Module 2 · Buildings and How They Handle Moisture
2.4 Modern Cavity Walls and Built-In Insulation
9 min read
From the 1970s the cavity stopped being an empty void and became a place to put insulation, and the failure modes moved with it. A partial-fill board with a residual cavity behaves quite differently from a full fill, which behaves differently again from something injected into a wall built for neither. Knowing which you are looking at, and where the resulting cold plane sits, predicts where any moisture will show up before you have found it.
Learning objectives
By the end of this lesson you will be able to:
- Distinguish partial fill, full fill and retrofit injected fill by inspection.
- Locate the cold plane for each and predict where condensation will occur.
- Recognise the signature of a slumped or voided fill.
- Assess whether damp at a specific location is a fill problem or a detailing problem.
Key terms
- Partial fill
- Rigid insulation board fixed against the inner leaf with a clear residual cavity — usually 50mm — maintained in front of it. Preserves the drainage path.
- Full fill
- Insulation occupying the entire cavity, typically mineral wool batts built in as the wall rises. No residual drainage void.
- Retrofit injected fill
- Bead, foam or blown fibre injected into an existing cavity through drilled holes. The commonest, and the one most dependent on pre-installation survey.
- Residual cavity
- The clear void kept in front of partial-fill insulation. Its whole purpose is to remain unbridged; mortar droppings and slipped boards defeat it.
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