The fabric first approach is the government-recognised strategy for large-scale domestic retrofit in the UK, promoted throughout PAS2035 and underpinning all major funded schemes. It is built on a single core principle: minimise a building's energy demand first, before making changes to its heating system or installing renewable technology.

Understanding what fabric first means in practice — and what it does not mean — is essential for everyone involved in retrofit planning and delivery.

What Fabric First Actually Means

A fabric first approach focuses improvements on the building's physical envelope — the walls, roof, floor, windows and doors that separate the interior from the outside environment. By improving the thermal performance of this envelope, the energy required to maintain comfortable indoor temperatures is reduced at source, regardless of how that energy is generated.

This is distinct from an approach that installs a more efficient or lower-carbon heating system in a poorly performing building. A heat pump in an uninsulated home will work harder, cost more to run and deliver a lower carbon saving than the same heat pump in a well-insulated home. The fabric first principle holds that insulating and sealing the building first makes every subsequent technology investment more effective.

The Five Key Principles

1. Optimising insulation

Improving insulation in walls, roofs and floors reduces heat loss in winter and heat gain in summer. The priority order typically follows the cost-effectiveness of each measure — loft insulation first (cheapest, highest impact), followed by cavity wall insulation where feasible, then solid wall insulation and floor insulation.

2. Reducing thermal bridging

Thermal bridges are points in the building fabric where heat escapes more readily than through the surrounding construction — typically at junctions between elements, around window and door frames, at wall-roof and wall-floor junctions. A fabric first retrofit addresses these systematically to create a continuous, effective thermal barrier rather than insulating the main areas while leaving significant heat loss pathways untreated.

3. Airtightness

Uncontrolled air leakage through gaps, cracks and joints in the building fabric is a significant source of heat loss in older UK housing. Improving airtightness — through draught proofing, sealing service penetrations and improving window and door seals — reduces the energy required to maintain temperature.

4. Passive design strategies

Where relevant, a fabric first retrofit considers passive design principles — maximising solar gain through south-facing glazing in winter, minimising overheating through shading in summer, and using the thermal mass of the building fabric to moderate temperature swings. These strategies use the physical characteristics of the building rather than active systems.

5. Balanced ventilation

Improving airtightness without addressing ventilation is not a fabric first approach — it is an incomplete one. As buildings become more airtight, controlled ventilation must replace the uncontrolled infiltration that previously provided background air movement. A fabric first retrofit always incorporates a ventilation strategy alongside the fabric improvements.

Benefits of the Fabric First Approach

• Long-term impact: Improving the fabric redefines the baseline energy demand of the property permanently. The benefits do not depend on resident behaviour or on the continued operation of any particular technology
• Technology agnostic: A well-insulated building performs better with any heating source — gas, heat pump, district heating or direct electric. Fabric improvements future-proof the property against changes in heating technology
• Comfort: Consistent indoor temperatures, elimination of cold surfaces and draughts, and improved moisture management all directly improve the living conditions of residents — independently of any reduction in energy bills
• Cost-effectiveness: The long-term operational savings from fabric improvements typically justify the upfront investment, particularly when combined with funded schemes that reduce or eliminate the capital cost to the resident
• Carbon reduction: Reducing energy demand reduces carbon emissions regardless of the carbon intensity of the energy supply, making fabric improvements valuable under any future energy scenario

Fabric First in the Context of PAS2035

PAS2035 promotes the fabric first approach throughout its framework. The Improvement Option Evaluation process requires that fabric measures are considered before heating system changes. The Medium Term Plan must sequence measures logically so that fabric improvements are completed, or at least planned, before low-carbon heating systems are specified.

This does not mean that heating systems can never be changed before fabric improvements are complete — there are circumstances, particularly in off-gas-grid properties, where replacing an inefficient heating system is a priority. But the PAS2035 framework requires that the whole-house strategy is planned coherently, with fabric improvements sequenced to maximise the benefit of subsequent heating and technology installations.

Common Misconceptions

The fabric first approach is sometimes misunderstood as meaning that all insulation must be completed before any other work begins. This is not the case. It means that the retrofit strategy should be planned with fabric improvements as the foundation, and that heating system changes should be sized and specified on the basis of the post-improvement fabric performance rather than the current performance.

It also does not mean that renewable technology cannot be installed in parallel with fabric works. Solar PV can be installed before fabric improvements are complete — but the system should be sized for the anticipated post-improvement energy demand, not the current demand.