Why Ventilate?


The problem of condensation, particularly in cold, pitched roof spaces is not new, but has been exacerbated over the last few years not only by the changes in building regulations, but also by some colder than average winters. The NHBC (National House-Building Council) in particular has seen an increase in the number of new-build properties where significant condensation has formed on the underside of the underlay, dripping on to the insulation and ceilings; causing complaints from new homeowners.

Although condensation can occur in any roof, new dwellings are particularly at risk. Complaints typically happen during the first winter season after completion, when new properties have just been occupied and the property is still in its drying out period, which can take up to 18 months. Although some of this moisture can be vented away from areas such as kitchens and bathrooms, homes are less leaky than in the past, which means more moisture migrates to the roof space. If this moisture becomes trapped in the roof space, condensation can, and will, occur when the temperature in the roof space falls below the dew point.

The question now arises as to the most effective means of removing moisture from the roof space in order to eliminate the problem. More and more properties are now being built, or re-roofed, using vapour permeable underlays (VPU) as an alternative to the more traditional bituminous 1F-type roofing felt.

While bituminous felts have always been subject to the need to ventilate the roof space as a minimum at the eaves, and sometimes also the ridge depending on the building configuration, VPU's promised to remove the risk of condensation without this need for additional ventilation; indeed some well-known VPU manufacturers continue to promote this solution. However, for VPU's to be used effectively in this manner, an almost "fully sealed ceiling" is required to prevent significant moisture entering the roof space; the VPU would then be able to deal effectively with the residual moisture that manages to pass through the ceiling.

The problem in practice is that a "fully sealed ceiling" is very hard to achieve given the many penetrations from the habited rooms into the roof void such as loft hatches, light fittings, cables/wiring, mechanical ventilation units and general cracks/gaps. Thus warm, moist air from the heated living space still has a tendency to enter the roof space and thus cause condensation. During the drying out period in a new home, simply due to the amounts of moisture generated, a VPU often is not able to cope with the resulting levels of moisture that end up in the roof space, especially during prolonged cold weather. For this reason, additional ventilation is still considered the safest and most robust solution.

Whatever the recommendations and solutions, the effects of condensation are well known - droplets on the underside of the underlay and damp insulation from drips at best - severe damp on walls and ceilings and potential damage to the actual roof structure at worst.

So what are the solutions?

Condensation on the underside of a Vapour Permeable Underlay
Condensation damage to roof timber