The Improvement Option Evaluation is one of the most important documents produced during a PAS2035 retrofit project — and one of the most frequently misunderstood. It is not simply a list of recommended measures. It is a structured analysis of multiple retrofit pathways for a specific property, produced by the Retrofit Coordinator on the basis of the pre-retrofit assessment data, and used to inform the client's decision about what work to commission and in what order.

Getting the IOE right matters. A poorly produced IOE — one that omits required measures, fails to model combinations correctly or does not address the whole-house strategy — creates problems at every subsequent stage of the project and is a common source of non-conformities at TrustMark lodgement.

The Purpose of the IOE

PAS2035 requires the IOE to evaluate a meaningful range of improvement options for the property, comparing their impact on the property's energy performance in a way that allows an informed decision to be made. The standard does not prescribe a single answer — it requires a genuine evaluation process that considers the full range of technically feasible options and presents the implications of each clearly.

The IOE must be based on the actual assessment data for the specific property. It cannot be produced from generic assumptions or template values. The RdSAP data from the pre-retrofit assessment, including the detailed condition data captured by the assessor, forms the foundation of the IOE analysis.

Required Content

A compliant IOE should contain all of the following:

Property Baseline

The current energy performance of the property, expressed as a SAP score and EPC band, forms the starting point. The IOE should clearly state the current position before any measures are applied, so that the improvement achieved by each option can be measured against it.

Assessment of Improvement Options

PAS2035 requires that a meaningful range of improvement options is modelled — not just the preferred or lowest-cost option. This should include combinations of fabric measures, heating system upgrades and renewables where relevant. Each combination should show the projected SAP score improvement, estimated fuel cost savings, carbon reduction and an indication of costs and payback.

Ventilation Strategy

Any measures that affect the airtightness of the property — including insulation measures, draught proofing and window replacement — must be accompanied by consideration of the ventilation strategy. The IOE should identify the ventilation risk and confirm that the proposed approach maintains adequate air quality. This is one of the most frequently missed elements in IOE production.

Overheating Risk Assessment

Where measures could increase the risk of overheating — particularly solid wall insulation and glazing improvements in properties with significant south-facing glazing — the IOE must address this. The overheating assessment does not need to be a full detailed analysis at IOE stage, but the risk must be identified and addressed in the measure recommendations.

Moisture and Interstitial Condensation Risk

Where insulation measures are proposed, the IOE should confirm that condensation risk has been assessed and that the proposed specification does not create moisture problems within the building fabric. Wall insulation in particular requires careful consideration of the moisture balance within the construction.

Medium Term Plan Relationship

The IOE feeds directly into the Medium Term Plan — the agreed phased approach to upgrading the property over time. The IOE should be structured so that the options it presents can be sequenced into a logical MTP, with earlier measures not compromising later ones. Insulating before addressing a failing heating system, or installing solar PV before improving the fabric, are examples of sequencing errors the IOE should identify and avoid.

Common IOE Failures

Documentation and Retention

The IOE is a required document for TrustMark lodgement. It must be retained as part of the project documentation and must be consistent with the measures that are ultimately installed. Where the client's decision results in a different package of measures being installed from those evaluated in the IOE, the reasons must be documented and the IOE updated accordingly.

A well-produced IOE is not just a compliance document — it is the technical foundation on which the whole retrofit project is built. Investment in getting it right at the outset avoids the delays, non-conformities and remediation costs that arise when it is treated as a formality rather than a genuine analytical exercise.