Retrofit Insulation: Risks and How to Mitigate Them

Retrofit insulation appears simple — add insulation, reduce heat loss, save money. Done badly, it causes interstitial condensation, rot, mould, and structural decay. Get it right and savings are real and durable.

The three main approaches

External wall insulation (EWI)

Insulation outside existing wall, finished with render or cladding. Best thermal performance, preserves internal space, no condensation risk to wall fabric.

• Cost: R2,000–R4,000 per m² of wall
• Changes external appearance — often refused in heritage protection overlay zones
• Window reveals, eaves, junctions require careful detailing

Internal wall insulation (IWI)

Insulation inside existing wall, finished with plasterboard. Suits heritage where external appearance must be preserved.

• Cost: R1,500–R3,000 per m² of wall
• Reduces internal room size
• Significant condensation risk — wall fabric stays cold and damp
• Specialist breathable systems required for solid-wall heritage

Loft insulation

Mineral wool over and between ceiling joists. Cheapest and highest payback retrofit.

• Cost: R300–R700 per m²
• Watch for ventilation gaps at eaves
• Storage above insulation requires raised boarding

Condensation risk

Insulating cools the wall outside the insulation layer. Warm internal air, if it reaches that cold layer, condenses. Common failure points:

• Junctions where insulation discontinues (window reveals, joists)
• Penetrations (services, sockets)
• Missed vapour control layer (VCL)
• Air leakage past insulation

Breathability vs vapour-tight

• Modern construction: vapour-tight strategy — VCL prevents moisture reaching cold layers
• Heritage solid walls: breathable strategy — moisture passes through wall without accumulating

Mixing strategies (vapour-tight IWI on lime-mortar wall) traps moisture and accelerates decay. Specify holistically.

Bridging

Thermal bridges around windows, at floor junctions, and at openings undermine insulation performance. Pre-fabricated insulated reveal kits help.

Mortgage and warranty

• EWI usually accepted by lenders if PAS 2030 installation
• IWI sometimes flagged as risk by surveyors
• SWIGA / IWA guarantees increasingly required

MCFAR advises on retrofit insulation strategy.

Frequently Asked Questions