The short answer
There is no universal winner. SEAI describes cavity, external and internal insulation as different approaches for different wall constructions and constraints. The safe choice starts with a survey of the actual wall, its exposure, moisture condition, ventilation and junction details.
| Check | Cavity wall | External wall | Internal wall |
|---|---|---|---|
| Starting question | Is there a suitable unfilled cavity? | Is an external system suitable for the walls, boundaries and elevations? | Can internal space and junctions be altered safely? |
| Access | Mostly from outside | External access and often scaffolding | Access to internal faces of external walls |
| Details to resolve | Cavity condition, exposure, defects and services | Eaves, sills, services, vents, damp and boundaries | Floors, ceilings, reveals, sockets, moisture and ventilation |
| Evidence to request | Survey findings and installed system specification | System specification, certification and junction drawings | Build-up, moisture strategy and junction drawings |
How to narrow the route
Cavity wall insulation
SEAI notes that not every home has a cavity and recommends checking where uncertain. A quote should follow an inspection, not a guess based only on the home's age. Ask what was observed, whether existing fill or defects were found, and why the proposed material is suitable for the wall and location.
External wall insulation
SEAI describes external wall insulation as a possible route for solid block or concrete walls without a cavity, and notes that planning permission or an ESB Networks service alteration may be relevant in some cases. The proposal should address boundaries, openings, roof edges, ventilation and moisture—not just the area of insulation.
Internal wall insulation
SEAI presents internal insulation as an option where there is no suitable cavity and external insulation is not possible or appropriate. It changes the inside of rooms, so the scope should state how reveals, floors, ceilings, services, airtightness, ventilation and finishes will be handled.
Checks for Cork and exposed locations
Exposure and moisture deserve a property-specific assessment. Ask the surveyor to record wall condition, damp or water-ingress defects, exposure, existing ventilation and any repairs required before insulation. Do not select a material from a generic county-wide rule.
Read SEAI's insulation, windows and doors guidance and the grant-route checklist. No route, grant, price, saving or outcome can be confirmed by this general guide.