External vs internal wall insulation: which is right for your house?
Both work. They suit different houses, and the wrong choice is expensive to undo. The decision usually comes down to three things: whether the outside of your house can be changed, how much internal space you can afford to lose, and how exposed the property is.
The short version
| External | Internal | |
|---|---|---|
| Floor area lost | None | Roughly 60–100 mm per external wall |
| Disruption inside | Almost none | Significant — rooms cleared, skirting and sockets moved |
| Thermal bridging | Largely eliminated | Harder to control at floors and party walls |
| Appearance | Changes the exterior | Exterior unchanged |
| Cost | Higher | Lower per m² |
| Can be done room by room | No | Yes |
When external is the right answer
External insulation suits solid-wall, stone and hollow-block houses with a render or pebbledash finish that nobody is precious about. It keeps the existing wall warm and dry, which matters more than people realise — a cold wall behind internal insulation stays cold and is where condensation risk concentrates.
It is also the stronger choice on exposed western sites. A continuous render system sheds wind-driven rain before it reaches the structure. On the Atlantic coast that is not a refinement; it is the difference between a wall that performs and one that does not.
When internal is the right answer
Internal insulation is often the only option on a brick house you want to keep looking like a brick house, or in a conservation area where the exterior cannot be altered. Dublin's Victorian terraces are the obvious case.
It is also the practical choice if you are renovating room by room, or if budget rules out doing the whole house at once. You can insulate the coldest rooms first.
The catch is detailing. Where an internal wall or an intermediate floor meets the external wall, the insulation is interrupted, and that junction becomes a cold bridge unless it is returned properly. A crude internal job can move a condensation problem rather than solve it.
What about cavity fill?
If your house has a cavity that is clean, dry and wide enough, filling it is far cheaper than either option above and causes almost no disruption. It should usually be the first question, not an afterthought.
The exception is severe exposure. Certification for bonded-bead systems restricts their use on the most exposed sites, because a wet cavity conducts heat rather than stopping it. A borescope survey establishes what you actually have.
Common questions
Internal is cheaper per square metre. External often gives better value overall because it eliminates thermal bridging and costs no floor area, but the upfront figure is higher.
It can if detailed badly. Insulating the inside face leaves the wall itself cold, so the build-up and the junctions at floors and party walls have to be designed to keep moisture out of that cold zone.
Typically 60–100 mm off each external wall depending on the system and the performance required. In a small room that is noticeable.
Rarely worthwhile. The SEAI wall grant is capped at €8,000 across all wall measures, and doing both gives diminishing returns for the cost.
External, in most cases. A continuous render system keeps wind-driven rain off the structure entirely, which matters greatly on the west coast.
Want a figure for your own house?
A measured survey gives you the actual cost and the grant that applies. €100, refunded in full when you order.
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