Can You Mix External and Internal Wall Insulation? – Can I Just Ask? | Ep.22

Mixing external and internal wall insulation on the same building is a question that comes up regularly — typically where a property has an attractive front elevation that the owner doesn’t want to clad, but plainer sides and rear that are well suited to external wall insulation. The short answer is yes, it’s entirely possible, but it requires careful thought at the junctions where the two systems meet.

The key principle is ensuring the two insulation layers overlap sufficiently to eliminate the risk of thermal bridging at the transition point. Rather than returning the full thickness of internal wall insulation all the way along the side walls to meet the external insulation, a thinner board — typically around 20mm — is used to make that overlap, with a return of around 500mm being sufficient to bridge the junction effectively. This keeps the detail manageable and avoids unnecessarily bulky returns on the internal walls.

Where you make that transition also matters. Choosing a natural break point such as a chimney stack or an internal partition wall gives a clean, logical stopping point for the internal insulation, rather than creating a junction mid-wall. The other general principle is to minimise the number of these transition junctions on any given project — the fewer points where the two systems need to meet, the simpler and lower risk the overall installation becomes.