Assess the house as one connected system before choosing any retrofit measure. A useful assessment links the building’s construction and insulation continuity with uncontrolled air leakage, deliberate ventilation, moisture risk, heating demand, controls and the way occupants use each room. The result should be a whole-house plan, even if the budget means completing it in stages.
Do not treat “more airtight” as an objective on its own. Sealing unintended leakage can reduce draughts and heat loss, but fresh air and moisture removal still need a deliberate route. Heating should be considered against the fabric and ventilation outcome you actually plan to create, not sized from the house’s current condition alone.
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What a whole-house assessment must answer
A retrofit decision is sound only when it explains how the proposed measures interact. The UK government’s 2014 Retrofit for the Future guide puts the relationship plainly: “Services (such as heating and hot water systems, lighting, renewable energy, controls) have to work with the insulation, airtightness and ventilation of a home to provide comfort and convenience.” It also states, “However, people still need fresh air to be brought in as ventilation.”
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In practical terms, your assessment should establish:
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- How the home is built, where heat is lost and whether insulation is continuous at junctions and penetrations.
- Which air paths are accidental leakage and which openings or systems are intentional ventilation.
- Whether damp, condensation, rain exposure or defective building fabric must be corrected before insulation.
- How the proposed fabric and ventilation work changes heat demand, room comfort, hot-water needs and control settings.
- Which measures depend on one another and how a staged programme can avoid making later work more difficult or expensive.
The purpose is not to select a technology in isolation. It is to define a compatible package for this building and this household.
Step 1: Establish the home’s baseline
Record construction and retrofit history
Start with the property’s age, construction type, exposed elevations, roof and floor forms, existing insulation and any previous retrofit. Identify whether walls are cavity or solid, which areas are accessible, and where the construction is uncertain. Record past repairs, extensions and changes to windows, doors, heating or ventilation; these often create discontinuities that are invisible from a room-by-room walk-through.
An Energy Performance Certificate (EPC) is a useful starting pointer, but it is not a substitute for a physical survey. Check that it is current and test its assumptions against what is actually present. An EPC may not reveal missing insulation, thermal bridges, blocked vents, altered chimneys or moisture damage.
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Describe how the household uses the building
Document occupancy patterns, rooms that are used heavily or kept closed, comfort complaints, hot-water demand, working-from-home or bedroom schedules, and any health or accessibility considerations. The same building can need a different package depending on whether rooms are continuously occupied, heated intermittently or used by people sensitive to cold, humidity or disruption.
Find defects before adding insulation
Inspect for roof and wall leaks, defective rainwater goods, damp, condensation, mould, blocked ventilation openings, failing finishes and structural defects. Solid-wall guidance advises resolving existing damp first and considering wall and roof condition and exposure to driving rain before installing insulation. Insulation can conceal a defect, alter drying behaviour or make a moisture problem harder to diagnose.
Step 2: Map heat loss and insulation continuity
Survey every envelope element
Create an element-by-element record for the loft or roof, walls, floors, windows, external doors and any areas adjoining unheated spaces. For each element, distinguish what is confirmed from what is assumed. Note thickness and condition where accessible, inaccessible voids, damaged or missing sections, and planned repairs or façade work.
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The objective is a continuous, buildable thermal envelope rather than the maximum nominal insulation in each component. Pay particular attention to junctions between roof and wall, wall and floor, extensions, dormers, corners, window reveals, service penetrations and places where insulation stops. These are common thermal-bridge locations and can become colder after other areas are upgraded.
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Material selection and placement must suit the wall and roof construction. Government best-practice guidance for internal wall insulation covers viability, design, material selection and hygrothermal considerations; those questions belong in the assessment before a system is specified. A proposal should explain how rain, vapour and drying are expected to behave, not simply state an insulation thickness.
Compare internal and external solid-wall options honestly
For a solid wall, compare internal and external insulation against the actual constraints:
- Internal insulation: may reduce usable floor area and require careful treatment of window reveals, floors, ceilings, services and junctions. It can be less disruptive to the external appearance but disruptive inside the home.
- External insulation: can preserve internal floor area and improve continuity across some junctions, but it changes the external finish, requires access and may affect gutters, sills, eaves, boundaries or heritage appearance.
- Both approaches: require the wall condition, rain exposure, existing damp and ventilation implications to be assessed. Energy Saving Trust notes that either form may reduce airflow and that the installer should determine whether additional ventilation is needed.
Do not approve insulation solely because its quoted thermal performance looks attractive. The assessment should show how it fits the existing wall, moisture strategy and future work.
Step 3: Separate airtightness from ventilation
Identify unintended leakage
Airtightness concerns uncontrolled air movement through cracks, gaps and service penetrations in the envelope. Inspect junctions around windows and doors, floor edges, loft hatches, pipes, cables, recessed fittings, chimneys, roof junctions and previous alterations. Record whether a proposed seal is part of the intended air barrier and whether it can be inspected and maintained after work.
Map intentional ventilation
List extract fans, trickle vents, passive vents, airbricks, chimneys and any mechanical ventilation. Establish which openings are intended to supply or remove air, whether they work, and which have been blocked or disconnected. Ask how fresh air will enter living and sleeping spaces and how moisture will be removed from kitchens, bathrooms and utility areas after each proposed measure.
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The government guide’s warning is explicit: “To be certain of good indoor air quality and low condensation, airtightness and ventilation have to work together in all seasons.” A plan that simply seals vents or fills gaps without replacing the ventilation function is incomplete.
Interpret pressure-test results correctly
A pressure test measures airtightness under controlled test conditions. It does not directly measure the infiltration that occurs during normal occupation. Everyday air movement also responds to wind, indoor–outdoor temperature difference, building height, location and sheltering.
Ask the assessor to state:
- What was measured and under what test conditions.
- Which parts of the building were included or excluded.
- What was estimated rather than measured.
- How weather, stack effect and sheltering could change normal infiltration.
- Which leakage paths are intended to be sealed and what ventilation remains afterward.
Do not present a divide-by-20 calculation as a precise forecast for an individual home. The Department for Energy Security and Net Zero’s September 2025 evidence review found limitations in existing infiltration-estimation methods and identified the need for further validation.
Step 4: Assess heating against the planned fabric outcome
Document the existing system
Record the heat source, hot-water arrangement, controls, distribution, emitters, maintenance condition and rooms that fail to reach or hold their target temperature. Include programmer or thermostat locations, zoning, cylinder or storage arrangements and any simultaneous hot-water demands. A heating assessment that ignores controls and emitters cannot explain the comfort outcome.
Model the post-retrofit demand
Ask for heating demand to be considered after the stated insulation, airtightness and ventilation package. Sizing replacement capacity from the current, leaky condition can produce a system that does not match the intended future load; sizing it without stating which fabric measures are assumed makes the result impossible to evaluate.
The proposal should show its assumptions about:
- Current and expected heat demand.
- Which roof, wall, floor, window or airtightness measures are included.
- The ventilation strategy and any electrical or ductwork requirements.
- Emitter temperatures, distribution changes and control arrangements.
- Hot-water demand and the effect on storage or recovery time.
- Comfort by room, disruption, maintenance and what changes if a later phase is deferred.
The official retrofit guidance supports this joined-up approach but does not prescribe one heat source for every home. A heat pump, boiler, communal system or other option must be judged against the building, the planned fabric outcome and the household’s requirements.
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Step 5: Turn the findings into a phased whole-house plan
A whole-house plan does not mean carrying out every measure at once. Energy Saving Trust says work can be staged, provided the intended future measures are considered so that today’s work fits the larger plan.
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- Explain the reason for each measure. Link it to heat loss, comfort, leakage, moisture, ventilation, durability or operating cost.
- Set dependencies. Identify repairs that must precede insulation, ventilation changes that must accompany airtightness work, and heating changes that depend on a stated fabric outcome.
- Define the ventilation and moisture provisions. State how air is supplied, extracted and monitored after each phase.
- State costs and uncertainty. Separate firm quotations from allowances and identify assumptions, exclusions and likely triggers for variation.
- Describe expected performance. Give the baseline, the post-work assumption and the uncertainty; do not promise a generic percentage saving.
- Plan disruption and maintenance. Include access, room downtime, future inspection points, filter or fan maintenance and responsibilities after handover.
- Specify checks after completion. Define what will be inspected, commissioned or monitored and how defects will be corrected.
Sequencing should protect future options. For example, a first phase should not block access needed for later insulation, remove a ventilation route without replacement, or install heating controls that cannot accommodate the intended system.
How to compare two retrofit proposals
| Comparison axis | Questions to ask |
|---|---|
| Heat-loss and comfort outcome | What baseline and post-work demand are assumed? How will room-by-room comfort change? |
| Airtightness and ventilation | What is measured versus estimated? Which intentional ventilation remains, is repaired or is added? |
| Moisture and building condition | Were damp, rain exposure, roof and wall condition and drying behaviour examined? Which repairs precede insulation? |
| Heating fit | Is the system sized and configured for the expected demand after the stated fabric work? Are emitters and controls included? |
| Phasing and compatibility | Will the first phase make later insulation, ventilation or heating work harder or more expensive? |
| Cost and uncertainty | Which figures are quoted, which are estimates, and what assumptions or exclusions could change them? |
| Quality and evidence | Who assessed and designed the work, which standards apply, and what inspection or post-work evaluation is specified? |
These are decision axes, not a universal ranking of technologies. The appropriate package depends on the property and household.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What assessment and retrofit work may cost
Energy Saving Trust’s table, accessed 13 October 2026, gives average installation costs for a semi-detached home and excludes potential funding discounts. The figures are source-specific averages, not quotations for an individual property:
| Measure | Published average | Qualification |
|---|---|---|
| Cavity wall insulation | £2,200 | Average installation cost for a semi-detached home; funding discounts may be excluded. |
| Solid wall insulation | £12,000–£15,000 | Average installation cost for a semi-detached home; internal and external solutions can differ substantially. |
| Air-source heat pump | £12,000 | Average installation cost for a semi-detached home; not a design or quotation for a particular property. |
Prices, grants and labour conditions change. Confirm current figures and applicable funding before commissioning. A low headline price is not comparable if it omits ventilation, repairs, electrical work, controls, access, making good or commissioning.
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Several published figures help explain the strength and limits of available evidence:
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| Figure | What it represents | What it does not prove |
|---|---|---|
| 40 homes | Homes whose retrofit learning informed the 2014 Retrofit for the Future guide. | It is not a representative national sample. |
| 14 case-study homes | Homes examined in the DEEP synthesis report (2021) for technical performance and risk. | The findings cannot establish representativeness for the UK housing stock. |
| 73 documents | Studies included in DESNZ’s September 2025 GRADE assessment of infiltration evidence. | The count does not make any single estimation method reliable for every home. |
| 12 methods | Infiltration-estimation methods evaluated in the same DESNZ review. | No one method gives a universal prediction across weather and building types. |
No nationally representative percentage saving is established for this assessment pathway. Case-study results vary with the starting condition and retrofit specification, so a proposal should explain its own assumptions rather than borrow a generic saving claim.
Choosing the right assessment service
For significant or interacting measures, use a qualified retrofit assessor who can produce a whole-house plan rather than a single-product survey. Energy Saving Trust recommends qualified assessment and says obtaining more than one assessment can help compare recommendations. It mentions TrustMark as a route for finding a qualified assessor.
Before commissioning, verify the assessor’s current qualifications, scope, scheme requirements and credentials directly. Ask what the deliverable includes: building inspection, moisture and ventilation review, airtightness testing or interpretation, heating-demand assessment, drawings or specifications, phasing, cost assumptions and post-work checks. Clarify whether the person assessing the home is also designing or installing the work, and how conflicts of interest are handled.
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Choosing insulation before investigating damp
Symptom: a wall system is proposed without a condition or rain-exposure survey.
Correction: repair leaks, defective rainwater goods and damp causes first; require an explanation of moisture and drying behaviour.
Sealing openings without a ventilation design
Symptom: vents or extract fans are blocked or removed because they appear to be leakage paths.
Correction: classify each opening as unintended leakage or intentional ventilation and specify replacement air supply and moisture extraction before sealing.
Using a pressure-test result as normal infiltration
Symptom: one test number is converted into a precise annual heat-loss or air-change forecast.
Correction: record test conditions, distinguish measured from estimated values and explain weather, temperature, height, location and sheltering effects.
Sizing heating from the old building condition
Symptom: replacement capacity is selected before the proposed fabric and ventilation package is defined.
Correction: state the post-work demand assumptions, emitter and control requirements, and what happens if a later insulation phase is delayed.
Treating an EPC as the survey
Symptom: recommendations are accepted without checking construction, inaccessible areas or previous alterations.
Correction: use the EPC as an initial pointer, then verify the physical building and household needs.
Questions to put in writing before work starts
- What is known about the construction, and what remains uncertain?
- Where are the insulation gaps, thermal bridges and air-barrier discontinuities?
- Which defects must be repaired before insulation or airtightness work?
- Which air paths are intentional ventilation, and how will they operate after each phase?
- What was measured, what was estimated and under what conditions?
- What heat demand is assumed before and after the proposed work?
- How do emitters, controls, hot water and ventilation fit the proposed heating system?
- Which future measures were considered when designing the first phase?
- Which costs are quotations, allowances or exclusions?
- What commissioning, inspection and monitoring will demonstrate that the work performs as intended?
If a proposal cannot answer these questions, it is not yet a whole-house retrofit plan.
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