This is the most common roof type in use even though it has most problems and is, hence, the most inefficient. You must provide adequate ventilation to this roof space (building regulations F2) as well as a high degree of insulation (building regulations L1) to create sufficient separation between habitable room space and roof space, usually at ceiling level. In order for condensation not to cause problems, it is critical that the roof space is ventilated thus dispersing any vapour that is present. Old roofs that have no under felt are draughty thus giving good ventilation, but the loss of a tile would cause a leakage;
Cold vented roof has also got a very high heat loss ratio. Recent history has seen the introduction of a bitumous membrane (1F) to combat heat and tile loss problems but the introduction of loft insulation at ceiling level together with inadequate ventilation has, in essence, created this condensation problem. Even when a roof has eaves ventilation the wool insulation is often jammed into the eaves thus restricting the ventilation or conversely the insulation is left short creating a thermal bridge in the corner of the ceiling in the room below, resulting in mildew. Both of these problems are a breach of the building regulations and are difficult to correct remedially when faced with local enforcement. Cold vented roofs will continue to be widely used throughout this country through existing properties that need re-roofing because of practical problems would make the job not cost effective.
Cold vented roof must be ventilated at eaves level by at least 10000mm/m2 equiv 10mm continuous ventilator on smaller roofs or more commonly 25000mm/m2 equiv 25mm continuous ventilator on standard roofs (ventilation volumes should be calculated on larger roofs such as public buildings). Ventilation can be achieved by many means but usually the use of soffit, eaves or over facia ventilators. This roof must create cross ventilation to eliminate un-vented voids and this sometimes requires connecting to ventilation at the ridge. Ridge ventilators are usually 5000mm/m2 min equiv 5mm continuous but care should be taken that they are not larger than the eaves ventilators or it can have a negative effect. Cross ventilation may also be achieved by using gable vents in the case of lean-to or tall roofs or over fascia vents above flat roof extensions (detailed).