Cold pitched roofs constitute 80% of domestic roofs in the UK and are commonly used when constructing new dwellings. They typically have horizontal insulation at ceiling level and a ventilated volume of cold air above. As the heat loss from low energy dwellings is reduced moisture risks increase, unless accompanied by ventilation and/or airtight construction. Current research suggests a ventilation slot is required in the U.K. climate – even in airtight dwellings. Whilst theoretical models can and should inform design, they are of limited use if their real-world performance is not verified. This paper examines the hygrothermal performance of a sealed (unvented), windtight, cold roof with horizontal blown mineral wool insulation located above an airtight house located in Northern England. The roof has been monitored over the duration of 24 months. The research has assessed the viability of such roofs in the UK climate. The results are somewhat surprising, counter intuitive and defy conventional expectations.