The importance of wind-tightness in ensuring the delivery of designed thermal performance.
I am sure most of us understand the expression ‘tea cosy approach to building’ this is where the thermal envelope surrounds all the exposed surfaces of a house or building. Just like us when we want to keep warm we should protect the insulation with a windproof layer, this is why, when we want to stay warm out on the hill, we protect a fleece with a windproof outer shell.
Leaving insulation susceptible to drafts can dramatically reduce its efficiency. This is where we encounter the word windtight, it is a word used to describe how a building is ‘wrapped’ and protected from the chilling effects of air moving through the fabric. Chilling not only reduces thermal efficiency it also can trigger the condensation of trapped moisture.
It stands to reason that keeping warm air inside a building combined with an effective (controlled) ventilation strategy will help it stay warmer. As a result, contemporary houses combine insulation and an airtight vapour control layer to prevent the free movement of warm air and vapour out of the building and reduce the risk of condensation. The simplest way to explain wind-tightness is to use the example of being out on the hill on a windy day wearing a lovely hi-tech fleece as insulation. The fleece may keep you warm for a while, but the wind will blow the warmth out of the fibres, reducing the fleece’s performance as an insulator. Now imagine the same scenario with a windproof jacket over the top. The fleece can do its job much more effectively because there is a windproof barrier that is not only preventing the heat from being blown out of your insulator by the wind but is also helping to ensure that the trapped warm air between the wool fibres stays close to your body. The same principle applies to protecting the insulating layer of a building.
The type of vapour control function is vital in the same way the breathability of Gor-tex transformed outdoor clothing, as in our example above. Wrapping a building up in non porous material (membrane or board) will trap moisture where it is not wanted and often in areas where it can condense to form liquid water, in either case this can lead to structural degradation or the production of moulds. This is why not only the principle of wind tightness is important but how it is achieved is critical in maximising the designed thermal efficiency. To read more you can download our handy technical guide at the end of this article, but before you do there is more to learn about how wind can cause problems for thermal performance.
For many build structures wind tightness can be achieved through the construction type such as brick and block or monolithic, however most UK houses have one timber frame section, the roof, and it is here that the levels of wind-tightness can be compromised. The most common source of wind tightness failure in roof build ups is wind washing, or thermal bypass. This occurs when exposure to wind causes air movement over and through insulation, stripping the heat from it, thereby reducing the insulation’s effectiveness.
Wind washing is a common problem in homes with insulated pitched roofs, with wind driving cold air through the attic insulation. The performance of fibrous insulation products, such as fibreglass and mineral wool is most likely to be compromised by wind washing because the cold air can move freely through the fibres, cooling the warm air with very significant negative impact on the overall U-value and heat retention of the roof’s insulation
Type of membrane Microporous vs Monolithic
Of course, 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, creating an increased risk of interstitial condensation. If rafters and battens have become wet during construction due to rainfall, the condensation risk and the threat of degradation of the building fabric is increased. Consequently, the building’s fabric could be at risk of failure unless a fully wind tight membrane has been installed, permanently preventing wind and rain from entering while allowing moisture to exit.
The conventional approach to achieving a wind tight barrier is to install a microporous membrane that allows moisture to escape by means of a passive air exchange (See Image). Microporous membranes are designed to offer protection against driving rain and enable both air and moisture movement, however the micro-pores are susceptible to becoming saturated in high humidity levels, particularly at the building stage. Saturation causes the membrane to ‘close’ so preventing it from transporting moisture to the outside effectively. As a result, homes constructed in very wet climates, particularly those exposed to extremely high humidity levels at the building phase, such as most of the UK, may be more vulnerable to condensation if a microporous membrane is used.
This is because microporous membranes function on the basis that water vapour and air can pass 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 pores of the microporous membrane can result in a film of moisture forming on the inside of the membrane, preventing vapour from escaping. This trapped moisture is a source of potential 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.
The alternative is to choose a wind and water tight membrane with a monolithic structure (See Image), providing 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. The combination of these two properties is the determining factor when it comes to both quality and the safety of the roof construction. This is a great advantage in climates where driving rain conditions are typical, such as here in the UK.
The pore-free structure offers maximum protection against driving rain and enables roofing contractors to use the membrane as both a wind proof barrier and a temporary covering.
The active moisture transport capability 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 a compatible tape system, and all cables/ pipe outlets are sealed with a suitable waterproof grommet before the tiling or cladding system 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.
Membranes referred to are
Solitex Plus and Intello are BBA certified
To download our handy technical guide to wind-tightness click on the image below
This is an adapted and edited version of an original article by Fintan Wallace from Ecological Building Systems, first published in May 2016.