Solutions


In the following we discuss the various solutions available to control condensation in domestic roofs covered by traditional "air-open" slates and tiles commonly used in the UK. It should be noted that some roof coverings like metal, fibre cement sheets or fibre cement slates are not generally classified as "air-open" and different solutions need to be employed as described in the British Standard, BS5250: Code of practice for control of condensation in buildings.

traditional solution

Traditionally, bituminous 1F-type felts have been used in the UK for domestic pitched roofs since the 1970's, along with roof void ventilation provided at low level or "eaves to eaves" ventilation (see image 1 below). For a cold pitched roof with a loft, and insulation at ceiling level - by far the most common type of pitched roof in the UK - a gap is created between the insulation and the underlay at the eaves so that air comes into the roof space and out the other side..

The only drawback with this method is that its effectiveness is very dependent on wind to create the through-draft necessary to remove water vapour; and often cold-snaps can have very little wind. Furthermore, when wind is present, it is not necessarily in the optimum direction and this can limit the passage of air into the roof void and subsequently the effectiveness of moisture removal. There is also a problem with steeply pitched roofs, where stagnant pockets of moist air can get trapped in the apex, untouched by the through-draft at low-level. Often, on steeper roofs (greater than 35 degrees rafter pitch), this low level ventilation is therefore supplemented by high level ventilation commonly provided at the ridge.

Image 1 - "Traditional Solution"
Image 1 - "Traditional Solution"

Popular Solution Using VPU's

As discussed previously, with the ever increasing use of VPU's in pitched roofing, it has become popular to use VPU's without additional ventilation both in warm pitched roofs - where the insulation is located at rafter level (see image 2 below) - as well as cold pitched roofs (see image 3 below). The use of VPU's in this manner has been promoted by the British Board of Agrément (BBA), the UK's leading independent authority for the approval of new construction products. For the use of these products to be successful in unventilated roofs, it is critical that water vapour is prevented to a very large extent from entering the cold roof space above the insulation; essentially an almost "fully sealed ceiling" is required. If this can be achieved then the VPU normally has the capacity to deal with any residual moisture that manages to pass through the ceiling.

Image 2 - Warm Pitched Roof with Insulation at Rafter Level
Image 2 - Warm Pitched Roof with Insulation at Rafter Level
Image 3 - VPU Popular Practice in Cold Pitched Roof
Image 3 - VPU Popular Practice in Cold Pitched Roof

In practice, in warm pitched roofs this solution is often quite feasible - indeed the preferred solution - particularly when vapour control layers are present as is often the case in new housing with rooms built into the roof. Unfortunately, problems can, and do arise, when these products are used in cold pitched roofs without additional ventilation. The difficulty arises because a "fully sealed ceiling" is very hard to achieve in practice in this type of roof construction. Cold pitched roofs almost never have an effective vapour control layer installed (since it is practically very hard to do). Moreover, numerous penetrations from the habited rooms into the roof void such as loft hatches, light fittings, cables/wiring, mechanical ventilation units and general cracks/gaps allow warm, moist air much more license to enter the roof space and thus cause condensation. This type of construction is therefore very vulnerable to roof condensation especially during the buildings' drying out period simply due to the volumes of water vapour entering the cold roof void. The VPU often is not able to cope with this level of moisture on its own during prolonged cold weather and additional ventilation in practice is still advisable.

New VPU Solution for Cold Pitched Roofs

High-level roof space ventilation, when situated at or near the ridge is an effective and proven form of passive roof ventilation in all weather conditions: it forms a stack effect, creating a draw that is not dependent on wind.

This method has always been recognised as an effective way to ventilate roof spaces, but up until recently the UK had largely experienced a decade or so of mainly mild winters. During this benign period there was a steady trend to use unventilated roof designs relying solely on VPU's to allow moisture to escape. The recent change in weather patterns over the last few winters has brought the wisdom of this approach into acute focus as the number of condensation complaints has soared in cold pitched roofs, especially in new housing.

Although the British Standard recommends that cold pitched roofs should have additional ventilation (either at high or low level) when using VPU's, the British Standard does not preclude the use of underlay products without ventilation used in accordance with their BBA certificates; and therein lies the problem.

The NHBC however, since January 1st 2011, now considers that roof space ventilation at high level should be provided in ALL cold pitched roofs using VPU's covered under their Buildmark Warranty and this requirement will take precedence over BBA certificates. (see image 4 below)

Image 4 - New NHBC Requirement with a VPU in a Cold Pitched Roof
Image 4 - New NHBC Requirement with a VPU in a Cold Pitched Roof