced through fabric improvements, the heat pump can:

Operate at lower flow temperatures
Be correctly sized (smaller and cheaper to install and run)
Deliver maximum SAP uplift
Operate more effectively alongside solar PV and battery storage, reducing reliance on grid electricity

This sequencing ensures both technical performance and cost effectiveness are optimised.

Stage 3 Whole House Retrofit Approach WHLG: Solar Photovoltaic (PV) Panels and Battery Storage

Measures installed

Roof mounted solar PV system (e.g. 3–4 kWp)
Domestic battery storage system

Technical impact

Reduction in calculated purchased electricity within SAP
Strong interaction with electric heating systems, particularly heat pumps
Battery storage enables surplus solar electricity generated during the day to be stored and used later
Stored energy can support heat pump operation, hot water production, and household loads during evening or peak tariff periods
Increased on site energy self consumption improves household resilience to energy price volatility

SAP / EPC outcome

Typical SAP increase: +5 to +12 points
EPC movement:
C → B (or strengthening an existing B rating)

Final EPC position
✅ E → C → B, achieved through a sequenced Whole House Retrofit Approach WHLG

Summary of EPC Band Movement

Retrofit Stage	Typical EPC Band
Before works	E
Loft & wall insulation	D / low C
Heat pump installed	Strong C / low B
Solar PV + battery added	B

Why This Example Matters for Warm Homes

This example demonstrates that:

Fabric first measures unlock later system improvements
Heat pumps deliver maximum SAP benefit only after insulation
Solar PV combined with battery storage acts as a final optimiser, improving SAP outcomes and supporting progression to higher EPC bands

Crucially, no single measure achieves this outcome alone. The combined, sequenced package delivers: