How to Evaluate Improvement Options Under PAS2035
PAS2035:2019 establishes a rigorous framework for whole-building retrofit, requiring a structured assessment of improvement options before implementation. Selecting the right combination of measures demands careful evaluation against technical, financial and practical criteria. This guide outlines the systematic approach housing associations and retrofit coordinators should follow.
Understanding the PAS2035 Context
PAS2035 mandates a tailored, risk-based approach to retrofit. Rather than applying standardized solutions, you must evaluate options specific to each building's characteristics, condition, occupancy and performance goals. This personalised approach ensures measures are effective and appropriate.
The standard requires:
- Building survey and detailed energy assessment
- Identification of viable improvement options
- Evaluation against defined criteria
- Selection of optimal measures
- Design development and risk mitigation
Stage 1: Establish Your Evaluation Framework
Define Clear Objectives
Before assessing options, clarify what you're trying to achieve. Is the primary goal energy performance improvement, compliance with future building standards, tenant comfort, or a combination? PAS2035 requires explicit clarity on performance targets.
Common objectives include:
- Reducing operational carbon emissions by X%
- Improving SAP ratings to meet funding criteria
- Addressing thermal comfort issues
- Reducing fuel poverty risk
- Extending building life and reducing maintenance costs
Identify Constraints and Opportunities
Document practical limitations early:
- Budget availability and funding routes
- Occupancy status (occupied, vacant, phased)
- Structural capacity and building condition
- Conservation or heritage restrictions
- Supply chain availability and lead times
- Tenant preferences and disruption tolerance
Key point: Document constraints in writing before evaluation begins. This prevents wasted analysis on technically sound but practically unfeasible options.
Stage 2: Generate and Scope Improvement Options
Your building survey should identify improvement measures across key areas:
- Fabric improvements: Wall insulation, roof insulation, floor insulation, window upgrades
- Heating systems: Boiler replacement, heat pumps, hybrid systems, district heating connection
- Hot water: Heat pump water heaters, solar thermal, storage optimisation
- Ventilation: Mechanical ventilation with heat recovery (MVHR), air tightness improvements
- Renewables: Photovoltaic panels, wind turbines (rarely suitable for retrofit)
- Controls and auxiliary: Upgraded thermostats, smart controls, lighting improvements
For each measure, establish basic technical feasibility and rough cost estimates at this stage.
Stage 3: Apply Systematic Evaluation Criteria
Technical Performance Assessment
Use your energy model to quantify the impact of each option:
- Estimated energy savings (kWh/year)
- Carbon reduction (tCO2e/year)
- Impact on SAP rating or EPC bands
- Improvements to thermal comfort (seasonal temperature ranges)
- Moisture risk and condensation impact
Model options both individually and in combinations. Interactions matter—insulation effectiveness depends on heating system capability, and heating system sizing depends on building fabric performance.
Financial Evaluation
Assess cost-effectiveness across multiple metrics:
- Capital cost: Full installed cost including design fees and contingency
- Annual savings: Quantified energy and maintenance cost reductions
- Simple payback: Capital cost ÷ annual savings (years)
- Lifecycle cost: Total cost over measure lifetime including replacements
- Cost per unit of outcome: £ per tonne CO2 saved annually
Consider grant availability. Some measures unlock funding that changes their cost-effectiveness profile.
Risk and Durability Assessment
Evaluate construction and performance risks:
- Moisture risk and interstitial condensation potential
- Thermal bridging and cold spots
- Building structure compatibility and structural loadings
- In-service durability and warranty provisions
- Maintenance requirements and future cost exposure
- Reversibility if performance proves inadequate
PAS2035 requires explicit risk assessment. Document assumptions and uncertainty ranges in your analysis.
Practical and Social Factors
Consider:
- Occupancy disruption during installation
- Tenant acceptance and behaviour change requirements
- Installation programme feasibility (critical path, sequencing)
- Supply chain maturity and installer availability
- Commissioning and handover complexity
Stage 4: Comparative Analysis and Selection
Create a comparison matrix evaluating options against your defined criteria. Weight criteria according to project priorities. For example, a carbon-focused retrofit may weight carbon savings heavily, whilst a fuel poverty programme prioritises cost-effectiveness and tenant comfort.
Typical selection process:
- Eliminate options failing basic technical or practical feasibility
- Score remaining options across weighted criteria
- Develop 2–3 shortlisted scenarios combining complementary measures
- Model full-building performance for each scenario
- Conduct sensitivity analysis (material price changes, energy price forecasts, discount rates)
- Present options to client with clear recommendation rationale
Stage 5: Document Your Rationale
PAS2035 requires clear documentation of evaluation methodology and decisions. Record:
- Building survey findings and performance baseline
- Options identified and reasons for exclusion (if applicable)
- Energy modelling assumptions and uncertainty ranges
- Cost data sources and lifecycle assessment approach
- Risk assessment conclusions
- Recommended package and justification
This documentation supports quality assurance, future retrofit stages, and accountability to funding bodies and tenants.
Conclusion
Systematic evaluation of improvement options under PAS2035 requires balancing technical rigour with practical pragmatism. By establishing clear objectives, applying consistent evaluation criteria, and documenting decisions transparently, retrofit coordinators can confidently select measures that deliver genuine performance improvement whilst managing costs and risks effectively.