Module 6 · Retrofit and Moisture Risk
6.3 Internal Wall Insulation and Interstitial Condensation
10 min read
Internal wall insulation is applied to millions of solid-walled homes because external insulation is often impossible — conservation constraints, cost, or a terrace where only one house is being done. It is also the measure with the highest failure rate and the most severe consequences, because it makes the masonry colder while leaving it just as wet, and puts the resulting cold plane where nothing can see it and nothing can dry it.
Learning objectives
By the end of this lesson you will be able to:
- Describe the mechanism by which internal insulation increases risk.
- Identify the junctions and details where failures actually occur.
- State the conditions that must be satisfied before it is specified.
- Assess an existing installation for the features that predict failure.
Key terms
- Interstitial condensation
- Condensation forming within a construction rather than on its surface, where it cannot be seen and often cannot dry.
- Insulation/masonry interface
- The plane behind the insulation and against the wall. The new cold surface, and where the risk concentrates.
- Junction detailing
- How the insulation meets floors, ceilings, party walls, reveals and joist ends. Where almost all failures occur.
- Embedded timber
- Joist ends, wall plates and bonding timbers built into the wall. Now colder and still damp, and out of sight.
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