Understanding Retrofit Risk Levels Under PAS2035

PAS2035:2019 (soon superseded by BS 8534:2025) establishes a risk-based approach to building retrofit work. The standard defines three retrofit risk levels that directly influence how projects are planned, surveyed, designed and delivered. Getting the risk level wrong at project inception can lead to inadequate surveying, unsuitable installer teams, and ultimately, poor retrofit outcomes.

The Three Retrofit Risk Levels

Low Risk Retrofits

Low risk retrofits involve straightforward interventions with minimal interaction between building systems and the retrofit measures. These projects typically:

• Target single building elements in isolation
• Have well-understood existing conditions
• Present minimal risk of unintended consequences
• Do not require complex integrated design solutions

Common examples include cavity wall insulation in standard construction, simple window replacement, or basic boiler upgrades. For low risk work, a standard surveying approach and competent installers (NVQ Level 2 or equivalent) are generally sufficient.

Medium Risk Retrofits

Medium risk retrofits involve multiple building elements or systems working together, requiring careful coordination but with manageable complexity. These include:

• Loft insulation combined with ventilation improvements
• External wall insulation with window upgrades
• Heating system replacement with controls upgrades
• Projects affecting moisture management or thermal performance in interconnected ways

Medium risk work demands enhanced surveying, including specialist assessments and building physics analysis. Retrofit coordinators must manage interactions between measures, and installers require higher competency (typically NVQ Level 3 or PAS2035 specific training).

High Risk Retrofits

High risk retrofits involve complex buildings, multiple interdependent systems, or significant unknowns that could lead to unintended consequences if not properly managed. These typically include:

• Historic buildings with unknown construction or structural concerns
• Solid wall insulation with complex moisture risk assessment
• Whole-building energy performance improvements requiring integrated design
• Projects where retrofit measures could affect structural integrity, fire safety, or health and safety
• Non-standard construction or buildings with limited retrofit history

High risk work requires thorough baseline surveying, specialist consultancy, detailed risk assessment, and close coordination throughout delivery. All team members must demonstrate advanced competency in building science and retrofit principles.

Determining Risk Level For Your Project

Risk level assessment depends on several factors working together:

Building Characteristics

1. Construction type: Standard modern construction is typically lower risk than solid wall, timber frame, or historic buildings
2. Age and condition: Well-maintained buildings with known construction details present lower risk
3. Complexity: Multi-storey or unusual geometries increase risk
4. Existing systems: Unknown or poorly documented systems elevate risk

Retrofit Scope

1. Number of measures: Single interventions are lower risk than integrated packages
2. System interactions: Measures affecting multiple building systems (thermal, moisture, ventilation, structural) increase risk
3. Known precedent: Retrofit approaches with established track record in similar buildings are lower risk

Technical Unknowns

1. Existing construction details: Lack of clarity about wall construction, hidden cavities or unusual specifications raises risk
2. Moisture risk: Uncertain moisture behaviour (particularly with external insulation on solid walls) significantly increases risk
3. Performance gaps: Uncertainty about actual vs. designed performance increases risk

Implications For Project Delivery

Surveying and Design

Low risk projects may use desktop assessment and visual surveys. Medium risk typically requires intrusive surveying and specialist input. High risk demands detailed baseline survey, building physics modelling, and potentially third-party review.

Team Competency

Ensure your retrofit coordinator and installers hold appropriate competency for the assessed risk level. PAS2035 and the emerging BS 8534 standard increasingly define specific training and accreditation requirements for retrofit work.

Quality Assurance

Higher risk projects justify increased QA spend—including independent inspections, commissioning testing, and performance monitoring post-completion. This investment protects against costly defects or performance shortfalls.

Programme and Budget

Risk level directly affects project cost and timescale. High risk retrofits require more planning, consultation, and quality control. Underestimating risk risks budget and programme overruns when unforeseen issues emerge during delivery.

Best Practice Approach

Adopt a systematic risk assessment process early in project definition:

1. Gather baseline information about the building and proposed retrofit scope
2. Consult with retrofit specialists to identify key risk factors
3. Document the risk assessment rationale and evidence
4. Confirm risk level with relevant technical authorities before finalising specifications
5. Ensure all team members understand the assessed risk level and its implications
6. Review risk assessment if scope changes significantly during design or delivery

Getting retrofit risk levels right ensures appropriate investment in surveying, design and delivery quality. This translates to better outcomes, fewer defects, and buildings that perform as intended for years to come.