hen discussing a fabric-first approach to thermal performance with specifiers, the concept of airtightness is widely understood; after all, it stands to reason that keeping warm air inside a building, combined with an effective ventilation strategy, will help it stay warmer. Wind tightness, on the other hand, has been widely overlooked, which often means that homes do not meet designed thermal efficiencies.

The most common source of wind tightness failure in roof build-ups is wind washing, or thermal bypasss. This occurs when exposure to wind causes air movement over and through insulation, stripping the heat from it, thereby reducing the insulation’s effectiveness.

It’s a common problem in homes with insulated pitched roofs, with wind driving cold air through the attic insulation, compromising the performance of fibrous insulation products, such as fiberglass and mineral wool.

Microporous vs monolithic

The problem with preventing cold air from entering a building through the roof due to the wind, is that keeping the wind out could potentially also result in trapping moisture within the building envelope, increasing the risk of interstitial condensation. If rafters and battens get wet during construction due to rainfall, this condensation risk and the threat of degradation of the building fabric is increased.

The conventional approach to achieving a wind tight barrier is to install a microporous membrane that allows moisture to escape by passive air exchange. While this is designed to offer protection against driving rain and enable movement of both air and moisture, the micropores can become saturated due to extreme humidity levels, particularly during construction.

This is because microporous membranes function on the basis that water vapour and air passes to the exterior through microscopic holes in a passive process that will work when there is a relatively high vapour partial pressure gradient. In highly-insulated homes, significant amounts of water vapour may need to escape quickly and blockages in the microporous membrane can result in a film of moisture forming on the inside of the membrane, preventing vapour from escaping. Trapped moisture could potentially cause interstitial condensation. What’s more, if the surface tension of a microporous membrane is compromised, by wood contents (e.g. turpentine) or solvents in the roof timbers, driving rain during the roof’s construction could penetrate the membrane, causing structural damage and mould.

An alternative is a wind and watertight membrane with a monolithic structure, providing a pore-free solution that prevents moisture from penetrating from the outside, while ensuring the transport of internal moisture vapour to the exterior via its molecular chain. The combination of these two properties is the determining factor when it comes to both quality and safety of the roof construction. This is a great advantage in climates where driving rain conditions are typical, such as the UK.

The pore-free structure offers maximum protection against driving rain and enables roofing contractors to use the membrane as both a windproof barrier and a temporary covering.

The active moisture transport of a monolithic membrane results in a dry structure with significantly less risk of condensation forming within the roof’s build-up. Even a slight difference in vapour pressure between inside and outside will engage the membrane’s chain reaction and ensure the monolithic membrane actually becomes more open to vapour as it becomes wet. This allows interior moisture to escape quickly and effectively through the roof structure. Meanwhile, the thermal performance of the roof’s insulation is not compromised thanks to the wind tight properties of the membrane.

Even if a film of water were to potentially build up behind a monolithic membrane, it can actively and reliably transport vapour along its molecular chains to the exterior.

Delivering design values

For maximum wind tightness performance, the membrane should be bonded as per manufacturer’s guidance, ensuring all overlaps are sealed with compatible tape, and all cables/pipe outlets are sealed with a suitable waterproof grommet before the tiling or cladding is installed.

Building Regulations have helped to improve the thermal performance and construction integrity of UK homes, but there is still much to be done to ensure that the design values of roof build-ups deliver their full potential. Installing an effective and permanently vapour-open, wind tight membrane is an important step in achieving that goal.