What is breathable underlay and can I use it?

There has been much confusion and misunderstanding on the subject of breathable underlays. Many people group these products together as one and don’t distinguish the different types including their differing capabilities. Breathable felts come in two types: 

Vapour Permeable

Vapour permeable underlays are often the cheaper of the two types. The fibrous structure of vapour permeable underlays is sufficiently dense to prevent liquid water from penetrating; while allowing water vapour to diffuse. Although water vapour can diffuse, there is still an argument for having additional ventilation to carry this vapour out of the roof space. In fact the NHBC has recently implemented guidelines that state when using a vapour permeable underlay, there should also be high level ventilation to provide sufficient air flow to draw this vapour out of the building.

Air Open

Air open underlays are generally the most expensive form of underlay. Air open underlays (according to their manufacturers) have the lowest vapour resistance and negate the requirement for any other roof ventilation. Whilst this is a claim that is supported by these manufacturers, there are still some questions over the long term performance and its suitability when specified with a close fitting roof covering. Where an external covering (such as fibre cement slates) is relatively airtight, there is a risk of interstitial condensation forming on the underside of the underlay and the external covering; to avoid that risk the batten space should be vented (See BS 5250:2011). There is also often a requirement for additional components such as sealant tapes.

In all cases the use of eaves vent system in conjunction with a ventilated dry ridge is not only the cheapest way to ventilate a roof, but also the most effective and assured in supplying over and above the minimum free air flow required to meet building regulations.