Understanding the "Wall of Death": Why Insulation in Masonry Buildings Fails

In many buildings, particularly those built with a single-skin masonry wall, it is common to add a steel stud frame on the interior, place fibrous insulation in the cavity, and finish the wall with plasterboard. This approach is appealing because it is inexpensive and easy to install. This build-up carries a significant and often overlooked risk, however. Because the insulation sits on the inside, the masonry remains close to the outside temperature, even when the interior is heated. The inner face of the masonry can therefore stay cold. When indoor air contains moisture, which is true in almost all occupied buildings, this cold surface can fall below the dew point and cause condensation to form behind the lining. Over time, this hidden moisture can lead to mould, dampness, odours, structural decay and unhealthy indoor conditions (“The Wall of Death – DCTech”).  

Why Common “Fixes” Often Fail 

A typical solution is to install a vapour barrier on the interior side of the insulation so that moisture from the inside cannot reach the cold masonry. Although the idea is straightforward, vapour barriers rarely perform as intended. Interior linings cannot be made perfectly airtight, and even minor gaps created by movement, penetrations and general construction tolerances allow moist air to bypass the barrier. Once moisture enters the cavity, it becomes trapped between impermeable layers and cannot dry (“The Wall of Slow Death” – DCTech)”. 

Some builders turn to rigid closed-cell foam insulation as an interior moisture control strategy. This is sometimes called the “Wall of Hope.” Rigid foam can act as a more continuous air and vapour barrier. However, many rigid foam products are combustible. This creates difficulties for projects that require non-combustible wall systems under Type A or Type B construction. Achieving compliance usually requires assessment by a fire engineer, which adds cost and complexity and can make the approach impractical. 

The Right Approach: The “Perfect Wall” 

A more robust and code compliant solution is to install insulation on the exterior face of the masonry. This creates a continuous layer of non-combustible insulation outside the structure, which is then protected by a ventilated rainscreen or cladding. With this configuration, the masonry remains inside the insulated envelope and stays much closer to indoor temperatures. This means the masonry stays above the dew point, which significantly reduces the risk of condensation and prevents hidden moisture damage. External insulation also allows the wall assembly to meet fire performance requirements because the insulation is non-combustible (“The Perfect Wall – DCTech”). 

Summary and Recommendation 

Internal insulation of single-skin masonry walls may look cost-effective, but the hidden moisture risk often outweighs the short-term savings. Vapour barriers and interior foam systems reduce risk only in theory and introduce their own limitations in practice. External continuous insulation combined with a ventilated rainscreen is the most reliable, durable and compliant solution. It prevents condensation, protects building materials and supports long-term building performance. 

For more details, diagrams and the original discussion, read Tech Note 04: The Wall of Death here.