What are the risks of unventilated cold roofs?

The risk of condensation in flat, unventilated roof constructions is well known, but in the context of climate change and evolving regulatory expectations, what was once a marginal risk has become significantly more serious.

Since early efforts in the 1970s to improve thermal performance, through warm and inverted roofs, lessons have been learned – but old problems persist, often exacerbated by newer materials or tighter building envelopes.

Nowadays, flat roofs can be very well insulated, meaning that a significant temperature gradient will probably exist through the structure, bringing about a consequent narrower margin for errors in design and construction. The consequences of a mistake can be profound.

However, even with modern membranes, the roofing system must accommodate larger thermal gradients, both from solar gain and from ambient changes, especially during heatwaves and more intense weather cycles.

By contrast, a 'cold roof' puts the insulation between joists or beneath the structural deck. If such a roof is unventilated, moisture ingress – from internal sources,  weather or construction moisture – may accumulate, with little opportunity to dry out. This leads to saturation, timber decay, structural damage, mould and loss of thermal efficiency.

In recent years, there has been a move towards what some designers have called a 'hybrid cold roof' where insulation is contained beneath the structural deck, as well as on top of it as in a warm roof. Don't be fooled by the terminology; a hybrid cold roof is still a cold roof and the risk of damage arising from condensation is just as acute.

Accurate condensation risk assessments are vital. Current Building Regulations in England, such as Part L and Part C  – which updated versions of BS 5250:2021 Management of moisture in buildings. Code of practice and harmonised European/ISO standards such as BS EN ISO 13788 – require designers to demonstrate condensation risk using appropriate modelling tools.

Hygrothermal modelling does provide a reliable assessment of building performance – but it is still performed using assumptions rather than actual data, and the outputs are based on a large number of user-selected variables.

Earlier versions of BS 5250 allowed roofs in cold deck form if vented – often requiring a continuous 50mm ventilation gap with cross‑ventilation – and a minimum vapour control layer resistance. However, more recent versions have tightened these requirements.

For example, there is now greater emphasis on minimising thermal bridges, ensuring air‑tightness and considering life cycle moisture performance. Effective ventilation is harder to achieve in large spans without engineered solutions, so unventilated cold decks are best avoided entirely where spans exceed about 5m, unless detailed modelling suggests otherwise.

By using a hygrothermal analysis tool, it might be possible to create a case for a cold or hybrid solution in which condensation does not occur. However, even the most reasoned case can be irrelevant if water is allowed to enter the roof during construction.

Exposure to rainfall before the waterproofing layer has been applied is not uncommon in the UK but if it happens, moisture can become trapped and take many months to dry. Constant cycles of evaporation and condensation enable moisture to be transported around the roof, with the resultant risk of widespread decay – usually to the deck and sections of joist in contact with it.

In another instance, a flat roof in a property in Oxfordshire with large spans and high insulation values had timber joist decay after three years because of impermeable finishes and a lack of drying provision.

Several other cases, also located in Oxfordshire, involved factory-assembled long-span roof cassettes – also known as super cassettes – that contained factory-fitted vapour control layers and glass wool insulation between the joists.

Factory assembly may be strictly controlled, but there is a risk of damage to the VCL during transport and installation, and a risk of water ingress before the building is fully watertight. If this occurs, the cassettes must be dried out properly.

Material selection is also critical. Modern membranes with higher vapour resistance may protect against rain or wind ingress, but may also impair drying capacity. Timber and hygroscopic materials remain particularly vulnerable.

When relative humidity in roof assemblies exceeds 80% for a few months or more, timber moisture content may rise above safe thresholds – usually around 20% – promoting fungal decay.

If you are inspecting and reporting on a building with a highly insulated flat roof – especially one with long spans – be very circumspect as to the risk of entrapped moisture, particularly in circumstances where leakage is known to have occurred or is suspected. It is better to instigate a trail of discovery than ignore clues and face the consequences.

A version of this article was originally published on 27 September 2019.