Fabric first approach to AIR AND WINDTIGHTNESS

Fabric first is a design philosophy that looks holistically at every aspect of the building envelope in terms of its thermal performance and ability to prevent energy wastage due to draughts and air leakage. It champions combinations of materials that will maximise a property’s service life and enhance comfort and wellbeing for the occupier.

The Importance of Air-tightness

While insulation is often specified based on the material’s insulating properties, with a particular focus on its Lambda value, often the finished project fails to meet its designed U-value due to gaps between pieces of insulation and poor levels of airtightness. Even a small gap between insulation panels of as little as a few millimetres can dramatically alter the thermal performance of the whole installation due to unforeseen air leakages.

Thermal bridging at the junction between insulated and non-insulated areas of the building envelope - such as around windows and doors – can also compromise the as-built U-value of a building. This, in turn, leads to a performance gap.

With a fabric first approach to considering the design of the whole building envelope, however, improved thermal performance can be achieved using natural materials, such as Gutex woodfibre and ThermoJute 100, which offer an enhanced ability to deliver airtightness and reduced thermal bridging. By selecting the right combination of insulation and installing it in combination with an airtight vapour control layer to prevent the free movement of warm, moist air out of the building, a fabric first approach can reduce both heat loss and the risk of condensation.

What is Windtightness?

Windtightness, is a widely overlooked aspect of best practice building envelope design, which can also result in a finished building failing to meet its designed thermal efficiency.

The simplest way to explain windtightness is to consider how a woolly jumper acts as insulation on a cold day. The wind will quickly blow the warmth out of the fibres, reducing its ability to insulate, unless you wear a windproof jacket over the top. With the windproof layer in place, the jumper can do its job much more effectively, preventing the heat from being blown out of the insulating layer and trapping stale, moist air inside the construction. The same principle applies to insulated roof and wall build ups.

The problem with preventing cold air from entering a building, however, is that this could potentially also result in trapping moisture from rain during construction within the building envelope creating an increased risk of interstitial condensation. Consequently, the windtight membrane selected must be able to allow moisture to exit. 

Microporous Vs Monolithic Membranes

The conventional approach to achieving a windtight barrier is to install a microporous membrane that allows moisture to escape by means of a passive air exchange, which works efficiently when there is a relatively high difference in vapour pressure between the inside and outside. However, the more significant the amounts of water vapour trying to escape, the more quickly moisture particles move towards the available escape route and the micro-pores are susceptible to becoming saturated and blocked, which may result in a film of moisture being formed. This prevents vapour from escaping, creating a condensation risk within the building fabric.

Conversely, a wind and water tight membrane with a monolithic structure, such as the pro clima Solitex range, provides a pore-free solution that prevents moisture from penetrating from the outside, while ensuring the active transport of internal moisture vapour to the exterior via its molecular chain. This molecular structure provides the most robust solution for breather membranes.

The pore-free structure offers maximum protection against driving rain so the membrane can be used both as a wind proof barrier and as temporary covering during construction to prevent moisture from penetrating the roof build up.

Written by Fintan Wallace Architectural Technologist, Ecological Building Systems

Posted on Wednesday, 11 September 2019
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