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.
What do the Building Regulations require?
When a house is renovated, energy savings must be carried out when they are cost-effective and do not create a risk of moisture damage (BR18 § 274). A measure is cost-effective when the annual saving times the lifetime divided by the investment is greater than 1.33 (§ 275). If there is a cavity with room for insulation, it must first be investigated whether it is cost-effective to insulate the cavity (§ 275(2)).
- 01External walls in renovation: U-value at most 0.18 W/m²K (§ 279 and annex 2, table 3).
- 02Foundations in renovation: linear loss at most 0.12 W/mK (same table).
- 03Listed buildings and buildings worth preserving are exempt if the requirements conflict with their architectural or cultural-historical values (§ 278).
Which houses are they?
The chart shows residential buildings with a brick facade by decade of construction. Notice the two largest bars: the 1960s and 1970s. That is the cavity wall generation. Many of these houses have a cavity wall that can be retro-insulated and that must be assessed against BR18's requirements on cost-effectiveness and moisture.
| Built | residential buildings |
|---|---|
| 1900–09 | 82,337 |
| 1910–19 | 62,231 |
| 1920–29 | 78,063 |
| 1930–39 | 96,249 |
| 1940–49 | 61,596 |
| 1950–59 | 97,951 |
| 1960–69 | 206,703 |
| 1970–79 | 268,563 |
| 1980–89 | 134,052 |
| 1990–99 | 62,154 |
| 2000–09 | 93,238 |
| 2010–19 | 63,355 |
| 2020–29 | 63,738 |
The brick houses from the 1960s and 1970s are the largest group. These are the cavity walls with wall ties that need inspecting, repointing and retrofit insulation — the trade's market is maintenance too, not just new build.
External wall material as registered in BBR. Buildings from before 1900 are not included.
Total for all of Denmark: 1,370,230 residential buildings.
Source: BBR (Datafordeler) via Grundfast · fetched
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').
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Next step: Mould inspection
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