Retrofitting of masonry is a growing part of the craft: cavity wall insulation, internal and external retrofitting. Here are the three methods, their thermal bridges and moisture risks — and why internal insulation is the one that most often goes wrong.
Introduction
The mason's role in energy renovation is growing with the climate agenda: every kWh that is not lost through the external wall matters. A masonry external wall can be retrofitted with insulation in three ways — by filling the cavity, by insulating from inside, or by insulating from outside. Each of the three methods has its own advantages, cold bridges and moisture traps, and it is especially moisture that determines whether a retrofit insulation becomes a success or causes damage.
Three methods
- 01Cavity wall insulation — insulation granulate is blown into the existing cavity between outer and inner leaf.
- 02External retrofit insulation — insulation on the outside of the facade, finished with a plaster system (ETICS) or a new skin wall.
- 03Internal retrofit insulation — insulation on the inside of the outer wall; often a last resort when the facade cannot be altered.
Cavity wall insulation
Many older houses have an uninsulated or only partially insulated cavity wall. Insulation granules — mineral wool granules or EPS pellets — are blown in through holes in the joints. It is cheap and fast, but requires a clean, continuous cavity of sufficient width without mortar bridges, and that the outer leaf is tight so that rain penetration via the new insulation does not create a moisture bridge to the inner leaf. The cavity width and suitability is assessed (e.g. with an endoscope) before work.
External additional insulation
External insulation is technically the best solution: it wraps the entire façade, so thermal bridges at floor separations and window sills almost disappear, and it moves the dew point out into the new insulation — away from the existing construction — so the moisture risk indoors is reduced. There are two main finishes: a plaster system on the insulation (reinforcing mesh in reinforcement mortar plus finish plaster, see 'Plastering') or a new brick skin in front of the insulation with wall ties and ventilated cavity. The disadvantage is that the façade's appearance changes — therefore the method is often excluded on heritage and listed façades (see 'Restoration of older brickwork').
Members only
The rest of the article is locked
Create a free user account to unlock all articles, calculators, templates and range — and get access to quizzes and classes. It takes less than a minute: name, email and a password. No payment card.
Want to try something first? See what you can use without an account — the junior zone, the encyclopedia, the videos and more.